Resting egg production and oviducal cycling in two sympatric species of alpine diaptomids (Copepoda:...

,

Journal of Plankton Research VoU7 no.ll pp.2049-2078, 1995

Resting eggproduction and oviducal cycling in two sympatricspecies of alpine diaptomids (Copepoda: Calanoida) in relation totemperature and food availability

c.D.Jersabek and R.Schabetsberger

University ofSalzburg, Institute ofZoology, HellbrunnerstrafJe 34, A-5020Salzburg, Austria

Abstract. Resting egg production and oviducal cycling were investigated for the calanoid copepodsArctodiaptomus alpinus and Acanthodiaptomus denticornis both in the laboratory and in a small karsticalpinelake by making a census of the number of eggs produced and the proportion of females in each offour morphologically distinguished reproductive conditions each day in the laboratory or during a 2-3week period in lake enclosures. In the laboratory, individuals were maintained on a mixed diet ofnatural phytoplankton at constant temperatures of 4, 10, 15 and 20oe, respectively. Both species differed considerably in their temperature requirements for reproduction. Lifetime fecundity was highestat lOoe inA.alpinus and at 200e in Aideruicornis, with up to 327 eggs ~-I spawned in the former and upto 582eggs ~ -I in the latter species. Unfavorable temperatures were further reflected in an increase inegg mortality and the allocation of time spent in a post-reproductive phase, as well as in a decrease oflongevity. Increasing temperatures enhanced egg production rates due to decreasing clutch productionperiods, although clutch size was negatively correlated with temperature. Maximum rates reached 5.88and 7.98eggs S?-I day-l in the laboratory, and 0.73 and 0.55 eggs ~ -I day -I in enclosures inA.alpinusandA.denticornis, respectively. Egg production rates and clutch size were clearly governed by nutritionalconditions in the lake, but were less affected by food supply in the laboratory. Here, rates of egg production were adapted to improving food supply by increasing the frequency of spawning events, ratherthan the number of eggs per clutch. No correlation was found between female body size and reproductive parameters in the laboratory. A very low proportion of total clutch production resulted inclutches composed of subitaneous eggs, Le. 0.14% in A.denticornis and 1.20% in Aialpinus. Oviducalphase duration allocations indicate that there exists a temperature optimum for gamete maturation.

Introduction

Arctodiaptomus alpinus (Imhof) and Acanthodiaptomus denticornis (Wierzejski)are the most common calanoid copepods in high mountain lakes of the Alps, whereAialpinus tends to be found at higher altitudes (mainly above 2000m a.s.l.) thanare usual for A.denticornis (mainly between 1500 and 2000ma.s.1.) (Kiefer, 1978).However, although there is an appreciable range of altitudinal overlap, cooccurrence of the two diaptomids is excessively rare (Bossone and Tonolli, 1954;Ravera and Tonolli, 1956;Kiefer, 1971). Both species are in general univoltine andproduce exclusively hibernating resting eggs at high altitudes, but populationshave also been described that alternate between subitaneous and resting egg production, although very rarely is there evidence that subitaneous eggs contribute asecond generation of adults to the population (Hacker, 1901;Ravera and Tonolli,1956;Eichhorn, 1957; Ferrari, 1971).

Towards higher altitudes, amplitudes of production cycles on all trophic levelsbecome increasingly narrow in temperate areas, so that organisms may face drastically shortened periods suitable-for reproduction. Thus, a temporal separation ofreproductive activities may hardly be a possible way out of competitive interactions in univoltine copepod populations in alpine environments. If similar sized

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© Oxford University Press 2049

to reproduce in a wide range of conditions (Jamieson and Burns, 19BB). Empiricalstudies of egg production may, therefore, lead to a better understanding of one ofthemechanisms linking the ecology and zoogeography of diaptomids, all genera ofwhichhaving restricted biogeographical ranges (Banarescu, 1990). As adult copepodSdo not rnolt after reaching sexual ma:urity, measuring the oocyte turnoverrate gives an insight into the net productIOn of female copepods (Watras and

Haney, 1980; Vidal and Smith, 1986),The necessity of field studies to evaluate the relative importance of laboratory-

derived factors in determining diaptomid reproductive parameters was recentlyemphasized by Chow-Fraser and Maly (1991). Ianora (1990) stressed the fact thatconventional methods to assess potential rates of egg production over a short timeinterval (generally 24 h) are likely to underestimate rates of egg production if dif-

ferences in reproductive conditions of females are ignored.In the present study, long-term variability in reproduction, including lifetime

fecundity, oviducal cycling patterns, clutch size variation, egg mortality, individualvariability of daily egg production, and longevity of A.alpinus and A.denticornisare compared at different constant temperatures in the laboratory, aswell as undernatural conditions in Lake Dreibriidersee over periods of more than 5 and 2months, respectively. This should allow for predictions to be made on their abilityto reproduce in a particular habitat due to temperature adaptations of reproductive capabilities, as it is hypothesized that either ofthe two species may becomecompetitively superior, depending on the thermal properties of a lake. A description of Lake Dreibriidersee, where the two diaptomids co-occur, is given in

Jersabek and Schabetsberger (1995).

Resting egg productionand oviducal cycling in diaptomids

Method

Laboratory experimentsSource and adaptation of animals. Zooplankton were collected from the oligotrophic alpine Lake Dreibriidersee in August 1990by vertically or obliquely towing a 0.5 m diameter plankton net (100 ..m mesh size) from near the bottom to thesurface of the lake, which was almost homothermic at 14°C. For a selective sampling of the predominantly epibenthically distributed population of A.alpinus, thenet was also horizontally towed by a diver just above lake bottom. Cod-end contents were gently diluted into 11 polyethylene vessels containing pre-cooled lake

water andtransported to the laboratory within5h of capture,Upon arrival in the laboratory, the plankton samples were placed in a 7°C cold

room, and immediately afterwards adult A.denticomisand A.alpinuswere pickedout with a wide-mouth pipet and for direct observation transferred individually to175ml beakers containing 30 ..m filtered eutrophic pond water. The wa~er~Ualltyand the food available in the pond water proved suitable for both speCies m previous experiments, when lake produced eggs could be reared to reproducuvematurity within a period of 3-4 weeks. Temperature acclimation was extendedover 3 days, so that temperature changes did not exceed 5°C day". Each femalewas placed with one male in a beaker and together they were allowed to acelun

ate

to experimental conditions until the female dropped its first clutch in the lab-2051

1It1IIiI,j1~

C.DJersabek andR.Schabetsberger

diaptomid speciesoccursympatrically in high-mountainlak .may be achieved by distributional differences within the Ie~, habitat partitioningdata). However,Maly (1973)foundintraspecific competitio

a.e (o~n unpublished

be stronger than interspecific competition, even in small hi h-ali Dzaptomus spp.toably dueto different food requirements of diff . g ltitude ponds prob-" 1 erent species D iff 'mg behaviors and associated morphologicalft' 1 erencesinbreed-between svmpatri ea ures were found t b, een sympatnc than between allopatricspecies of Ps . 0 e greatermg that selection reinforces both behavioral and mor ho~UdodzaPtomus,indicar.leadmg to more complete reproductive isolatio ? ogical divergences, thus(Jacoby and Youngbluth 1983) The adapt' n .pn~fir to the waste of gametesdu ti tratezi " ive sigm cance of diffc ive s rategies of co-existingCalanoid ' di 1 erent repro-(1988). 1 a IS iscussed by Chow-Fraser and Maly'

Th~ Diaptomidae belong to the minority of calanoids that tvoi ' teggsm egg sacs untilthe nauplii are ready to h t h ds that typically carry theirdropped, rather thanreleasingthem continuo al c, or a clutch of resting eggs isment (HuysandBoxshall, 1991),Thus the ha~~~ mto t~e surrounding environ.theycanadapt eggproduction ratesto foolsu I w~ mam mech~nisms bywhichameters being constant. First rates f pp y, Wlt~ other environmental par-increasing the frequency of cl~tch depoo .et?g pr~ductlOn may be increased byre ,. SI IOn with the numbe f

mammg constant. Second,increasing clutchsizes m r 0 eggs persacoutput at constant spawning rates. In ener I ay.enh,ance the reproductiveencountered in diaptomid copepods Th a , a combination of the two will beall f d ' e mean clutchsize of di t ids i

Y oun to be a negativefunctionof t lap orm s IS gener-and Maly, 1991) anda positive functi emperature (Elmore, 1983; Chow-FraserButler, 1987; Chow-Fraser and Mal;,I~~9~f f~~~d~oncentration ~~ill~amson andbetweentemperature andfood eff t ). nee of synergistic mteractionslidus was presented by Willi ec s on egg productionrates of Diaptomuspal-ture-independent food li~~::son .andd Butl~r (1987), who developed a tempera-t IOn m ex which permits 'emperature effectsbymakinguse f th k separation of food and

between gravid and non-gravid c~ndit~ now(ledge thatmaturefemales oscillate1967; Katona, 1975; Moore and Sande IOns ~arshall and Orr, 1952; Conover,1983; Runge, 1985b; lanora 1990) andr~~~7:, Watra.s and Haney, 1980; Watras,one fertile clutch per matin~ (W t da emale diaptomids can only produceproportion of females present' a r~tshan Haney, 1980; Watras, 1983).Thus the

m et er the dark ph (i 'mature oocytes) ortheclearph C' ase Le, oviducts filled withegg production due to inadequ~~: :.e..~~~~cts transparent) may indicate limited

In order to understand life I vai a inty of mates or food, respectivelyductive characteristics may cY~de strategies, information gathered on r~pro-.[:' prOVI e valuable arg t f ' ,

prererences m both ecological d I' umen s or explammg habitat. an evo utionary ter Thi

species thathave to achieveg th ms. IS IS especially true oflimited periods of time as i~oW t' an(~rep:oduction (or energy storage) withinHirche, 1989' Smith 19'90) arlc ,IC omita, 1956; Hirche and Bohrer 1987'

" or a pme (thi t d ) , "adaptations of reproductive ea bili , IS s u y environments. Temperaturedi I pa 1 ities may constitut k fispersa of alpinecopepodsal 1,. e a ey actor actingon thereproductive characters such:ngt h,t~dI~al or latitudinalgradients. Variance interns, maybe justasimportant s p asticity m clutch size and oviducal cycling pat

asmeanreproductive t t2050 s ra egy, and allow copepods

M = G + 0 + B x 100

Nf = 0 + G +B +N x 100

G

under given conditions and the total fecundity of singlefemales is simply estimated

bycounting all produced eggs,To relate reproductive parameters to the body size of females, prosome lengths

weremeasured between the anterior tip ofthe cephalothorax and the lateral end ofthelast thoracic segment upon their death. Measurements are accurate to the near-

est 10~m.The pattern of changes in the oviducal cycle was monitored by relating the stateofthe oviducts to either a dark phase (i.e. gravid) or a clear phase (i.e, non-gravid),Depending on the presence or absence of an external ovisac and the degree ofoocyte maturity, female diaptomids can be in one of four major states of reproductive condition: G = gravid/non-ovigerous; 0 = non-gravid/ovigerous; N =nongravid/non-ovigerous; B ~ gravid/ovigerous (cf. Williamson and Butler, 1987).When a change in reproductive state was noted, the point midway from the pre-

viousobservation was designated as the time of occurrence.Applying these criterions to the indices developed by Williamson and Butler

(1987) for DiaptomUS pallidus, an attempt was made to quantify the relative effectof food and/or mate limitation on reproductive rates, Here, the food limitationindex (f) and the mate limitation index (M) are based on quantifiable responses ofthe oviducal cycle and are expressed by the following equations:

Food availability. To exclude inedible phytoplankton species like large colonies ofCyanophyta and Chlorophyta, as well as herbivorous metazoans from the foodsuspension, pond water was 30 urn filtered prior to further handling. Aliquots of100ml, fixed with Lugol's iodine, were taken in weekly intervals for phytoplanktonandprotozoan counting, Biomass estimates are based on counts according to Utermohl's method and calculations by volumetric approximation to geometric shapes,

The phytoplankton assemblage in the filtered pond water was almost entirelycomposed of small unicellular species, with cryptomonads (Cryptomonas spp.Rhodomonas minute) being the dominant group during the first 3 months,Depending on weather conditions and pond temperature, the phytoplankton biomass fluctuated between 0.14 and 0.68mg freshweight ml' (0.33-2,78 x 10

3

cellsml') during the main spawning period from August to November (Figure 1). Thesubsequent decline of biomass was mainly due to the decrease in cryptomonadspecies. From December to January, the algal biomass remained consistently low,but the qualitative spectrum offood items varied considerably. Diatoms (Synedraacus, Synedra sp., Cyclotella sp., Fragilaria construens), ",-algae (small «5 urn)Chlorellaceae and unidentified flagellates), green algae (Scenedesmus spp.Koliella cf. longiseta) and dinoftagellates (Gymnodinium spp., Peridinium sp.)

2053

These indices should be generally applicable to other diaptomids, provided that

life history characteristics are similar to those of D.pallidus.

Resting egg production and oviducal cycling in diaptomids

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Experimental design. Twenty experimental beakers (volume 1and temperature, each with one female and one 1 75 ml) per species

h Id' ma e were pla d i

o mg 30 ~m filtered pond water. The latter were arran , ce m 301

basinsperatures of 4°C (:':O.2°C), 10°C (:': 0.2°C) 15°C (+0'f;~m water baths atternrespectively. A direct exchange between th~ beaker-;- . I ) and 20°C (:':0.10c),water, kept in slight turbulence by air bubbling w s vbol udme and ambient pond100 ' as ena e by 15 2'

J.Lm mesh nylon gauze, One-third of each basin's 1 cm windows offresh filtered pond water. All laboratory w k vo u~e was replaced dailybyVot h' I' , or was carried out m 'Ho se climatic chamber at constant temperature of 10 00 + o a eraeus,

light-dark cycle. Illumination was provided by 'COOI~W;( -;0.2 C) on a 12:12hmounted 1m overhead (4 x 58 W), ite fluorescent tubes

oratory, at which point scoring began from the first cle hfull reproductive capability, only egg-bearing femalesarPd ase onward, To warrant

t h' an mature m I (

op ore recogmzable) were initially used for expe . a es sperma-t ' nments Each e .ermmated after all females had ceased reproducti ' xpenment wasIon,

C.DJersabek and R.Schabetsberger

Eggproduction and reproductive conditions Obse 'August to December 1990,and after termina~ionOf~~atIOns ,:ere made daily fromevery second day from January to February 1991 T~ee;p~n~ents at 15and 20°Cand scored: female reproductive state ( .d' 01, owm~ were measuredclutch extrusion, clutch size, proportion ;:~~n~v7~~::avld,o~Ige,rous), time ofopaqueness) and mortality of females Diff " ggs (as indicated by theirized and unfertilized clutches or oocy~e e

It ere,ntlatl,On was made between fertil

result of laboratory constraidts som f xrlusIOn without ovisac formation, As a(i , , ' e ema es may be abno 1 'i.e, parasitized, damaged or infertile) Th rma m some respect

of the whole laboratory population, but ~:~ntotonly the averag.e ~gg productioneggs IS of interest Whenever the he maximum individual output of

f" re was reason to sus t '"

emale (l.e. abnormally prolon ed id .pec mate Iimitation of alethargic male), the male was;e 1;~~~I phase, e~trusIOnof unfertilized oocytes,(eggs S? -I day') were calculated ~s the ~v~re:n daI,ly :g? production rates (EPR)vation interval normalized to 1 da Th . di g~ of individual EPR, and the obsering clutch size by the correspond·Y' le m

hividual ~PR were calculated by divid

checks were conducted on femal swh: utc 1production period (CPP). Additional

T' esw osec utch extrusi d

o obtam spawning frequencies (clutches S? -1 -1 r~SIO? ~as ue at any moment.by their corresponding individual clutch si day), individual E~Rwere dividedpods were transferred to a fresh f d z~, After each spawmng event, copewere poured through a 100 m ~ol suspenSIOn, and the contents of their beakersmean CPP and mean clutch :ze fO~~: SIeve t~ collect all eggs, When calculatingclutch produced by each fern 1 c~ expenmental temperature, the last viable

As both copepod pOPulati~:s::~ ~t~Ittedt~ red~ce ~he influenc~of senescence,th,e sampling program was p f ctly umvoltme m Lake Dreibrudersee and

, erformed at the ti ffi 'females m the lake it may b d ime 0 rst occurrence of ovigerous, e assume that '

more than one or two clutches before c expenmental females had not producedoutput should approximate th apt~re. Thus, the observed reproductive2052 e reproductive output during lifetime spawning

Enclosure experiments

Experimental design. To determine the in situ egg production of both species,enclosure experiments were carried out in Lake Dreibriidersee from September toNovember 1989. Transparent tubes (height 50 cm, volume ~5 I) provided withremovable 100 urn nylon nets on either end were adapted as plankton cages. Eachcage was equipped with two opposite 100cm2 windows of 200 urn nylon gauze toensure a sufficient exchange with surrounding lake water, and a small pipe fortaking water samples from the enclosure. Eight cages of either species werestocked with 70 animals (approximate natural densities; ~: 0 =1:1), and then suspended in the lake at a depth of 4 m (3 m above lake bottom). Three empty cageswere used as reference controls to correct EPR for a possible input of non-cageproduced eggs (egg diameter 115-135/-Lm). A quick and reliable species- and sexspecificseparation of copepods used in experiments was feasible by discriminatingfreely swimming adults according to different posture of antennulae. In this way,no optical aid was necessary and the stocking of one cage could be completed in<10min. Only mature females (gravid or ovigerous) were selected, to facilitate abetter comparison of the two species. Thus, our results may overestimate the in situeggproduction, as immature females were present in the open lake during the first3 weeks of the experiment. In general, each set of animals remained unchangedthroughout the whole experiment. However, if the number of females in a cage fellbelow 20, or sex ratios deviated considerably from one, initial densities or sexratios were. re-established.


attained a varying relative importance. The abundance of ciliates in general waslowat times of low phytoplankton concentrations. While peaks in ciliate biornassduring summer and autumn were related to planktonic species (small urotrichids,Strombidium sp., Strobilidium sp.), the increase at the end of experiments wasrelated mainly to the appearance of a large hypotrichid (Stylonychia sp.) species.

Eggproduction and reproductive conditions. Cages were inspected at intervals of17-19 days which, in view of comparatively long clutch renewal times in the lake,proved to be fully sufficient. Animals and eggs were simultaneously retained by thebottom net while raising the cage carefully above the water surface. At the end ofeach interval, numbers of eggs produced, clutch size and reproductive state offemales were controlled in each cage. The latter two were compared with that fromopen-water samples and from the near-bottom layers, respectively. Mean EPRwere calculated by dividing the number of produced eggs by the number offemalesand the duration of the time interval in days. In the event of animal losses, thedegree of decomposition of dead females was used to weight the mean number offemales present in a cage. It was assumed that strongly decomposed females diedduring the first, moderately decomposed females during the second, and weaklydecomposed females during the last. third of the mentioned time interval. Thisassumption seems to be justified by prior observations of copepods decomposingat comparable conditions. Mean clutch sizes were derived separately from intactegg batches dropped to cage bottom and from clutches attached to females when

2055

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20 ;0 40 60 70 60 90 100 110 120 130 140.: 'eo."e~." ";:_"8" ,,;;,n

~~ 1.Rate?'~f!:!U!~! ~~!::!~!~:! and.. enticomis (b) ~t d.Ifferenttemperatures in the laboratory. Symbols represent 5 da~ means' quanti

~atl~e (c) and.quahtat~ve (d) changes in the biomass of natural phytoplankton and ciliated protdzoansin00 suspensions dunng laboratory experiments; fw == fresh weight.

2054


inspected. Mean times of clutch renewal (clutch production period or CPP) werecalculated using the formula:

nCpp=~ (

1

where d, ... n is the duration of experiments in enclosures 1 to n, negg1 .•. n is thenumber of eggs produced in enclosures 1 to n, nF1 .. , n is the weighted number offemales in enclosures 1 to n, cl is mean clutch size and n is the number of enclosures.

Female reproductive states and indices of food and mate availability were considered by applying the same criterions as described above.

Food availability. On each occasion, water samples from two alpinus and twodenticornis enclosures were drawn off by a diver using a 100 ml syringe. Concurrently, plankton samples were taken from the same depth near the enclosures witha plankton trap. Biomass estimates of algal and protozoan plankton were obtainedas described above. The large inedible dinoflagellate Ceratium hirundinella(O.F.M.), which accounted for 75-85 and 20-40% of total phytoplankton biomassin September and October, respectively, was not included in biomass estimationsof food organisms.

Statistical analysis. Reproductive parameters of both species were exploredbetween different temperature treatments using ANOVA and post hoc multiplecomparison of means [modified LSD (Bonferroni) test] and the regression andcurve-fitting procedures of SPSS for Windows 6.0.1 statistical package (1994).Levene's test for homogeneity of variances and the Kolmogorov-Smirnov goodness of fit test were applied to each data set. In cases where no heteroskedasticitywas observed and data were normally distributed, data were analyzed untransformed. In most cases, data transformation or the application of non-parametrictechniques was necessary. Survival analyses were performed with the Kaplan andMeier product-limit method provided by SPSS.

Results

Laboratory experiments

Generalpattern ofeggproduction. The females of both species differed individually under laboratory conditions, but shared a common pattern of egg production,which was superimposed by different thermal preferences (Table I, Figure 1).With only two exceptions, females of A.denticornis were unable to produce viableclutches at 4°C, but apart from that, only two out of 140 females failed to commence breeding: one alpinus female at 20°C and one denticornis female at 10°C,the latter being heavily affected by a fungal parasite. While individuals ofA.denticornis were randomly distributed within the experimental vessels whilefeeding, those of A.alpinus predominantly concentrated at the bottom, withfemales showing a more close affinity to bottom than males. There was no diurnalperiodicity in phase onset or spawning.

2056

• Resting egg production and oviducal cycling in diaptomids

. ters and egg viability in laboratory experiments. CV = coefficient ofR oductlVe parame .

Table I. epr d d error of the mean; l.s. = life spanvariation; SE - stan ar

A.denticomis A.alpinus

Characteristic 4°C 10°C 15°C 20°C 4°C 10°C 15°C 20°C

;:n number of clutches S?-I 0.2 16.4 25.0 30.1 4.9 12.5 10.0 6.4

(1.10) (1.56) (1.50) (0.55) (0.90) (1.03) (0.60)

(SE) ° 21.4 20.4 22.2 30.9 25.9 31.5 24.6Clutch sizeCV ( Yo) ° 0.33 1.03 0.80 - 4.10S bitaneous egg clutches ( Yo) 12 331 520 633 114 253 207 131~ mberof oviducal cycles 33.3 92.5 96.5 94.0 80.7 96.7 94.6 90.1s:ccessful clutches (%) 25.0 85.2 90.2 83.9 68.4 90.1 84.5 77.1100% viableclutches (%) 8.3 7.3 6.3 10.1 12.3 6.7 10.1 13.0+some non-viable, eggs (%) ° 25.0 5.1 1.7 2.7 6.1 1.6 1.4 3.1Amorphous mass III egg sac ( Yo)

41.7 2.4 1.7 3.3 13.2 1.6 3.9 6.9Oocyte extrusion, no egg sac (%)

1.14 0.77 1.43 3.45 3.69 0.62 1.96 3.68Non-viable eggs in clutches (%)

56.1 132.5 113.7 102.3 129.9 106.2 66.1 38.2Lifespan in experiment (days)

12.9 89.4 93.1 89.0 79.8 87.7 76.5 64.7Spawning period (% l.s.)post_reproductive-phase 1

72.4 8.3 4.9 9.8 13.1 6.4 18.7 31.0(%l.s.)post.reproductive- senescent

14.7 2.3 2.0 1.2 7.1 5.9 4.8 4.3(%1.s.)

31 31 14 21 18 5 22 20Number of 0 replacements

(3.2) (67.7) (71.4) (61.9) (66.7) (60.0) (63.6) (30.0)(%successful)

in clutch size and spawning frequency, and thus in mean daily ~gg pro-p~aks ith me delay followed times of abundant food supply (FIgure.1).

*t,~[~~s:~e:dw~el::was causally related t~ a~e~p~~::~~efr:~:tc:~~ches:~~for converdsion .of~~ge~:~~~~~:~e(~::~~cl~ne~r regressi~n coefficients reflectareplotte agams 00. . ence between the two: A.alpinus 4°C:a rather low and non-:I~n~ficantc(orr~~p~~~150C r2 = 0.11 (P = 0.67); too few datar2 =0.08 (P=0.34); 10 C.: ::=0..13 t: ~-0'23 P·=0.08)'150C: r2::=0.26 (P::=0.07);obtained at 20°C; A.dentlcorms 10 C. r bt·' ~ at 40C As the linear relationship20°C: r2=0.12 (P=0.27); too fe.w data 0 ame u 1 is' si nificant A.alpinus 4°C:between rates of egg productIOn andOf~~~ ;5Fg~ == O.9~ (P < 0.05); A.denticorr2==0.31 (P < 0.05); 10°C: r2=.0.3~ (~;-o 54 (P <'0.01); 20°C: r2::=0.53 (P <0.01)nis 10°C: r2=0.41 (P<0.05), 15 C. r - . [on to im roving food avail(Figure 2) the two species seem to adapt e.gg productI H P ver if individualability m~inly by increasing their spawnm~ frequenc~~di~;~lut~hproductionclutch sizes are plotted together with th~Ir c~~r~~s a larger clutch to be prodperiods (Figure 3), it can be shown that the tIme.I dfo the production of a smalleruced is not necess~rilylo~gerthan the ti~e reqUIret : spawning rates seems to beclutch. Thus, also mcreasmg the clutch SIze.at cons an

a possible way of enhanci~g the reproductlVel~~~:~'fecundity, the slope of theWhen maternal body SIze was rel~ted to . A significant relationship

regression line was not significantly ~Ifferent fro~ ~~:~' could be traced only atbetween prosome length and maximum elute

2057

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5 10 15 20 25 30 35

o 10 15 20 25 30 35

20 L...-l--,---,---,-...L:-L..--L-'--.l.-l---.:.'":"""---:::""o

140 • a.lpinus n=115• 0 dentic. n=599

8 0 0 200C120 .o.~ i· "\

16010 15 20 25 30 35

-. o,lpinus n=1960 dentic. n=500.. .. 15°C.

.o •• ... .., . . .••••••- • I· . :..• · 1 ... ... ·..·I.! ·.•·.··.1.. ..

• o,lpinus n=92° dentic. n= 4

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. ti es but were easily dropped at disturbance, like experimentaldutch-carrymg im ,handling or mating attempts of males. . f time

As a further criterion affecting the reproductilve sUIccefsfs, thteedPrboyp~:~pne~ature.d f phase was c ear y a ec

spent in the post-repro uc l:,e. he relative extent of the spawningTogether with a drastic reductlOn of hfespan, t. . d b 100CinA alpinus

. d' .. h d t4°CinA.dentlcornlsan a ove '.penod was strongly lI~l1ms ea. T bl I) Within the post-reproductive penod(P < 0.001, Mann-Whltney V-test, a e. . batch to death) two dis-(here considered as th~ t.ime~rom the l~st s~~~e~:~~~~~~e,females retai~ed theirtinct phases may be dlstmgUls~ed. D~nng it re eated male replacements,capability for oocyte maturation which, despi ~ in fhe clear phase after the lastresulted in no viabl~ clutches. If females r~~am:t 1 be senescent (second phase).emptying of the oviducts, they were consi ere ith

0unable to refill their ovaries

However at 4DC A.denticornis females were el er, . t t xtrude mature oocytes.

during the whole exp~nmen or 0 e. des resulted in clutches composed ~fA very low prop.ortlOn of reprod~ct1vfe ~al clutch production of A.denticornzs

subitaneous eggs, l.e. 0.14 and 1.~0 ~ 0 .td

. . only one out of 80 females. ti I While m A entlcornlS

and A.alpznus, respec ive .y. h f llowing28 clutches of resting eggs at(1.2S%) produced two subltaneous clutC

IeS(7

0S0I< ) of A alpinus contributed subi-

20°C prior to death six out of 80 fema es '. ° ., 2059

clutch size.' . f clutch size at different temperatures in the laboratory.

Fig.3.Clutch productIOn penod as a ~und~t~~n ~ clutch Please note the different scales of the y-axes.Each data point corresponds to one III IVI ua .

iII

A. denticornis

70 lO·C

6• l5·C •[J 20·C 20°C

5 [J

[J •

4- 15°C'0

* •4°CM>

A. o,lpinus

1 l.t;,....

b

21 0 21

~e

0

.~ 18... .. 0 18 0

~0

0 •rJl

~ 15 15..~

;:;~

12 12D

9 90 e• •

7.... .. 4"C

o lO·CI 6 • l5·Ci>-,

c.:l •'d 5.... 15°CI()!o 4-

0rtl~ 3bDllJ


0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2-1

food (mg fw 1 )

Fig.2. Mean rates of egg production and mean clutch size in relation to food availability (natural phytoplankton + ciliates) at different temperatures in the laboratory. Only values obtained within the mainspawning period (>75% of females reproducing) are considered; fw =fresh weight.

20DC in A.denticornis (r2 =0.25, P<0.05) and at 15°C in A.alpinus (r2 =0.S3,P < 0.01; Table II).

Interclutch times (i.e. the time between the detachment of one clutch and theappearance of another) were usually short and, if successful mating occurred,could be minimized to less than 1 h. Minimum interclutch times of 1-2h wereobserved across the whole temperature range in A.alpinus, and from 10 to 20DC inA.denticornis. Gravid females that suspended egg production or produced abnormal egg sacs (nearly empty or filled with an amorphous egg mass), or even discarded unfertilized oocytes without egg sac formation, frequently recoveredquickly after replacement of the male (Table I). Evidence of mating was seen byattached spermatophores. As female diaptomids require an insemination prior tothe production of each clutch, it may thus be concluded that these females weremate limited rather than temporarily infertile or injured by laboratory handling.However, the proportion of abortive oocyte extrusions was generally low if compared with the total amount of reproductive cycles. At 4DC in A.denticornis and at20°C in A.alpinus, reasons other than frequency of mating events, probably somekind of physiological stress on males or females, seemed to be responsible for prolonged G phases, as only 3% of male replacements resulted in extrusions of viableclutches in the former species and 30% were successful in the latter one, respectively (Table I). Some females extruded only few unfertilized oocytes, with theiroviducts remaining dark. In both species, unfertilized oocytes shed into ovisacs ingeneral remained loosely attached to the female for only a few hours up to normal

20S8

i) P < 005]' alpinus 4°e versus alpinus 20°C, denticornis 15°C versus(Bonferrom , .' . ° (Fi 5 ) Th um-d

. . 200e alpinus 100e versus denticornis 10 e FIgure a. e mean nentlcornzs , . F h t . ds A al-

ches roduced er female is shown III Table 1. or s or peno '.'b~r of clut I Papable olenhancing its daily EPR at high temperatures (FIgurepinus was a soch' F 5b5b). Among several curve-fitting models applied to the data s own III igure ,

the function

A 10°C D 10"C


-.t'J

~ 7 ••••• denticornis>< ..... aZpinus

"""-6Mell

,aa 5::l~ 4~~ell

ell 3;>

~ 2~a::lCJ

y =e(bll + bllt)

. . 093 df -56 F-766 P<OOOl) as well asYielded the best fit in A.dentlcornzs (r

2 =. , .. - '. - , . b - 2847. I' ( 2 - 081 d f = 69 F=291 P < 0.001) with the constants 0-' ,III A a .pinus r -. , ..,' . At 4 d2074 db - -1638 -7.821 in A.denticornis,A.alpinus; tlS temperatu~e. . an. ° an \~us had' hi her EPR (ANOVA, P < 0.05), while A.dentlcornzs was

10 e, Auit: 15 d 20~e (ANOVA P < 0.001). Maximum EPR reached 5.88supenor at an , + 52) ~ -1 da -1 in the(±0.63, 95% Cl) eggs ~ -1 dayv in the former and 7.98 (_0. eggs y

latter species.

Clutc~~ze a7~~lut~9~):~~~~~~:~~~t~~:)~te:e,:~b:~p~~:;'~n~e:r~~~r(~~:) a;~~ t; it7(n = 287) at io-c in A.denticomis. It was inversely co_rre~;~ff

. . ' (10 20°C) (ANOVA F=5.44, d.f.-2, ,with temperature III A.dentlcornzs -. 'h' between theP < 0.005) with a statistically significant decrease of mean elute SIze [ANOVA

15 and zo-c, but not between the 10 an~ 15°~~~te~~~::~;a:::tclutch·size~Post hoc modified LSD test (Bonferrom), P .]. ) d' t h' h (15/20oe'

. I (4/10oe' P- 091 an a ig er ,were not significantly different at owe~ .' -. ° the lar er females ofP _ 060 Student's t-test) temperatures III A.alpznus. At 10 C, . g.-. , h (F 5d) which despite a some

A.alpinus produced distinctly larger clutct es .igure R (P' ' 5b) At higher. It d i a nigher EP igure .

what lower spawmng frequency, :esu e III . (from 16 8 at 100e to 12.2 attemperatures, a drastic decrease ~n mean ellltc~SIze c ortion of abortive150e) together with an increase III eggmortahty and the pr p .

, 2061

s

A.denticornis Aialpinus

Variable Tee) n Range p.l. ,2 b P n Range p.l. ,2 b p(urn) (urn)

Largest 4 16 1216-1335 0.060 0.011 n.s.clutch 10 17 1165-1231 0.044 0.028 n.s. 16 1207-1311 0.050 0.029 n.s.

15 18 1125-1236 0.114 0.020 n.s. 18 1040-1404 0.533 0.040 <0.0120 19 1123-1229 0.249 0.031 <0.05 15 1124-1378 0.132 0.017 n.s.

Total egg 4 0.003 -0.073 n.s.number 10 As above 0.088 1.155 n.s, As above 0.010 0.046 n.s.

15 0.156 1.121 n.s. 0.027 0.110 n.s.20 0.040 0.745 n.s. 0.009 0.045 n.s,

n.s., not significant.

Table 11. Results of simple least squares regressions on copepod egg production (y) versus fe 1prosome length (x). p.l. =prosome length.r? =coefficient of determination; b =slope of regression7~n =number of observations. Largest clutch size and total egg production related to lifetime spawninIntieach individual go

C.D.Jersabek and R.Schabetsberger

taneous eggs across the whole temperature range from 4 to 200e (Table I). Whilethe production of a subitaneous clutch was followed by resting egg production,again at low temperatures, three females terminated breeding with subitaneousegg production at 20oe. One clutch of A. alpinus, produced at 4°e, was apparently amixed clutch composed of both resting and subitaneous eggs (8r/2s). Together witha random sample of resting eggs from all experiments, the former could be successfully hatched after a period of 8 months, and thus proved to be truly diapausing.Spawning of subitaneous eggs seemed to be randomly distributed within the reproductive period and clutch-carrying periods of subitaneous eggs did not differ fromthose of resting eggs.

Fecundity. At 20oe, only A.denticornis was capable of maintaining high reproductive rates over a long period, with 100% of the animals still reproducing after 2months. Up to 568 eggs per female [mean 378.8 :t 49.5, 95% confidence interval(Cl)] were produced within 4 months. In contrast, female A.alpinus were exhausted after a few weeks, with only 50% of the animals still reproducing after 16 days(Figure 4). Up to 135 eggs per female (mean 70.2 ± 13.2) were produced. At thelowest temperature investigated, A.denticornis appeared lethargic and unproductive with only two females producing 11 and 18 eggs, respectively, whereasfecundity of A.alpinus at 4°e (up to 185 eggs per female; mean 77.3 ± 26.0) surpassed that observed at 200e (non-significant, Student's r-test, P= 0.63; Figures 4and 5a), although, due to smaller clutches at 20oe, less clutches were produced(Table I). At 10oe, egg production rates as well as total reproductive output duringlifetime spawning closely resembled each other in both species (Figures 4 and5a-c), with up to 325 eggs (mean 203.1 ± 35.3) produced per female in A.alpinusand up to 303 eggs per female (mean 220.8 ± 31.5) in A.denticornis, respectively(Student's t-test, P=0.46). The corresponding output at 15°e was 451 (mean326.6 ± 43.7) and 313 (mean 117.3 ± 31.4) eggs per female in A.denticornis and inA.alpinus, respectively. No statistically significant difference could be tracedbetween the following means of fecundity [ANOVA, post hoc modified LSD test

2060

~~l==,=::q 70

605040302010

30252015105o

20 ...........~

15

10

5

20

15

10

l>"l5c

~Q)

~0"Q)

M.....


5

-1eggs clutch

. for A al inus and A.denticornis egg batches produ~ed atFig.6.Frequency distri~utlOnsof egg numbers i di~te median, first and third quartile, respectively,different temperatures III the laboratory. Arrows nPlease note the different scales of the y-axes.

I h of e gs was strongly temperatureThe mean time needed to produce ~ne c ~~c f70 + 0 07 days n =579, A.al-

dependent and lowest at 2~oC (A~e;;~o~~~~8~e;~5 'day~, ~ = 3, and 17.34 ± 0.99pinus: 3.03 ± 0.24 days, n - 95). .'. d A alpinus respectively. In all but

d d 'n A denticornis an. ,days, n =72, were nee e I '. . inus si nificantly longer to producethe 4°C temperature treatment It took A.alp gd' . 6 54 + 021 days

. + 19 da s (n = 227) (A. enucorrus . .,.-' ,one egg batch, With 0

6.95- O. + oy 20 d s (n = 178) (A.denticornis 3.83 ± 0.08n=286) needed at 10 C and

04.45:-.ay

days, n =480) required at 15 C (FIgure 5c).

. roduction as measured as the proportion ofEgg viability. The effi~Ie~cyof egg p., of a'viable clutch, was highest at 150Coviducal cycles resultm.g III the fo~matlO~~. . A al inus. Viability of eggs from(96.5%) in Aidenticomis and at 10 C (9? o)hlll '. P ith 0 77% of eggs within

hi h t t 10°C III bot species, w .successful clutches was ig ~s ~ .. dO 62% in A.alpinus, respect-viable clutches being defective III A.dentzcornzs an .ively (Table I).

. . ducin females oscillated between gravidGametogenic cycle (Figure !). Repro N J) conditions. InA.alpinus, these flue-(Phases G + B) and non-gravid (phases +

2063


...400

BI'0 a0

7'Cv

6

Po. 300

~

{J)

Ol:l

Q)

oqPo.

!Jl><

200

'0

Q)...I

I

...o-

3 0.e>

lIJ

"<l~ 100

2 .!.~v

0

0800

18600

500

16400

~ 30014 n'"'

2'='0

....e 20012 g-c,

!Jl0..

10 ~.o

100

B80

60

65 10 15 20 5 10 15 20

Temperature (oC)

Fig. S. Means of fecundity (a), maximum rates of egg production (b), clutch production period (c) andclutch size (d) as a function of temperature in the laboratory. Mean rates of egg production wereobtained over a 1 week period at the time of maximum reproduction. To minimize age-specific effectson mean clutch size, the last clutch of each female was omitted from calculations. Circles with error barsare estimates and 95% confidence limits (CL). Curves were fitted by applying a second-orderregression line to data points.

clutch extrusions, were observed in A.alpinus (Table I). The same applies toA.denticornis at the opposite end of the temperature range investigated. However,too few data were obtained to confirm the effect of low temperature on clutch sizein this species. At higher temperatures, the smaller A.denticornis females produced larger clutches than A.alpinus [ANOVA, post hoc modifed LSD test (Bonferroni), 15°C P<0.05, 20°C P>0.05; Figure 5dJ, maintaining a nearly linearincrease in EPR with increasing temperature (Figure 5b). The individual clutchsize was less variable inA.denticornis than in A.alpinus and the coefficient of variation (CV) was typically near 20% of the mean (range 9.0-49.3%) in the formerand near 30% of the mean (range 13.3-45.4%) in the latter species (Table I). Themaximum clutch sizes observed were 36 (lO°C) in A.alpinus and 27 (loOe) inA.denticornis, respectively. These values are far above the highest egg numbersever observed in the lake, which were 21 in A.alpinus and 13 in A.denticornis. InFigure 6, frequency distributions of all viable clutches produced during experiments are illustrated.

2062

C.DJersabek and R.Scbabetsberger

Survival (Figure 8). The lifespan of both species was significantly affect.ed bytemperature (Kruskal-Wallis one-way ANOVA, X2 = 87.6, P < 0.001). Sur:Ival ofA.alpinus females decreased with increasing temperature (P < 0.001), while thatofA.denticornis decreased above 10°C, but was lowest at 4°e (P < 0.001). Survivalat higher temperatures (10-20°C) was significantly higher in A.denticornis, while

2065

Resting egg production and oviducal cycling in diaptom:ids

conclusionthat conditions are not appropriate for reproduction in this species. Thehighproportion of clutch-carrying females present during the first days of experimentare solely a consequence of female stocking prior to experiments. However,it is not possible to decide to what extent food concentration and mating activityareresponsible for observed proportions, as a great many female A. denticornis didnot continue a full reproductive cycle at 4°C. No copulations were observed duringthewhole experiment, and at any time most animals appeared weak and with clearguts.At higher temperatures, environmental factors become increasingly influential and the synchronization of oviducal cycle and CPP may become blurred attimes of low or highly fluctuating food concentrations.

At first glance, the proportion of females in reproductive phase B gives a goodimpression of breeding intensity, being increasingly sensitive to nutritional conditions with rising temperatures. Maximum proportions (5 day means) wereobserved at 15°C (70%) in A.alpinus and at 200e (73%) in A.denticornis,

respectively.The relative amount of non-ovigerous clear-phase females (N), and thus the

food limitation index, usually remained low at times of stable reproduction. Welldefined peaks of 19% at 10°C (days 90-95) and of 24% at 200e (days 25-30) inA.alpinus succeeded declines of food concentration below 0.20mg freshweight 1-1,indicating a feeble food limitation of egg production, although minimumlaboratory values still exceeded maximum values encountered in Lake Dreibriidersee. Regarding the amount of females in phase N, A.denticornis seemed not tobe food limited at 10 and 15°C, and also proportions of 13 and 10% at days 20-25and 90-95, respectively, where only a slight response to food decline at 20°e. However, absolute rates of egg production, clutch size variation and spawning frequency (Figure 1) revealed a considerably more sensitive correspondence tofluctuating food supply, as is shown by changes in oviducal cycle patterns alone.The end of each experiment was preceded by a strong increase of females in phaseN, which was a consequence of females entering the post-reproductive periodrather than of food-limited reproduction.

The percentage of non-ovigerous gravid females (G) was lowest at temperaturesofhighest reproduction (A.alpinus: 10°C,A.denticornis: l5-20°C), presumabl.y as aresult of low interclutch times and rapid fertilization of ripe oocytes. Availabledata indicate that the allocation of time spent within either phase N or phase G isdiminished with increasing temperature, and reaches a minimum at the temperature of most successful spawning. Beyond 10°C, the phase lengths considerablyincreased in A.alpinus, implying, that values around this temperature are optimalfor gamete maturation, provided that food supply and mate availability areadequate. In A.denticornis, additional observations above 20

0ewould be needed

to confirm this general pattern of the cycle's response to temperature.

lI4·C

15"C

20·C

~

lE 1tW'~1/\

\\ J\ ~I\ hIf IfOCI

tllJ 1\ 1\ 1\

11\ 1/1\ 1\1/

1\ h

liN!~rIr

II'~ r-f\ '\..

l..- f.,

A. alpinus

~a v vf\ E1O"C

~h VIIlJ

1\ 1/\

hlJ f\!J j". 1\I)

:~l/ 1\ //

~~I/~~I/~1,,1\ lI i\ ~

0.8

0.2

0.4

0.6

0.8

0.6

0.4-

0.2

rn~ 0.8(lj

...<:: 0.6Pot

Cl) 0.4.~..,~ 0.2

"do"'"PotCl)

"'"'+-fo~ 0.6o.~

~

"'"oPot 0.2o

"'"Pot

o 20 40 60 80 100 120 140 160 180 20 40 60 80 100 120 HO 160 180

Aug Sep act Nov Dec Jan Feb Aug Sep act Nov Dec Jan Feb

days of observation

Fig. 7.Effect of ter;nperature on allocation of reproductive phases in A. alpinus and A. denticomis in thelaboratory. The dlst~nce bet~een two vertical solid lines (shown in 4 and lOoe experiments) corresponds to the mean time required for clutch production. Dotted vertical line: only 25% of females stillreproducing. Indices o~ food limitation are given by circles. Reproductive phases: N, non-ovigerousclear phase; G, non-ovigerous-gravid; 0, ovigerous-clear phase; B, ovigerous-gravid.

tuations showed a conspicuous synchronization with the mean duration of CPP at4°C. ~lternatin? proportions of reproductive phases therefore simply reflect patterns m egg laymg and vitellogenesis rather than the effects of food availability.Internal constraints like the time needed to distribute nutritive material to theoocytes, as well as the clutch-carrying period, are suggested to be the main determinants that govern egg production in A.alpinus at given conditions. In A.denticornis, the pattern observed in gametogenic cycle at 4°C lends credence to the

2064

70

Nov

6050

III IV

... -r----- ......" ···.a:lpsed

• alp femDden.Lsedo dent fem

_ _ _ _ alp lake.........c:;·;·;·:·'. d'efi't I ilke

II

30 40days

Oct

Resting egg production and oviducal cyclingin diaptomids

20

I

10

openlake

encl.D

• encl.A'Oenc1.Do lake

eencl.A

b.........alpinuso denticornis

"'"""""'EPR,···.,·..Cp·p ....

..:::::::~~:::~;;; ...:::::;~::~:::.:.:.:~ ...;:.:.; ..

o

6

8

E 6I-

4

0.10

~ O.OBI

~ 0.06....eo 0.04-El

0.02

0.00

decrease of ~1.5 eggs between succeeding clutches ofA.alpin~smay be estimated,asmean durations of CPP (13-16 days) indicate that females did not ex~rude~orethan one further clutch (still attached to the mother animal) between inspecnons

2067

Sep_ greenalgae+desmids 11cyanobacteria il~~[!mm cryptomonads:::::::::,u.-algae dinoflagellates

Fig. 9. Clutch size variations in enclosures and in the open lake (a), egg production ra~es .and clutchproduction periods in enclosures (b), lake temperature at .d~pth of enclosure.s (c), quantitative (d) .andqualitative (e) changes in biomass of phytoplankton and ciliated protozoans m enclosures of Aalpinus(encLA) and A.denticornis (encl.D), and in the open lake at the depth of en~losure~; sed ~ droppedclutches fern = clutches attached to females. Shaded area accentuates decreasing or mcreasmg.trendsof clutch size between successive clutches. Error bars are 9S% CL. I-IV = times of control; horizontalblack bar indicates ice cover; fw = fresh weight.

A. ctlpinus20

18

16

14

12

10

8

Q) 6>

',.-l 4-cd2

rJ)Q) 0-cd 20

S 18Q)

l+-4 16~ 14

12

10

6

6

4

2

A.alpinus lived longer at 4°C [Kaplan and Meier product-limit analysis of survivaldistributions; P < 0.001 (4, 15, 20°C), P < 0.01 (10°C)]. Comparisons of survivaldata between the two species revealed no statistical significant differences, andthus similar survival times, in the following data sets: alp4°C-dent10°C (P = 0.97),alp10°C-dent15°C (P=0.49), alp10°C-dent20°C (P=0.81), alp20°C-dent4°C(P=0.26) (Mann-Whitney V-test).

0L---'----l--.L_L-.--'----l----'--L---'---L--''---€1&----'-~-''___<~----'----.J_JfJ

o 20 40 60 60 100 120 140 160 180

days of observationFig. 8. Temperature-dependent mortality of A.alpinus and A.denticornis females in laboratoryexperiments.

Enclosure experiments

Egg production. In the course of enclosure experiments, both species exhibitedconsiderable differences in breeding characteristics, which were even more distinct in the open lake. With the exception of slightly smaller clutches in September(Mann-Whitney V-test, P < 0.05) clutch sizes in A. denticornis did not significantlydeviate in enclosures from that observed in the open lake (Kruskal-Wallis oneway ANOVA, X2=3.26, P=0.51; Figure 9a).This was also reflected by similar proportions of reproductive phases (Figure 10). In Aialpinus, clutch size and EPRstrongly decreased until the end of October in enclosures (Figure 9a and b),although pelagic feeding conditions improved quantitatively and qualitatively(Figure 9d and e) and a general trend of increasing clutch size with decreasingtemperature would be expected from laboratory results. If sedimented clutchesare plotted separately from that still attached to the females in enclosures, a

2066

c.O.Jersabek and R.Schabetsberger


2069

Gametogenic cycle (Figure 10). High proportions of non-ovigerous clear-phasefemales are considered evidence of food limitation in A.alpinus. While this proportion exhibited a baseline of between 1.9 and 7.0% in the open-lake population,it increased to 51.4 % in enclosures. Similarly, the reduced reproductive capacity inthe cages is emphasized by a decreased proportion ofB females, which fell t03.7%in November, as opposed to 60.7% in the open-lake population. In contrast, inA.denticornis the food limitation index in plankton cages remained somewhatbelow that observed in the open lake in October. In the ice-covered lake, itincreased to a maximum of 19.9 and 14.7%, respectively. The wide discrepancybetween the amounts of N females in the experimental and the open lake population in September simply originates from a high proportion of immature femalesstillpresent in the lake and the fact that the very first mature females were initiallyselected for experiment. Gravid phase duration was distinctly longer inA.denticornis than in A.alpinus, as reflected by a higher proportion of females inphase G. Although resulting mate limitation indices (36.3-64.2%) would suggestprolonged gravid phases due to inadequate mate availability, similar patterns ofoviducal cycles at low temperatures in the laboratory (Figure 7), as well as maledensity in experiments, make M likely to overestimate the actual importance of

mating at these probably non-optimal conditions.

dominated (Gymnodinium spp.) aspect in October (Figure 9d and e). Maximumvalues of 0.55 ± 0.05 eggs 9 -I day:', corresponding to 0.095 ± 0.009 clutches 9-

1

day". were recorded at the beginning o~Octo?er (Figure 9a). Since gravid femaleswere initially selected for cage stocking, highest rates of egg production andspawning frequency of A.alpinus in September (0.725 ± 0.056 eggs 9-I ,day-I,0.079 ± 0.007 clutches ~ -I day") must be to some extent a result of feeding conditions prior to capture. The mean total egg production within enclosure experiments, which covered the main spawning period, amounted to 22.20 ± 1.48 eggs~_I day:' (3.91 ± 0.25 clutches 9 -I day:') in A.denticornis and to 23.41± 1.43 eggs~-1 day" (3.06 ± 0.21 clutches S? -I day") in A.alpinus, respectively. The time ittookan average female to produce one clutch of eggs ranged from 10.5 to 34.3 daysinA.denticornis and from 12.7 to 75.2 days in A.alpinus (Figure 9b).

Eighteen days after the start of the experiment, the presence of second metanauplius stages in one alpinus enclosure suggests that at least one female switchedto subitaneous egg production. Considering the mean CPP (12.67 ± 0.98 days) andthe eggdevelopment time of ~7.5 days at 10°C (derived from subitaneous clutchesproduced in the laboratory), it becomes clear that this subitaneous clutch must

have originated from pre-experimental conditions.


Egg mortality. The proportion of non-viable eggs within successful clutchesincreased uniformly from 0.71 % in September to 1.73% in November inA.denticornis, whereas it remained nearly constant (0.56-0.69%) inA.alpinus during the ice-free period. In November, however, egg mortality in the latter species

suddenly came to 2.18 % after freezing.

2

20 ~0

40 ::g+

60 g.G ~

N80 ~

open lakeI II III IV

0.8

0.6.::l 0.40

:5 0.2Cl> .s~ ].... 60.Eo

Cl)tl ~ 45El 7. 30

41 15-000....I 0.8[/lCl) 0.6[/l

III~ 0.4-., P<

'f Cl) 0.2.~

0 .....

~C)

~ 60&

-e0"" 45'T:l P<%~ 30

15

15 30 45 60 75 15 30 45 60 75days

Fig.l~. ~ll?cat!on.ofreproductive phases in A.alpinus and Aideruicomi(M) limitation indices in enclosures and in the open lake I-IV _ ti lS f~males, food (f) a~d matebetween reproductive phases =proportion of egg-carrying f~ I -Nm~es 0 ~ontrol; c~nne.ctlOg linefemales. Error bars are 95% CL Re rodu ti ma es. + imrn. - non-gravid + Immatureovigerous-gravid; 0, ovigerous-cl;ar p~ase;~,~~i;~~~~~g~~i~~n-ovigerOUS-clearphase; G, non-

~~git~~e ~a and b )1' After 5 weeks in enclosure, A.alpinus females still carried1 arger c utches (6.25 ± 0.28 eggs, 95% Cl) as did A.denticornis(5.79 - 0.35), but after 8 weeks egg batches of A.alpinus (5 65 + 0 36) 11tha th f . . . -. were sma er

nose 0 1tS relative (6.12 ± 0.35) (Figure 9a). However size differences~et~een at~ached clutches were neither significant between n~r within the two::c~~d~~l t~~last 6 weeks of the enclosure experiment (Kruskal-Wallis oneAY' . .x -9.49,d.f.=5,P=0.08).Reversetrendsinclutchsizevariationoft .~?~n.~s 1~ the open l~k~ ~upport the assumption that distinct dietary habits, due

to 1s.n LutklOnal p~c~~lantles, are a main factor that modifies egg production paterns m a e Dreibrudersee.

In A.denticornis EPR remai d 1 duri(0.17-0.55 e s _; -I me ow unng the whole period of reproductiongg 9 day), but there was a clear response to an increase of food

SU.p/ctlr ~lthOUgh the mean lake temperature fell from ~9°C to below 5°C from the(;-0~~) Sedtembe~ to/he end of October (Figure 9c), rates of egg production

atth . ti an (sFP~wmng requency (P = 0.87; Student's t-test) were nearly identical

ose imes igure 9b) S' It 1 .t 63 . imu aneous y, the mean clutch size increased from 5.30010'mey~s pe~.~lutCh, and pbytoplankton concentrations increased from 0.04 to

. . g ,w 1e . the. qualitative spectrum changed from a chloro h ceandommated (Cruclgema rectangularis) aspect in September to a dinophycean-

2068


Discussion

The sympatric populations of Aulentlcomis and Aialpinus were found to prefdifferent temperature ranges for reproduction in the laboratory, but displayed

er

similar reproductive capacity at 10°C, which approximately equals the mean tem

perature d,urin~ the main spawning peri~d i~ their natural habitat. The inability ofmost dentlcorn~s females t~ spawn at 4 C m t~e laboratory is remarkable, as insome well-studied populations from lower altitudes, where the species has twoprincipal periods of reproduction, ovigerous females were present during almostthe whole year, reaching a minimum during the cold period (Eichhorn, 1957; Lair1977;Devaux, 1980), Also in Lake Dreibrudersee egg batches are carried until th~population collapses as a result of oxygen depletion in winter. Thus, breeding continues even in the ice-covered lake. It is still unclear, however, whether experimental constraints that do not become noticeable until temperatures becomeunfavorable are responsible for the reduced breeding capability of A. denticornis atlow temperatures in the laboratory. As no copulations were observed at 4°C, mateavailability may have been one limiting factor, as it would also be reflected by ahigh proportion of non-ovigerous gravid females and thus a high mate limitationindex (cf. Williamson and Butler, 1987),

Sporadic switching from resting egg production to the production of subitaneous eggs, even in univoltine populations which will hardly ever encounter conditions appropriate for a second generation to develop in nature, might berevealing about the preservation of a species-specific reproductive plasticity.Although induction of subitaneous egg production appeared to occur somewhatacci~entally in the laboratory, the potential to produce both types of eggs mayprovide a valuable adaptation when colonizing new habitats, Significant differences between the two clutch types were not detected in either durations of reproductive phases or in clutch-carrying periods, as claimed by Watras (1980) forDiaptomus leptopus, in which it took nauplii 41 % longer to hatch, than a clutch ofdiapausing eggs to be dropped, regardless of temperature. Reduced clutch-carrying periods would imply a higher corresponding rate of clutch production if interclutch times remain constant. However, too few subitaneous clutches wereobtained for firm conclusions. A similar equivalence in carrying times of subitaneous and resting egg clutches at experimental conditions was also discerned inD.pallidus (Williamson and Butler, 1987),

Even if, in terms of population growth, it is the rate of egg production that ismore important in populations producing subitaneous eggs (Corkett and Zillioux,1975), increasing the total number of eggs spawned in the lifetime of an averagefemale may become the superior strategy in univoltine copepods if predictableadverse conditions are survived by diapausing eggs. Although A.alpinus wass~ow.n toen~ance its instantaneous rates of resting egg production by several timesWIthmcreasmg temperatures, the total output of this cold-adapted species duringlifetime spawning was lowest at 20°C, whereas in the laboratory population ofA.denticornis the preference for higher temperatures found expression in maximum values of egg production rates as well as in the highest lifetime fecundity at20°e. It seems reasonable to suppose that Aidenticornis will never be able to take

2070


d tage of its potential fecundity in high-elevation lakes that only exce?-f~l a lvan reach suitable temperatures, and that initial densities of calanoidaonall,~ ~lalYbe strongly modified by the thermal regime of the lake during the pre-naup 11 WI ' . if h .. both in terms of absolute values and of relative proportions 1 t ere IScedingyear, h . d t Isnoth -occurring species, A similar discrepancy between t e estimate na uraano er co . duri lifeti . d thdit f the marine Calanus [uunarchicus unng I etime spawmng an efu~nlYO ' dT d. tal results obtained by Hirche (1990) was observed by Diel an an eexpenmen 'd '92) d also Runge (1985a,b) found consistently lower egg pro uction rates~19h 'fian

ldthan in the laboratory in Calanus pacificus. In Lac Pavin (France),

mte e . ' f 'A d

ti rnis was found to vary ItS reproductIve effort rom year to year m re-en lCO .' d h ibl. t nVI'ronmental variables by Lair (1984), who discusse t e pOSSI e pos-

sponse 0 e .., which can be assumed along the r-K continuum. .It10nS ' I'd d t onThe present laboratory-derived observatIOns on ongevity a~ egg pro uc 1

t th t females of both species may reproduce at a relatively constant r~tesugg

es.iod of several weeks or months and die within a few days of ceasing

over a pen ' . . 'd

tion Thus reproductive senescence IS assumed to be of mmor Import-repro uc 1. , . '

d. g the main reproductive period of diaptomld copepods. ThIS argument

ance unn . . ' . his corroborated by Williamson and Butler's (1987) similar e:penence WIt. repro-ductive persistence of Diaptomus spp. as well as by Maly s (1983) findmg t~atclutches produced by univoltine Diaptomus shoshone 6 weeks after matur~tlOncan be as large or larger than the first clutches pr~duced,A.sreported by J~mlesonand Burns (1988), clutch sizes of Boeckella speCIes also did n~t vary sIg~Ificantlybetween early and late clutches, but appeared to fluctuate Wlt~ fluctuatmg foodlevels. On the other hand, clutch sizes of marine Ca.lanus species were found t.ovary with the age of females, as in senescent o.vanes the supply of oocytes ISbelieved to slow down with age (Hirche, 1989; Dlel and Tande, 19?2).

In contrast to populations generating more than one genera~IO~ per. rep~oductive period, correlations between female body size and clut~hSIze~n univoltinepopulations are not likely to become blurred by changes associated WIththe meanage of females that show a synchronous development ofl~rv~l~tages.Here',effectspredominantly associated with feeding requirements and individual fec.undlty maybe expected to govern the temperature-dependent rates of egg productIOn. Gene:ally the clutch size of calanoids is a clear function of female prosome length mfreshwater (Ravera and Tonolli, 1956; Davis, 1959; Cole, 196?; Smyly, 1968; Mal

y:

1973; Hofmann, 1979; Jamieson and Burns, 1988) and man~e (McLaren, 1965,

Hki 1977' Runge 1984' Smith, 1990; Hirche, 1992) enVIronments, and the

op ms, , " . f I (C k tt andhighest variation in clutch size is associated WIththe largest ema es or ~McLaren 1969' Watras and Haney, 1980). However, Maly (1?83) gave evidenceI

, " I if diti favoring either large or smalthat negative correlations may resu t 1 con 1 IOnsbody size during development change to favor the opposite ,:hen copepods arereproductively mature. For example, having a higher potential fecundity, largefemales may produce smaller clutches or none if they are caught short on .nutntI?n

, for mai (Maly 1973) A high vana-and need a higher proportion of food or mamtenance " .bility in individual clutch size may also be reflected in a low correlat.ion co~fficle~twith prosome length (cf. Hirche, 1989). The coefficient o~ clutch SIzevana~~~~~A.alpinus in the laboratory was similar to that found by HIrche (1989, 1990)

2071

C.OJersabek and R.8chabetsberger

arctic marine species Calanus glacialis (30%, range 12-50%) and eft. .(35%, range 14--77%), but was lower in A.denticornis. Neverthele~sn;::rch.

CUJ

was a function of prosome length ~nly at 15°C in A.alpinus and' at ~~~A.dentzcornzs. Probably m most expenments ranges in female bod . In

t. . y SIzecovered a

00 narrow range to reveal significant effects on reproductive feat h ..ddi 11 . ures t at werea mona y modified by alternating nutritional conditions.

The negative influence of temperature on clutch size has been well df 1

· f di . ocumentedor severa species 0 iaptomids. Particularly strong effects of t

d h f. emperature

ecrease were s own or some North Amencan species of Diapto b El(1983) and by Chow-Praser and Maly (1991) who noted that a demc

usy. moreo ' rease In tem-

perature from 30 to 20 e and from 25 to 20oe, respectively produced t 2. . 1 . ' up 0 a -foldIncrease m c ut eh SIze. However, although principally the same tend . .. he two alni . ency IS evidentIn t e two a pme species, temperature effects are rather feeble with' thei

d f. 1 . m err pre-

sume pre erentia ranges m the laboratory. In D pallidus a decrea . tf

0 • , se m ernpera-ture .ram 25 to 15 e had no effect on clutch size, but caused a doubling of the timerequired for clutch renewal (Williamson and Butler 1987) Thus th ff. ... ", e e ect oftemperature on clutch SIze m diaptornids may vary from species to s .

1. . pecies, and

genera ities c~nnot be made regarding the family. Jamieson and Burns (1988)foun.dclutch SIzes to v~ry with species, food, temperature, and the interactions ofspecies-food and species-temperature in Boeckella spp. In high food clut h .

f h sneci ked i ' c SIzeso eac specieswere ran ed mversely WIth temperature.In enclosures, inade~uate feeding ~onditions are proposed to be the reason for

~he obse~ed decrease m egg production rates, spawning frequency and clutch sizeIn A.alpznus, as the mean ~umber of eggs per clutch simultaneously increased in~he op~n lake. The sam~ IS also reflected by a highly increasing food limitationmdex In enclosures, which remains negligible in females caught immediatelyabove lake ~ottom. Thus, observe? differe~cesbetween breeding capabilities of~he two specIes.must be ~ resul~ o~ interspecific differences in reproductive capacity as ~ell as of intraspecific vanation due to experimental constraints. The latterises~eclall~~rueofA.alpinus, as this usually bottom-associated species was deprivedof ItS familiar habitat as forced into the plankton cages. Here, the animals concen-trated at the net bottom for feeding (observed by a SeUBA diver) hile iAd ti . .. . ' W 1 e III

. en zcornzs s,wlmmmg beh.avIOr and distribution within the cages did not indi-cate a~y conta~ner effect. Dzaptomus spp. feeding selectively and exclusively onbenthic and epibenthic food items have already been described (Lowndes, 1935;Kadota, .1960).As ~e do not know on what items A.alpinus feeds in the lake, it isnot pos~Ible to decide whether qualitative or quantitative nutritional aspects arer~spo~slble for th.eobserved decrease in egg production. At the same time, A.dentzcornzs doubled ItS rates of egg production in spite of decreasing lake temperatures. Apart. from a less pronounced increase in food abundance, this enhancede.g~ production seems to be closely related to changes in the qualitative composition o~ the algal ,diet, which changed from a chlorophycean-dominated to a~ymnodmean-d?mmated aspect. Accumulating evidence of the potential dietaryImporta~ceof dmoflagellates (Kleppel et al., 1991) corroborates this observation.

[F~rran (1971) fou~d the mean clutch size ofArctodiaptomus bacilli/er (Koelbel)

venfied to be A.alpznus (Imhof) by Kiefer (1971)] to be closely linked with the

2072


. onal succession of the whole biocoenosis and with the thermal regime of the:.The amount of decrease in clutch size, whic~ was hig~est at th~ beginni~g of

oduction, corresponded with the amount of mcrease m numencal density ofr~;ophagous cladocerans and rotifers in different lakes. Ravera and Tonollil1956)suggested the positive ~orrelationb~tweenaverage.bod~l~ngth.and clutch. e among univoltine populatIOns of A.alpznus and A. dentzcornzs m alpme lakes to

: a consequence of the selective ~ction of a high ra~e of water renewal ins~pportinglarger individuals capable of hlg~er egg productI~n to compensate for high lossdue to lake outflow. Relating body SIzeof centropagid calanoids and egg numbersto rates of water renewal in Australian freshwater reservoirs, Timms (1968)reached the same conclusion. However, in view of the large amount of developmental flexibility, care should be taken in attributing size differences to. geneticdifferences among populations (Maly, 1973). Further, although a consIderablediversity of nanoplanktic algae may be present in high-altitude lakes of the centralAlps throughout the year, very low crops are generally found during the period ofcopepod reproduction (Pechlaner, 1964, 1971; Jersabek and Schabetsberger,1995). In such environments especially larger copepods. seem to be at risk .ofbecoming severely food limited if the phytoplankton standmg stock decreases WIthincreasing water renewal. Chow-Praser and Maly (1991) mentioned that theimportance of length as a predictor of clutch size may be eclipsed bY,a strongcovariance between prosome length and food concentratIOn m Dzaptomus

minutus and D.oregonensis.Several aspects should be considered when interpreting the very low c,orrespon-

dence between the amount offood available and the rates of egg productIOn as wellas the clutch size in A.alpinus and A.denticornis in the laboratory. First of all,weekly controls of nutritional conditions seem to be too widely ~paced in view ofthe large fluctuations observed in food abundance and the relatively short clutchproduction times under experimental conditions. Further, t.he~xact knowl~~ge ofthe lag phase for conversion of ingested food to egg productIon IS a pre~eq~I.sIte forstudies aimed at understanding the interactions between food avaIlabIlIty andreproduction. Using 14C-labeled food mixtures, Tester and Turner (1990) ~easured this time lag in continuously spawning marine copepods and fo~nd co~sId.erable interspecific variability. In the present study, the clutch productIOn p.e:IOd IS arather rough estimator of this lag phase and approp~iate only ~n condition thatingested food is used for the following clutch. ThIS assumptIOn seems to bestrengthened by the results obtained by Checkley (1980) for Paracalanus parvusand by Checkley et al. (1992) for Centropages jurcatus, w~ich show th~t the rate ofegg release is a function of the food ingested on the previ~us ?ay..Smith and Hall(1980)find even a shorter delay for Temora longicornis (cited m Nival e! al., 19?0).The minimum duration for the conversion of ingested energy to eggs m contmuously spawning marine copepods is significantly <24h, since. Marshall ~nd ?rr(1961) and Comita and Cornita (1966) reported that the IllcorporatIOn mtospawned eggs of 32p of phytoplankton occurred as rapidly as 8 h'.However, th~pattern of gamete maturation is quite different in diaptomid species, whose 0:1ducts contain oocytes which are approximately the same age and ~tate of matu~Ity(Ishikawa, 1891; Watras and Haney, 1980;,Watras, 1983) and III some manne

2073

2075

ReferencesBanarescu,P. (1990) Zoogeography ofFreshWaters. GeneralDistribution and Dispersal ofFreshwater

Animals. Vol. 1, Aula-Verlag, Wiesbaden, pp. 1-511. . 'Bossone,A. and Tonolli,V. (1954) Il problema convi;enza di Arctodiaptomus bacillifer ~KOELB.) di

Acanthodiaptomusdenticornis e di Heterocopesaliens (LILLJ.). J:!~m.I~t. Ital. Idr~bLOl., 8, 81-~4.Cahoon,L.B. (1981) Reproductive response of Acartia tonsa to variations m food ration and quality.

Deep-Sea Res., 28A, 1215-1221. . . ..,Checkley,D.M. (1980) The egg production of a manne planktomc copepod m relation to Its food sup-

ply: laboratory studies. Limnol. Oceanogr.,25, 430-446. . ., .Checkley,D.M., Dagg,MJ. and Dye,S.1. (~992) Feeding, ex~retion and egg production by mdividuals

and populations of the marine, planktomc copepods Acartiaspp. and Centropages [urcatus. J. Plank-

ton Res.,14, 71-96. . thChow-Fraser,P. and Maly,E.J. (1988) Aspects of mating, reproduction, and co-occurrence m ree

freshwater calanoid copepods. Freshwater Biol., 19, 95-108.. . . .Chow-Fraser,P. and Maly,E.J. (1991) Factors governing clutch SIZe In two species of Diaptomus (Cope-

poda: Calanoida). Can.1. Fish. Aquat. Sci., 48, 364-370. . .Cole,G.A. (1966) Contrasts among calanoid copepods from permanent and temporary ponds m An-

zona. Am. Midl. Nat., 76,351-368. ..' 3 576 591Comita,G.W. (1956) A study of a calanoid copepod population m an a.rctlc~ake. Ecology, 7, - .Comita,G.W. and Comita,J.J. (1966) Egg production in Tigri?pusbrevlcornts. In Barnes.H. (ed.), Some

ContemporaryStudies in Marine Science. Allen and Unwm, London, pp. 171-185. .Conover,R.J. (1967) Reproductive cycle, early development, and fecundity in laboratory populatlOns

of the copepod Calanus hyperboreus. Crustaceana, 13, 61-72. .Corkett.CJ. and McLaren,l.A. (1969) Egg production and oil storage by the copepod Pseudocalanus m

the laboratory. J. Exp. Mar. Biol. Ecol., 3,90-105. .Corkett,C.J. and Zillioux,E.J. (1975) Studies on the effect of temperature on th~ egg ~aymg of three

species of calanoid copepods in the laboratory (Acartia tonsa, Temora longlcorntS and Pseudo-calanuselongatus). Bull. Plankton Soc. Jpn, 21, 13-21. .

Davis,C.C. (1959) Breeding of calanoid copepods in Lake Erie. Verh. Int. Ver. Theor.Angew. Limnol.,

14, 933-942. '

Acknowledgements

The authors gratefully acknowledge Professors Dr A.Goldschmid and DrH.Adam for providing funds and laboratory facilities. Thanks are due to ProfessorDr A.Herzig for advice and discussion on vari?us points, and to two a~onymo~sreferees for their invaluable suggestions that Improved a former version of thismanuscript. C.J. wishes to thank Dr A.Liaunigg for assistance in the field and her

help during the laborious laboratory work.


ti 0 One potential source of error is introduced by dietary differences amongcau 10 . d h f . I I' hi b

od species and to understan t e unctiona re ations ip etween egg pro-copep , .ductiooand food quality needs furthe~effort. ~ever~heless,.these observ.atIOnsare

f Ifor understanding egg productIOn of alpme diaptornids and have Important~e l~catI'ons for interpreting their distribution in high-altitude lakes. In con-unp 1 .'

I'00 the experimental data show that, accordmg to lake temperature, either of

ellSI , d . I . d . f hth two species may become repro uctrve y supenor an co-existence 0 t e twoe .es seems to have a thermal compromise as a prerequisite. It would be interest-

specI . I' f h . diff . hei h. to examine whether allopatnc popu ations 0 t ese species 1 er m t eir t er-109 . f f . I' dtmaltolerances and reproductIve pe: ormanc~s ram sympatnc po~u ations, ~n 0

whatextent variations in reproductive potential represent adaptation to particular

environments.


species where a variety of maturation states is found in the oviducts as a result fthe gradual maturation of a batch of primary oocytes (Conover, 1967; Razou~1974;Raz?ulsetal., 1986;.Ia~ora"": 1989; Ia~lOra, 1990). Thus, while ripe oocyte~are shed simultaneously m diaptornids, a contmuous recruitment of ripe oocyte .provided by this sta~gered pattern. of ~amete m~turation in marine copepo~~Because a short duration of phase N implies potentially rapid oocyte turnover thiparam~ter has a signi~cant impact on r~tes of egg production (Watras, 1983)~Assummg egg production to be related WItha 10 day time lag to nutritional statusof different Bo~ckel~a and Calar:zoecia species, Maly (1991) could observe multipleclutch production WIthout feedmg these freshwater Centropagidae. An additionalsource of uncertainty may be introduced by individual, age-specific and/or temporal (i.e. seasonal) differences in the response to a fluctuating food supply. It iswell known that changes in food quality may be as important as quantitative alterations (Cahoon, 1981; Kleppel, 1992; ~as~ett.et al., 1993). Sciandra et al. (1990)suggested that the frequency of food variation IS a parameter as important as quality and quantity, which must be considered to predict adequately the egg production offemales during their lifespan. Collectively, these factors may contributet~ ca.mouflage the .actual effect of ~oo~ on the instantaneous reproductive output.Pickmg up the subject of food quality m laboratory experiments, it did not seem tohave a decisive influence on reproduction, as the relative composition ofphytoplankton was very similar over the main experimental period. However asA.denticorniswas shown to be an omnivorous species that also feeds on protozoans and rotifers (Lair, 1992; Lair and Hilal, 1992; Hartmann et al., 1993), ciliateabundance may play a particular important role if phytoplankton is scarce.

Consistently short interclutch times might be indicative of comparatively littledep.end.e~ce of this in~erval on temperature, provided that food supply and mateavailability are sufficient. There are different views on how to apply the term'intercl~tchtime'. If it is considered as the period between two successive layings(cf.Jamieson and Burns, 1988; Chow-Fraser and Maly, 1991), it includes the wholeoviducal cycle and will therefore be a clear function of temperature. The definitionapplied here agrees with that used by Watras and Haney (1980), who discriminatebetween clutch-carrying time and interclutch time. As female diaptomids have tomate prior to the production of each clutch (Eckstein, 1964; Watras and Haney,1980; Watras, 1983; Williamson and Butler, 1987), very short interclutch timesimply that gravid females either become inseminated with only a short delay afterclutch deposition or that ovigerous dark-phase females may also become successfully inseminated.

.Differences in egg viability possibly result from different qualities of insemination. Ianora et al. (1989) noted that egg viability increased if a new male was introduced aft~r the death of an old one. A decrease in sperm quality of old males maythus provide an acceptable explanation for the increasing egg mortality observedat the end of lake experiments in both species. Recently, Ianora and Poulet (1993)dem~nstrated that food type also strongly influences egg viability.

ThISstudy shows that field and laboratory data should be combined to obtain acle~r understanding of reproductive characteristics, but the extrapolation of datadenved from the laboratory to events occurring in the field must be done with

2074

2077


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Dtel,S. and. Tande,K. (1992) Does the spawning of Calanus finmarchi y. . ogta, ~, 17-34.reproducible pattern? Mar. Biol., 113, 21-31. cus m high latItudes follow a

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Received on March 28, 1995; accepted on July 11,1995

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J,QIIl1Ull ofPlanktoD Research VoU7 no.Ll pp.2079-2091, 1995

IDterspecific variability and environmental influence on particulateorganic carbons13e in culturedmarine phytoplankton

C.Leboulanger l, CDescolas-Gros-", M.R.Fontugne3, l.bentaleb' and H.Jupinl

ILaboratoire de Biologie Vegetale, Universite de Perpignan, F-66860 Perpignan,zLaboratoire d'Hydrobiologie, URA CNRS 1355, Universite de MontpellierII,F-34095 Montpellier Cedex 5, 'Centre des Faibles Radioactivites, LaboratoiremixteCEA-CNRS, BP 1, F-91198 Gifsur Yvette Cedex, France

4To whom correspondence should be addressed

Abstract. The stable carbon isotope composition of particulate organic matter expressed as S13C wasmeasuredin cultures of 13 species of marine micro algae in different phylogenetic groups. The effects ofsalinityvariations and changes in photoperiod were also assayed for three of them (i.e. Skeletonemacostatum; Amphidiniuni operculatum and Isochrysis galbana); the effect of nature of nitrogen supply(nitrate, ammonium) was studied for one (S.costatum). These environmental parameters were chosenbecause of their variability in the ocean and their possible effects on !)13C values of phytoplanktonorganiccarbon. Batch culture conditions and sampling time after inoculum were strongly controlled inorder to provide cells in good physiological state which were comparable from one culture to the other.In the same way, sampling was limited to the first 2 days of exponential growth, in order to avoid apossibledissolved inorganic carbon (DIe) limitation. Carboxylase activities [of the enzyme ribulose1,5-bisphosphate carboxylase oxygenase (Rubisco), and the three 13 carboxylases: phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase(PC)] and total chlorophyll a concentrations were assayed simultaneously. The S13C values observedwere between -30.2%0 and -12.7%0, i.e. comparable to those observed in the world's oceans. The isotopic composition of phytoplankton organic carbon was shown to be under the influence of the parameters tested but !)13C variations are specific to the species considered. The nature of ~ carboxylasefound in each species, or systematic position, could not be linked to the isotopic composition of organiccarbon. No linear or single correlation between 813Cvariations and environmental modifications wereobserved and there is no evidence for a simple and universal relation between !)13C of phytoplanktersand their environment. In monospecific cultures as in the field, 013C fractionation by Rubisco (andeventually by PEPCK) may be counterbalanced by other mechanisms.

Introduction

A strong interest in the global carbon cycle caused by increasing human influenceand potential greenhouse effect leads the oceans to be defined as a 'great sink' foratmospheric CO2 (Shackleton, 1990). Phototrophic communities are principallyinvolved in the phenomenon of inorganic carbon fixation in the open ocean. Apowerful method to trace carbon flux from the atmosphere to the deep-sea sediment is the study of 13C/12C ratio variations (expressed as BBC) in inorganic andorganic carbon (Fry and Sheer, 1984;Kiyashko, 1987) as photosynthesis results in achange in isotopic composition of organic matter relative to the inorganic source(Park and Epstein, 1960).A wide range of particulate organic carbon (POC) BBC isobserved in the ocean (values between -14%0 and -35%0). Such variations werelinked to water temperature (Sackett et al., 1965) then estimated as +0.1%0 °C-l(Fontugne and Duplessy, 1981), related to paleoclimates (Descolas-Gros and

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