Research article

Female and male contribution to egg size in salmonids

SUSANNA PAKKASMAA1*, NINA PEUHKURI, ANSSI LAURILA1,

HEIKKI HIRVONEN and ESA RANTAIntegrative Ecology Unit, Department of Ecology and Systematics, Division of Population Biology,

P.O. Box 17 (Arkadiankatu 7), FIN-00014 University of Helsinki, Finland1Present address: Department of Population Biology, Evolutionary Biology Centre, Uppsala

University, Norbyvagen 18D, SE-75236 Uppsala, Sweden

(*author for correspondence, tel.: +46-18-4716496; fax: +46-18-4716484;

e-mail: susanna.pakkasmaa@ebc.uu.se)

Received 23 November 2000; accepted 8 October 2001

Co-ordinating editor: H. Kokko

Abstract. Egg size contributes to other life history traits of an individual. It is traditionally con-

sidered as a maternally determined characteristic to which the male does not have any direct

contribution. However, a recent finding in insects suggests that males can affect egg size also

directly. In fish, the male effect could take place only during egg swelling, as the final egg size is

reached after that. We studied egg size in four freshwater salmonid species (the land-locked Atlantic

salmon, the brown trout, the Arctic charr and the lake trout) right after fertilisation (initial egg size)

and after the swelling phase (final egg size). The results showed that the final egg size is affected not

only by the initial egg size but also by both the female and the male through the process of egg

swelling. This study suggests that paternal contribution may form a previously largely ignored

source of variation in early life history traits in salmonid fish.

Key words: early life history, egg size, egg swelling, female effects, male effects, Salmo, Salvelinus

Introduction

Egg size is a central early life history character influencing the fitness of both

the mother and her offspring (Bernardo, 1996a). Egg size may correlate not

only with the status (e.g., age, size and condition) of the female producing the

eggs, but also with the size and other attributes hatching offspring (Chambers,

1997). In fish, for example, large eggs generally result in large larvae and fry

(Chambers, 1997) having, among other things, wider mouth gape, longer visual

reactive distance and greater swimming speed than is typical for their smaller

conspecifics (Blaxter, 1986; Miller et al., 1988). These traits, in addition to large

size as such, are likely to have survival value in nature, e.g., in terms of in-

creased efficiency to forage and/or to avoid predators.

Traditionally, egg size has been considered as a maternal character deter-

mined by the female’s genotype and the environment she has been exposed to

Evolutionary Ecology 15: 143–153, 2001.� 2002 Kluwer Academic Publishers. Printed in the Netherlands.

(Bernardo, 1996a, b; Chambers and Leggett, 1996; Mousseau and Fox, 1998).

The male’s influence on egg size has been acknowledged in some animals,

where the male contributes to female nutrition through food or nuptial gifts

provided before egg laying and affects thus egg size indirectly (e.g., Gwynne,

1984; Sakaluk, 1986; Daan et al., 1990). Recent observations in the cricket

Gryllus firmus, however, showed that the male can affect egg size through a

direct contribution to the water uptake and/or metabolism of the embryo

before hatching (Weigensberg et al., 1998). In fish, maternal effects on egg size

have been widely recognised (e.g., reviewed by Heath and Blouw, 1998), but

the potential for the male affecting egg size has practically been neglected. In

the present study, we examine the effects of both the female and the male on

egg size in salmonid fishes.

In salmonids, like in most teleost fishes, the final egg size is attained after

swelling following egg activation by water contact or fertilisation. Swelling is

caused by an osmotic influx of water from the surrounding medium, resulting

in the formation of a fluid-filled perivitelline space underneath the expanding,

outer egg membrane (Alderdice, 1988). The extent of swelling differs markedly

among fish species so that the perivitelline space can make up most of the egg

volume (Lønning and Davenport, 1980) or be hardly noticeable (Bolin, 1930).

It is well established that the amount of resources provided by the female

influences egg size (Bernardo, 1996b; Chambers and Legget, 1996). There is

some indication from Atlantic cod (Gadus morhua) that the female may con-

tribute to the final egg size also by influencing the extent of swelling (Kjesbu

et al., 1996). The male contribution to egg size can only be manifested through

the swelling process as fertilisation in salmonids is external and the male does

not provide any resources to the developing embryo, neither directly nor via

the mother. We studied the parental contributions on egg size in four salmo-

nids by including the pre-fertilisation size (female effect) and the post-fertili-

sation size, i.e., the amount of swelling (female and male effects).

Materials and methods

We studied egg size in four freshwater salmonids: land-locked Atlantic salmon

(Salmo salar m. sebago), brown trout (S. trutta m. lacustris), Arctic charr

(Salvelinus alpinus) and lake trout (Sa. namaycush). Salmon, brown trout and

Arctic charr are native to Finland, but lake trout is an imported species

originating from North America. Salmon and brown trout were captured as

breeding adults from the Vuoksi water system, eastern Finland, where both

species occur naturally. In addition to wild fish, we used parental salmon from

a hatchery stock originating from the same area. For Arctic charr and lake

trout, only hatchery-reared parents were available. For each species, we used a

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complete factorial mating design and created fertilisation matrices by crossing

five to eight females each with five to seven males (Table 1). The matrices

consisted of 25–56 families, each with approximately 300 eggs. The parental

fish from the hatchery stock were randomly chosen, with the condition that the

males and females of each matrix were always from different year classes in

order to avoid matings with close relatives. Individuals from the wild were

randomly assigned to each matrix.

All crossings were made in the hatchery in October–November 1996. Eggs

and milt of the wild fish were stripped at the place of catch and stored on ice for

a variable period but always less than 2 weeks. The eggs were stored together

with ovarian fluids, and milt was stored in plastic bags filled with pure oxygen.

The motility of spermazoa was checked under microscope before being used

for fertilisations, and milt showing at least 70% of progressively swimming

spermatozoa was used. The fish from the hatchery strains were stripped in the

hatchery, and fertilisations were made on the day of stripping or the following

day. The eggs of each female were divided in equally large batches (about 300

eggs) in 0.2 l plastic vials and fertilised. They were photographed immediately

after fertilisation, allowed to swell, and photographed again 4 h later, by which

time the swelling is completed (Kamler, 1992).

Maximum diameter of a sample of eggs (usually >100 eggs per family) was

measured from the photographs with an image analysis programme. Family-

specific average egg sizes before and after swelling were calculated from these

data, and they were used to analyse the parental effects on egg swelling and

thus on the final egg size. Linear regression was used for each matrix separately

in order to analyse how much of the variation in the final egg size can be

explained by variation in the initial egg size. We used matrix-specific residuals

of the linear regression of the final egg size against initial egg size as the

response variable in the analyses. Thus, we controlled for the potential effect of

initial egg size and the associated female effects when analysing egg swelling.

Each matrix was analysed with a two-way analysis of variance (ANOVA)

without replication. The female � male interaction was calculated according to

Table 1. The number of matrices of wild and hatchery fish in each study species and the lengths

and weights of the parental fish. The origin of the fish (hatchery or nature) is indicated.

Species Matrices Origin Females Males

Length (cm) Weight (g) Length (cm) Weight (g)

Salmon 3 Nature 67–78 3050–5770 62–96 1900–7500

7 Hatchery 48–69 1220–4830 12–51 20–1410

Brown trout 3 Nature 48–77 1180–5410 36–86 590–7060

Arctic charr 7 Hatchery 47–65 880–2800 53–67 1590–3230

Lake trout 2 Hatchery 56–66 2630–4370 59–78 1870–5980

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Tukey (1949). The probabilities obtained from the ANOVAs were combined

using Fisher’s meta-analytic method described in Sokal and Rohlf (1995). Here

the method created an overall test for the significance of female and male

effects on egg swelling. Probabilities were calculated separately for wild and

hatchery-reared salmon.

Results and discussion

As expected, most of the variation in final egg size was explained by the initial

egg size (Table 2). Nonetheless, we found statistically significant male effects,

female effects, and female � male interactions on the final egg size corrected by

the initial egg size (Table 2, Fig. 1A). The female had an effect on the final egg

Table 2. Two-way ANOVA without replication testing the effect of male (M), female (F) and their

interaction on final egg size.

Species Origin M F p

M F M · F r2

Salmon Nature 7 8 0.248 0.045 0.677 0.73

Nature 5 7 0.387 <0.001 0.620 0.97

Nature 5 7 0.133 0.094 0.604 0.80

Hatchery 6 6 0.089 0.065 0.094 0.92

Hatchery 6 6 <0.001 0.145 0.643 0.82

Hatchery 6 6 0.089 0.520 0.744 0.93

Hatchery 6 6 <0.001 0.046 0.019 0.97

Hatchery 6 6 <0.001 0.537 0.640 0.60

Hatchery 6 6 0.833 0.259 0.689 0.57

Hatchery 6 6 0.003 0.140 0.775 0.93

Brown trout Nature 6 5 0.474 0.002 0.238 0.98

Nature 6 7 0.305 <0.001 0.108 0.02

Nature 6 5 0.701 0.089 0.685 0.97

Arctic charr Hatchery 6 6 0.025 0.347 0.780 0.82

Hatchery 6 6 0.002 0.797 0.251 0.83

Hatchery 6 6 <0.001 0.238 0.006 0.88

Hatchery 6 6 <0.001 0.430 0.143 0.72

Hatchery 6 6 <0.001 0.002 0.286 0.85

Hatchery 6 6 <0.001 0.038 0.467 0.87

Hatchery 6 6 0.022 0.431 0.054 0.56

Lake trout Hatchery 6 6 0.623 0.051 0.198 0.82

Hatchery 5 5 <0.001 <0.001 0.883 0.74

For each matrix (line), the origin of the parental fish, number of males and females used in the

crossings, as well as probability values for males, females, and their interaction are shown. As the

analyses were performed on regression residuals of final egg size (after swelling) against initial egg

size (before swelling), also r2 values from regression analyses are indicated.

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size in two wild salmon matrices and one hatchery-reared matrix, two brown

trout matrices, two Arctic charr matrices, and one lake trout matrix. Significant

Figure 1. (A) An example of a salmon matrix in which we found a significant female effect

(F5;24 ¼ 2:69; p ¼ 0:046), male effect (F5;24 ¼ 13:19; p < 0:001) and female �male interaction effect

(F1;24 ¼ 6:32; p ¼ 0:019) on final egg size corrected by the initial egg size. Vertical lines are the

residuals from the regression of final egg size on initial egg size grouped for each female. The

symbols represent different males. (B) Egg swelling for the same matrix as in A. Each circle

indicates the swelling in percentage in the family.

147

male effects were found in four salmon matrices, all of hatchery origin, in all

Arctic charr matrices and one lake trout matrix. The interaction between the

female and the male was significant in one salmon matrix (hatchery-reared fish)

and two Arctic charr matrices. Combined results (Table 3) showed that the

female effect was significant in each species separately, and the male effect was

significant in three out of four species. The female�male interaction was found

to be significant only in Arctic charr.

Salmon had the largest eggs, with the average egg diameter being 6.42 mm

(SD � 0.20) and swelling increased egg diameter on average by 5.2% (SD �1.46) (Fig. 1B). In brown trout, average egg size was 6.30 mm (SD � 0.34) and

swelling 4.2% (SD � 4.22). The Salvelinus sp. had smaller eggs. The average

size of Arctic charr eggs was 5.55 mm (SD � 0.22) and swelling corresponded

to an increase of 4.6% (SD � 1.61), whereas lake trout eggs were on average

5.08 mm (SD � 0.21) and swelling 8.0% (SD � 1.96).

Egg size and its effects on offspring size and size-related fitness character-

istics has been the subject of much research in a number of oviparous organ-

isms (e.g., insects: Fox and Mousseau, 1998; amphibians: Kaplan, 1980, 1998;

Table 3. Combined probability values for female, male, and interaction effect on final egg size for

each species separately. Results for hatchery and wild fish are presented separately as well as

grouped together.

Hatchery Nature Hatchery + nature

�2R ln p 2k p �2R ln p 2k p �2R ln p 2k p

Salmon

Male 57.32 14 0.001 8.72 6 0.238 66.62 20 0.001

Female 20.08 14 0.130 10.92 6 0.125 31.00 20 0.055

M�F 18.85 14 0.207 2.74 6 0.920 19.02 20 0.521

Brown trout

Male 4.58 6 0.677

Female 17.28 6 0.009

M · F 8.07 6 0.238

Arctic charr

Male 50.81 14 <0.001

Female 27.49 14 0.019

M�F 27.11 14 0.019

Lake trout

Male 21.20 4 <0.001

Female 21.60 4 <0.001

M�F 3.49 4 0.558

All species

Male 129.55 32 <0.001 13.30 12 0.348 142.84 44 <0.001

Female 69.18 32 <0.001 28.20 12 0.005 97.38 44 <0.001

M�F 46.88 32 0.043 10.81 12 0.545 57.69 44 0.081

148

fish: Hutchings, 1991; Reznick, 1991; Heath and Blouw, 1998; Einum and

Fleming, 1999). Maternal effects on egg size are widely documented and the

studies have traditionally considered egg size as reflecting the amount of the

resources provided by the mother for the developing embryo (Bernardo,

1996b), thus constraining the size of the hatching individual. In the present

study, we found both the female and the male to affect the final size of the eggs

in three of the four salmonid species. This was despite the fact that we had

controlled for variation in initial egg size by using regression residuals as re-

sponse variables in the analyses. Hence, our results suggest that, at least in

salmonids, the view of egg size as a character entirely determined by the ma-

ternal genotype and phenotype should be revised. Taking into account the

possible male effects on egg size and other life history characters may open new

avenues for research for studies of early life history variation.

Our results indicate that both parents contributed to the final egg size

through the swelling process. Li et al. (1989), on the contrary, suggested that in

salmon swelling is an inherent property of the eggs and is of equal extent even if

the unfertilised eggs differ in size. However, their study design did not allow for

examining the effect of females, nor those of males. In the study of Kjesbu

et al. (1996) with cod, the eggs of individual females tended to show a similar

swelling pattern (measured as egg dry weight/diameter ratio) from year to year.

The authors interpreted such a female effect on the extent of egg swelling to

reflect a strong genetic component. Although the study design we used does not

allow the interpretation of our results in terms of genetic differences among the

females, they are in accordance with those of Kjesbu et al. (1996). The sig-

nificant female � male interaction observed in some matrices in the present

study shows that the final size obtained by the eggs of a certain female may also

depend on the male with whom the eggs have been fertilised. However, the

exact mechanisms by which the female and male affect egg swelling are at

present unknown. The osmotic properties of the egg change when it comes into

contact with water (Alderdice, 1988) and, for example, the sperm of certain

males may have caused the egg to absorb water more efficiently.

The results of our study agree with those of Weigensberg et al. (1998) who

found that, in addition to the indirect effect, the male also had a direct effect on

egg size in the cricket G. firmus. In their study, this effect was a result of

differences in the water uptake and/or metabolism of the embryos fathered by

different males. In fish, egg swelling increases the volume of the perivitelline

fluid and thus depends on the amount of water absorbed by the egg (Alderdice,

1988).

Salmonids spawn on the stream (or lake) bottom, and the eggs develop

buried in the bottom gravel. Therefore, they are likely to be exposed to me-

chanical disturbances and pressure changes by gravel and water movements

(Groot and Alderdice, 1985). The fluid-filled perivitelline space protects the

149

growing embryo from such external disturbances, and thus likely influences its

normal development and survival (Laale, 1980). If the parents influence the

extent of swelling, as our results suggest, this might be reflected in differential

survival of offspring from different families even if the initial egg size might

have been the same. All else being equal, this should select for increased

swelling of the eggs, especially in stream environments where the external

mechanical disturbances are likely to be stronger than in lakes. So far, this

remains speculative as the families in our study could not be incubated sepa-

rately after the photographing and no data on egg survival is therefore avail-

able. A recent study by Lahnsteiner et al. (1999) with brown trout showed a

positive relationship between the average increase in egg size during swelling

and egg viability in batches of eggs from different females. However, it is not

clear whether the process of egg swelling itself produced this outcome or

whether the less swelling eggs were initially less viable. On the other hand, there

is indication that in environments with limited oxygen large egg size may be a

disadvantage (van den Berghe and Gross, 1989). This could be of evolutionary

significance, as incubation of salmonid eggs may take place in low oxygen sites

(Chapman, 1988).

It is unlikely that the volume of the perivitelline fluid (i.e., the extent of

swelling) directly influences larval size. However, it is known that the fluid-

filled perivitelline space (together with the outer egg membrane) acts as a re-

sistance layer for the gas exchange of the developing embryo (Rombough,

1988). Diffusion distances of oxygen will be different in differentially swelled

eggs and may thus influence the oxygen uptake of the embryo and thereby its

growth and well-being. The embryo excretes its metabolic wastes in the peri-

vitelline fluid (Alderdice, 1988), and there may be a minimum volume and thus

minimum egg size needed for normal metabolism. The perivitelline fluid may

serve also other functions (Laale, 1980), but these may not be so clearly con-

nected with the magnitude of swelling (e.g., polyspermy prevention), or be

relevant for salmonids (flotation).

We studied eggs of both hatchery-reared and wild fish. All Arctic charr and

lake trout parents were of hatchery origin, because wild parents were not

available. All brown trout were wild fish, but in salmon, we used both wild and

hatchery-reared parents. We found female effects among both types of fish, but

male effects only among hatchery-reared fish. A possible explanation for this

observation is that the storing may have affected the properties of the gametes

from the wild fish. However, the eggs and milt were stored on ice, and fur-

thermore, sperm motility was checked before fertilisations and only high-

quality sperm was used. On the other hand, the procedures we used for

hatchery fish were closer to the natural as the eggs were fertilised shortly after

stripping. An alternative explanation for finding male effects on egg size only in

the hatchery-reared fish might be related to the fact that the hatchery envi-

150

ronment differs in several respects (e.g., feeding, temperature regimes, inter-

actions between individuals, to mention a few) from the natural environment of

these fishes. For example, Crill et al. (1996) studied the effect of parental en-

vironment on morphological and physiological traits in Drosophila melano-

gaster. They found that the parental environment affected certain traits, and in

particular, egg size was affected by the rearing temperature of the male, so that

the eggs sired by males raised in high temperatures were smaller. Although

Crill et al. (1996) observed cross-generational environmental effects on off-

spring traits, the actual mechanisms how they were mediated are unknown.

Clearly, further studies are needed to settle out this question.

To conclude, we found that, in addition to the female, also the male in

several salmonids influences the egg size through his effect on egg swelling after

fertilisation. Our study thus suggests that paternal contribution may form a

previously largely ignored source of variation in early life history traits.

Keeping in mind the broad scientific interest on maternal effects (Mousseau

and Fox, 1998), to find out the causes and consequences of male effects calls for

further studies. The obvious next steps are to study the occurrence of this

phenomenon in other fish species, as well as the significance of egg swelling and

male-mediated egg size variation on offspring performance and survival in

salmonids. Our study was performed at the family level, and to find out the

magnitude and fitness consequences of egg swelling at the individual level is

another challenge for future research.

Acknowledgements

We thank the Saimaa Fisheries Research and Aquaculture unit of the Finnish

Game and Fisheries Research Institute for providing the facilities for this

study. We also thank Teija Seppa for help in photographing, Jorma Piironen

for discussions and other help, and Erik Petersson for comments on an earlier

version of the manuscript. Our research was funded by the Academy of Fin-

land, Finnish Ministry of Education and the European Commission (FAIR-

CT96-1957).

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