www.elsevier.com/locate/jembe

Journal of Experimental Marine Biolog

Patterns of adult abundance in Chthamalus stellatus (Poli) and

C. montagui Southward (Crustacea: Cirripedia) emerge during

late recruitment

A.M. Power a,*, J. Delany b, D. McGrath c, A.A. Myers d, R.M. O’Riordan d

a Department of Zoology, Room 226 Martin Ryan Institute, National University of Ireland, Galway, Irelandb Dove Marine Laboratory, University of Newcastle, Cullercoats, North Shields, Tyne and Wear NE30 4PZ, UK

c Department of Life Sciences, Galway-Mayo Institute of Technology, Galway, Irelandd Department of Zoology, Ecology and Plant Science, University College Cork, Lee Maltings, Prospect Row, Cork City, Ireland

Received 15 September 2004; received in revised form 18 October 2005; accepted 15 November 2005

Abstract

The aim of this study was to investigate when adult distribution patterns are established in the barnacles Chthamalus stellatus

and C. montagui. Adult dzonesT were identified by analysing field counts of both species at mid and upper shore heights. Monthly

collections of cyprids, b1 month old metamorphs and recruits (all metamorphosed individuals older than approximately 1 month)

were made for C. stellatus and C. montagui in natural barnacle beds at six shores in SW Ireland. This was carried out over one year

in 1996/1997, using a hierarchical sampling design. Abundance of total recruits (0–3 months old) was compared between adult

zones after the main settlement season had ended. In addition, scales of variability in 0–3 month recruitment into adult zones were

compared between the species at two scales: shores (1000s of metres) and sites within shores (10s of metres). Older recruits of each

species, up to 11 months of age, were also compared between adult zones.

The majority of settlement (measured as attached cyprids) occurred between August and October 1996. In October, there was

no effect of adult zone on the abundance of total (0–3 month) recruitment up to that point in either species. Despite this

homogeneity in recruitment between adult zones, significant spatial variation was found in 0–3 month recruits of both species at

both of the scales examined. In C. stellatus the amount of variation associated with the larger scale (shore) was more than twice that

of sites or of the residual variation (replicates within sites). 0–3 month recruitment in C. montagui was also most variable at the

scale of shores but the residual variability (between replicates within site) was of similar magnitude to that of shores. Variability in

0–3 month C. montagui recruitment was relatively low at the scale of sites.

There was a small but consistent input of recruits to adult zones over 9 months of the year, complicating the assessment of when

adult patterns were set-up in these species. By June 1997, characteristic patterns of adult dominance had been established at all

shores. Settlement had completely ceased by this time and individual barnacles were potentially 11 months old. Neither settlement

nor early recruitment are significant in determining adult zonation patterns in these species. Instead, differential mortality patterns

in individuals up to the age of 11 months are implicated in determining patterns of distribution of both species.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Barnacles; Chthamalus; Recruitment; Regulation; Spatial variation

0022-0981/$ - s

doi:10.1016/j.jem

* Correspondi

E-mail addr

y and Ecology 332 (2006) 151–165

ee front matter D 2005 Elsevier B.V. All rights reserved.

be.2005.11.012

ng author. Tel.: +353 91 493015; fax: +353 91 494526.

ess: annemarie.power@nuigalway.ie (A.M. Power).

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165152

1. Introduction

Identifying patterns of distribution of species and the

factors underlying these patterns is a fundamental goal

in ecology. Barnacles have open populations, i.e., local

recruitment is potentially uncoupled from local repro-

duction by a dispersal phase (Roughgarden et al., 1985;

Hughes, 1990). Larval recruitment in open populations

is analogous to birth in closed populations (Caley et al.,

1996), therefore attempts to understand how adult dis-

tribution patterns arise in barnacles have concentrated

on determining whether adult patterns were set-up at

larval settlement (i.e., dbirthT) or instead, at a later stagein benthic life due to post-settlement mortality (or

ddeathT) (see Connell, 1985; Caley et al., 1996 for

reviews).

Investigations into whether small or variable settle-

ment can regulate species’ distribution and abundance

are carried out by seeking evidence of density-depen-

dence between the abundance of settlement and subse-

quent adult densities (see Caley et al., 1996). Settlement

regulation has been predicted from a theoretical model

for small settlement densities (Roughgarden et al.,

1985) and subsequently shown in field studies (e.g.,

Connell, 1985; Gaines and Roughgarden, 1985; Ken-

dall and Bedford, 1987; Minchinton and Scheibling,

1991; Carroll, 1996; Jeffery, 2003). However, contrary

to predictions, settlement regulation has also been

shown at large settlement densities (Raimondi, 1990;

Menge, 2000). In addition, mortality factors such as

predation were important in determining density of

adults even when settlement densities were low (Car-

roll, 1996; Menge, 2000). Where intertidal vertical

zonation of adults of species exist, another approach

has been adopted. In this case, processes that contribute

to the establishment of vertical zonation patterns have

been examined by comparing the vertical limits of

settlement and recruitment, with the upper and lower

vertical limits of adults (e.g., Hatton, 1938; Connell,

1961a,b; Denley and Underwood, 1979; Strathmann

and Branscomb, 1979; Wethey, 1983; Raimondi,

1988). In some cases, both approaches have been si-

multaneously examined, by seeking evidence of densi-

ty-dependent relationships between different life stages

at several shore heights (e.g., Minchinton and Schei-

bling, 1991; Carroll, 1996; Menge, 2000).

All open populations are driven by recruitment in the

sense that recruits are required to have arrived and

survived at some time in the past to set the initial condi-

tion (i.e., density of recruits), on which post-settlement

factors may act to produce adult populations (Caffey,

1985; Carroll, 1996). Nevertheless considerable evi-

dence has accumulated to indicate that mortality due to

predation, competition, disturbance, or a combination of

these three is important in structuring barnacle popula-

tions (Hatton, 1938; Connell, 1961a,b; Moyse and

Knight-Jones, 1967; Lubchenco and Menge, 1978;

Wethey, 1983, 1984; Menge, 1991, 2000; Lively et al.,

1993; Carroll, 1996; Pineda et al., 2002). However, not

all of these studies examined the relative contribution

of settlement and early post-settlement mortality (e.g.,

Menge, 1991, 2000; Lively et al., 1993). Connell

(1985) defined settlement as the point when an indi-

vidual first makes permanent contact with the substra-

tum. This has rarely been observed directly in the

field; more usually, sampling occurs at an unknown

time after settlement. Strictly, all studies where indi-

viduals have survived a period of time between set-

tlement and census are more correctly defined as

recruitment studies and not settlement studies (for

further discussion see Connell, 1985; Underwood

and Denley, 1984).

In a review, Underwood and Denley (1984) pro-

posed that settlement patterns could be used to frame

an alternative hypothesis to explain regulation of popu-

lations. The importance of events at settlement such as

larval supply or arrival at adult zones (Gaines and

Roughgarden, 1985; Gaines et al., 1985; Gaines and

Bertness, 1993; Minchinton and Scheibling, 1991),

larval distribution in the water column (Grosberg,

1982), larval settlement rate (Denley and Underwood,

1979; Bushek, 1988; Raimondi, 1990; Sutherland,

1990), larval quality (Jarrett and Pechenik, 1997;

Miron et al., 1999) or settlement cues (Raimondi,

1988) have been shown (see review by Connell, 1985

and discussion by Underwood and Denley, 1984; Un-

derwood and Fairweather, 1989).

Dense populations of the barnacles Chthamalus stel-

latus and C. montagui occur in SW Ireland, which

offers an opportunity to examine simultaneously how

distribution and abundance of both species are estab-

lished. Populations of these species approach the north-

ern edges of their ranges in SW Ireland and although

they can overlap vertically over shore heights and

horizontally over gradients of exposure, zones of nu-

merical dominance of each species do occur. C. mon-

tagui usually dominates the upper barnacle zone and

can be significantly more abundant there than it is on

the midshore, where C. stellatus may be dominant

(Power, 2000; Delany et al., 2003). C. stellatus-domi-

nated zones tend to be found on more wave-exposed

shores (2–3 Ballantine scale (Ballantine, 1961)) while

C. montagui-dominated zones tend to be at less wave-

exposed locations (3–6 Ballantine scale) (Power, 2000).

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 153

It has been postulated that population distribution

and abundance of these two species may be established

at settlement: larger cyprid size and a greater dispersal

of larval phases in C. stellatus relative to C. montagui

may favour its increased settlement on offshore islands,

while less distantly-occurring larvae and larval reten-

tion in more sheltered conditions may favour C. mon-

tagui (Crisp et al., 1981; Burrows, 1988; Burrows et al.,

1999; Ross et al., 2003). However, Power et al. (1999a)

and Delany et al. (2003) have shown in SW Ireland that

neither attached cyprid abundance (dsettlementT), normetamorphs (b1 month old) favoured conspecific

adult zones compared to zones dominated by adults

of the other species (although this contrasts with a

recent report from Plymouth, S England (Jenkins,

2005)). In addition to no distributional differences be-

tween the species at settlement/metamorph stages, there

was significantly higher post-settlement mortality in the

dwrongT adult zone in both species between 6 and 12

months after settling on the shore (Burrows, 1988;

Delany et al., 2003). This poses the question: when

do zonation patterns emerge in these barnacle species?

In order to examine at what stage adult zonation

patterns were established, the present study examined

recruitment variation between adult zones, separately in

C. stellatus and C. montagui. Total recruitment was

compared after the main settlement season had ended

in different adult zones for each species (i.e., the anal-

ysis compared the sum of cyprids plus metamorphosed

individuals plus recruits to a maximum age of 3 months

old). All sampling was carried out in natural barnacle

beds and spatial variation between adult zones was

examined at two scales: shores (1000s of metres) and

sites within shores (10s of metres). A similar compar-

ison, this time omitting sites, was carried out for older

recruits (to a maximum possible age of 11 months old).

Previous studies have noted that total abundance of

C. stellatus cyprids was significantly higher than that of

C. montagui cyprids, despite the fact that metamorphs

of the two species did not significantly differ (Power et

al., 1999a; Power, 2000; Delany et al., 2003). This will

not affect the interpretation of recruitment into adult

zones in the present study because total recruitment was

examined separately in each species. However if, as has

been suggested, a different proportion between settlers

and metamorphs exists in the two species and this is

due to a different rate of metamorphosis (Power et al.,

1999a), it has important consequences for comparing

settlement in the species since the attached cyprid as a

settlement unit would not be directly equivalent be-

tween the two. On the other hand, if the species differ

in their proportion between cyprids and metamorphs

because of different early post-settlement survival, this

is also a potentially important source of variation to

measure. We examined the difference between the spe-

cies in cyprids (expressed as a proportion of cyprids

plus metamorphs) to see whether these proportions

differed consistently, or whether the pattern was due

to disproportionate effects at particular adult barnacle

zones, shores or dates.

2. Methods

2.1. Study sites and sampling

Zones of adult dominance were established by sam-

pling counts of each species as adults in 15 independent

replicate (10�10) cm quadrats at six shores in Co.

Cork, SW Ireland. The shores were: Bullens Bay East

(BBE), Bullens Bay West (BBW), Old Head of Kinsale

(OHK), Garrettstown (GARR), Nohoval (NOH) and

Castlepark (CPK) (for further details see Power, 2000;

Delany et al., 2003). Comparisons of species abundance

were made between mid (1.5–2.4 m above CD) and

upper (3.75–4.65 m above CD) shore levels at all

shores except for one shore, CPK, where the upper

level was 2.85–3.75 m above CD due to the barnacle

zone occurring over a reduced vertical extent at that

shore.

Settlement and recruitment of C. stellatus and C.

montagui were assessed using a hierarchical experi-

mental design within zones where either C. stellatus

or C. montagui were more abundant as adults. We

wished to test the hypothesis that recruitment in a single

species was different between adult zones therefore we

chose randomly-selected representative replicate shores

within each zone. Zones were thus kept independent by

being sampled on different shores. Sampling was car-

ried out from July 1996 to June 1997 within the C.

stellatus (=mid shore) zone at three of the six shores

(BBE, NOH and OHK) and similarly within the C.

montagui (=upper shore) zone at three remaining

shores (GARR, BBW and CPK). Shores were located

within 30 km of each other. Three sites were sampled

per shore, these sites were defined as areas of (2�2) m

within the designated zone and were haphazardly se-

lected on each visit to be at least 5 m but no greater than

30 m apart. Sampling was carried out monthly on each

shore within 5 days either side of the lowest spring tide.

On each sampling occasion, all cyprids and metamor-

phosed individuals within six replicate (10�10) cm

quadrats per site in natural barnacle beds were removed

and placed in 70% ethanol. Replicate quadrats were

stratified to exclude pools and crevices.

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165154

In addition to sampling recruitment at independent

adult zones (at different shores -see above), we exam-

ined recruitment of each species between adult zones

within shores. Two shores were monitored each day at

the beginning of the Chthamalus settlement season in

1998. Once cyprid settlement was observed, two

patches were sampled in each adult zone on each

shore between 28 July and 13 August. Patches were

approximately 5 m wide and were sampled in the mid

and upper shore zones, corresponding to zones of

adult abundance in C. stellatus and C. montagui. In

each patch 30 replicate collections were made of

cyprids and metamorphs in (10�10) cm quadrats.

During sampling occasions with heavy settlement rep-

licate size and number was reduced to 18 (5�5) cm

quadrats.

2.2. Identification of cyprids, metamorphs and recruits

Identification of cyprid to species was possible fol-

lowing measurement of each individual’s carapace

length (see O’Riordan et al., 1999; Power et al.,

1999b). Metamorphs were identified according to Bur-

rows (1988) and determined to be b1 month-old based

on criteria of size, colour and degree of calcification

(Power et al., 1999a).

The beginning of the settlement season in 1998 was

observed to within a day of its initiation at the study

shores. Therefore for the experiment comparing settle-

ment/recruitment between adult zones within shores,

the age of metamorphs could be assumed to be b17

days old. Note that this was slightly younger than

potential maximum age of metamorphs in the spatial

variability study, in which metamorphs were aged to b1

month old.

In analyses of individualsN1 month old and b11

months old, maximum recruit age was determined as

the time elapsed between the initiation of settlement in

1996 and the sample collection date for each of the four

dates analysed.

2.3. Statistical analyses of age classes

2.3.1. Adult zones

We tested the hypothesis that adult species abun-

dance differed between mid shore zones, where C.

stellatus was the more abundant barnacle species and

upper shore zones, where C. montagui was more abun-

dant. A three-factor analysis of variance (ANOVA) was

carried out: both zone and species were fixed, orthog-

onal factors with two levels each; three random shores

were nested within each zone.

2.3.2. Variation in 0–3 month recruits between adult

zones and at spatial scales

Total recruitment was compared in each species

separately between two adult zones after the main set-

tlement season had ended. This was carried out at two

spatial scales. Because all age groups were removed and

identified this single point estimate of recruitment

includes cyprids, metamorphs and recruits of up to a

maximum age of 3 months old. Sites were haphazardly

selected on each date, so to include this spatial scale in

ANOVA, analysis was restricted to a single point after

the main settlement season had occurred in both species

(October). Zone had two levels and was considered as a

fixed, orthogonal factor. Shore had three levels, was

nested in zone and was random. Site was nested in the

shore�zone interaction, was random with three levels,

there were six replicate quadrats sampled within each

site. Spatial scales of recruitment were examined sepa-

rately in each species so that the magnitude of random

effects of scale could be examined for each species

separately in the model. From the mean squares esti-

mates generated by ANOVA, we calculated the magni-

tude of effects according to the components of variation

associated with the random factors in the model (shores,

sites and residual) (Winer, 1971).

2.3.3. Abundance of cyprids+b17 day old metamorphs

between adult zones (within shores)

To examine whether abundance of settlement or

early recruitment (cyprids+b17 day old metamorphs)

varied between adult zones within shores we compared

mid (=C. stellatus zone, 1.5–2.4 m above CD) and

upper (=C. montagui zone, 3.75–4.65 m above CD)

for each species. Zone and species were considered to

be fixed orthogonal factors. Shores were random and

orthogonal and patches were random and nested within

shores. All four factors had two levels and nine inde-

pendent replicates were used for each species.

2.3.4. Recruit abundance (N1 month and b11 months

old) between adult zones

Patterns of species dominance in recruits between

adult zones were examined across the sampling dates

after the main settlement had occurred. Bad weather

made sampling impossible at all six shores in some

months. This missing data applied equally to both

species so, although it did not compromise the hypoth-

esis under investigation, this did have the effect of

making the design unbalanced. Therefore the analysis

was simplified to a comparison between zones (two

levels, fixed, orthogonal), shores (two levels, random,

nested in zone), species (two levels, fixed, orthogonal)

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 155

and dates (four levels, i.e., November, December, Jan-

uary and June; fixed and orthogonal). Nine independent

replicates were used for each species.

2.3.5. Cyprids as a proportion (of cyprids+b1 month

old metamorphs) between adult zones, shores and

sampling dates

We tested the hypothesis of a different proportion

between the two species of abundance in cyprids rela-

tive to b1 month old metamorphs. This comparison

was carried out across different adult zones and across

shores within zones over the main settlement season.

The dependent variable was a proportion between

cyprid abundance and metamorph abundance from

one month later. Since sites were haphazardly chosen

on each sampling occasion, we could not match sites

between successive visits so this factor is omitted from

the analysis. There were 4 factors in the ANOVA, zone

and species each had two levels and were orthogonal

and fixed. Date was also orthogonal and fixed and

included four levels representing four sampling occa-

sions when cyprids were sufficiently abundant, i.e.,

early August, late August, September and October.

Shore was nested in zone and was a random factor

with three levels. There were 18 replicate proportions.

Table 1

Analysis of variance of abundance in two barnacle species

Source of variation df

Zo 1

Sh (Zo) 4

Sp 1

Zo�Sp 1

Sp�Sh (Zo) 4

Residual 168

Transformation Ln (X +1)

Cochran’s C 0.190

P Significant ( P b0.05)

SNK multiple comparisons:

Zo within levels of Sp

C. montagui

C. stellatus

Sp within levels of Zo

Upper shore

Mid shore

Sp within (Sh�Zo)

Mid shore BBE

Mid shore OHK

Mid shore NOH

Upper shore GARR

Upper shore BBW

Upper shore CPK

Adult zones of dominance were compared between Chthamalus stellatus on

upper shore (3.75–4.65 m above CD). Sampling was carried out on six shores

of model terms: Zo=zone, Sh=shore, Sp=species. Abbreviations of shores

OHK=Old Head of Kinsale, NOH=Nohoval, BBE=Bullens Bay East.

For all analyses, homogeneity of variances were

checked using Cochran’s C and where variances were

found to be heterogenous, transformations were made.

In cases where transformations failed to homogenise

variances, we proceeded with the analysis on untrans-

formed data. Analysis of variance is robust to departures

from the assumption of homogeneity of variances if the

experiment is large and balanced (Underwood, 1997). In

addition, since violations of this assumption increase the

risk of Type I errors, non-significant results are accept-

able (Underwood, 1997). Multiple comparisons within

levels of significant factors performed according to

Student–Newman–Keuls (SNK) tests. Analysis of vari-

ance was conducted GMAV5 software for windows

(Institute of Marine Ecology, Sydney, Australia).

3. Results

3.1. Adult zones

Cochran’s C indicated significantly heterogeneous

variances between samples (P b0.05) which transfor-

mation failed to correct, but based on a large balanced

experimental design (in this case there were 15 repli-

cates) we proceeded with the analysis. C. stellatus were

MS F P

8.75 0.47 0.531

18.62 10.71 0.000

9.96 1.15 0.345

221.88 25.54 0.007

8.69 5.00 0.000

1.74

UpperNmid shore

Upperbmid shore

C. montagui NC. stellatus

C. montagui bC. stellatus

C. montagui bC. stellatus

C. montagui bC. stellatus

C. montagui bC. stellatus

C. montagui NC. stellatus

C. montagui NC. stellatus

C. montagui NC. stellatus

the mid shore (1.5–2.4 m above CD) and C. montagui zones on the

in SW Ireland, n =15. SNK=Student–Newman–Keuls. Abbreviations

: GARR=Garrettstown, BBW=Bullens Bay West, CPK=Castlepark,

C. stellatus zone C. montagui zone

0

50

100

150

200

250

S O N D J F M A M J

garr

bbw

cpk

0

50

100

150

200

250

S O N D J F M A M J

bbe

ohk

noh

Cyprids N= 4628

Metamorphs N= 14420

0

50

100

150

200

250

earlyAug

lateAug

earlyAug

lateAug

earlyAug

lateAug

earlyAug

lateAug

S O N D J F M A M J

bbe

ohk

noh

Cyprids N=4568

0

50

100

150

200

250

S O N D J F M A M J

garr

bbw

cpk

Metamorphs N= 5630M

ean

Abu

ndan

ce/d

m2

Fig. 1. Mean abundance (FS.E.) of Chthamalus stellatus cyprids and b1 month-old metamorphs in adult zones of C. stellatus and C. montagui at

six shores in SW Ireland in 1996/7. BBE=Bullens Bay East, OHK=Old Head of Kinsale, NOH=Nohoval, GARR=Garrettstown, BBW=Bullens

Bay West, CPK=Castlepark.

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165156

significantly more abundant in the mid zone compared

to upper zone but the opposite pattern occurred in C.

montagui, which was more abundant in the upper zone

than in the mid zone (significant zone� species effect,

df =1, F =25.54, P=0.007, Table 1). For multiple com-

parisons of species within zone, C. stellatus was more

abundant than C. montagui on the mid shore and on the

upper shore C. montagui was more abundant than C.

stellatus (Table 1). This pattern was apparent at all three

shores nested in each zone (significant species� shore

(zone) effect, df =4, F =5.00, P=0.000, Table 1).

C. stellatus zone

0

20

40

60

80

100

120

140

earlyAug

lateAug

SO N D J F M A M J

bbe

ohk

noh

0

20

40

60

80

100

120

140

earlyAug

lateAug

S O N D J F M A M J

bbe

ohk

noh

Mea

n A

bund

ance

/dm

2

Cyprids N=382

Metamorphs N=5768

Fig. 2. Mean abundance (FS.E.) of Chthamalus montagui cyprids and b1 m

six shores in SW Ireland in 1996/7. NOTE Different scale from Fig. 1. Ab

3.1.1. Annual settlement and recruitment in C. stellatus

and C. montagui 1996/1997

Cyprids of both species were found from early Au-

gust 1996 at all six shores and in both adult zones,

having been absent from all six shores a month previ-

ously (July 1996). The cessation of settlement as indi-

cated by an absence of cyprids was less synchronised.

Cyprids were found on at least one shore each month

except in March, May and June 1997, when no cyprids

of either species were found at any of the six shores.

Only cyprids of C. stellatus were found after November

C. montagui zone

0

20

40

60

80

100

120

140

earlyAug

lateAug

S O N D J F M A M J

garr

bbw

cpk

0

20

40

60

80

100

120

140

earlyAug

lateAug

S O N D J F M A M J

garr

bbw

cpk

Cyprids N=810

Metamorphs N=5766

onth-old metamorphs in adult zones of C. stellatus and C. montagui at

breviations of shore names as in Fig. 1.

Table 2

Analysis of variance of total 0–3 month recruitment (cyprids+metamorphs+recruits) in Chthamalus stellatus and C. montagui in SW Ireland

Source of variation df C. stellatus C. montagui

MS F P Variance component MS F P Variance component

Zo 1 257.98 1.47 0.292 3.80 0.08 0.797

Sh (Zo) 4 175.11 7.69 0.003 8.46 50.11 8.41 0.002 2.43

Si (Zo�Sh) 12 22.76 6.64 0.000 3.22 5.96 2.74 0.003 0.63

Residual 90 3.43 3.43 2.18 2.18

Transformation Sqrt (X +1) Sqrt (X +1)

Cochran’s C 0.184 0.461

P NS Significant

SNK multiple

comparisons

for Sh (Zo):

C. montagui zone C. stellatus GARRNBBW C. montagui GARRNBBW

GARRNCPK GARRNCPK

BBW=CPK BBW=CPK

C. stellatus zone OHK=NOH=BBE NOHNOHK

BBENOHK

NOH=BBE

Sampling was carried out 3 months after initiation of settlement at three shores nested in each zone and at three sites nested in each shore, n =6.

SNK=Student–Newman–Keuls. Variance components only calculated for random factors. Abbreviations of model terms: Zo=zone, Sh=shore,

Si=site. Abbreviations of shores as in Table 1.

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 157

1996, apart from GARR where a few C. montagui were

found in January 1997. Cyprid occurrence of C. stella-

tus was extremely sparse after December 1996 and only

a few cyprids of this species were recorded at either one

or two of the six study shores each month. At one of the

shores (BBE), cyprids of both species were found in

April 1997, having not been recorded for several

months previously (Figs. 1 and 2).

Metamorphs (b1 month old) of both species were

abundant from early August onwards at all six shores

but missing data points from Spring 1997 made the

cessation of metamorph recruitment difficult to assess

precisely. Metamorphs of both species were found on at

least one of the six shores in almost every month

sampled except June 1997, when they were absent

from all shores (Figs. 1 and 2). Despite settlement

and recruitment being observed over a prolonged peri-

C. stellatus zone

0

20

40

60

80

100

120

140

bbe ohk noh

C. stellatusC. montagui

Mea

n A

bund

ance

/dm

2

Fig. 3. Mean abundance (FS.E.) of Chthamalus stellatus and C. montagui 0–

Ireland. Abbreviations of shore names as in Fig. 1.

od, the majority of recruitment to b1 month old age

group had happened by the end of October, by which

time 77% and 66% of the annual recruitment to this age

class had been reached in C. stellatus and C. montagui,

respectively. (Percentage annual recruitment was calcu-

lated as (number of cyprids+b1 month old metamorphs

collected by end October) / (total of cyprids+b1 month

metamorphs collected in one year)� 100).

3.2. Variation in 0–3 month recruitment between adult

zones and at spatial scales

This analysis compares variation in recruitment be-

tween adult zones after the main settlement season. As

this occurred by October, the comparison considers the

sum of cyprids, metamorphs plus older recruits (to a

maximum age of 3 months).

C. montagui zone

0

20

40

60

80

100

120

140

garr bbw cpk

C. stellatusC. montagui

3 month recruits in adult zones of C. stellatus and C. montagui in SW

Table 3

Analysis of variance of recruit abundance (cyprids+b17 day old

metamorphs) in mid (1.5–2.4 m above CD) and upper (3.75–4.65 m

above CD) shore zones corresponding to zones of adult abundance in

Chthamalus stellatus and C. montagui, respectively

Source of variation df MS F P

Sh 1 160.48 112.07 0.009

Pa (Sh) 2 1.43 0.23 0.792

Zo 1 151.31 1.01 0.499

Sp 1 111.56 48.29 0.091

Sh�Zo 1 150.08 7.52 0.111

Sh�Sp 1 2.31 0.23 0.679

Zo�Pa (Sh) 2 19.96 3.26 0.042

Sp�Pa (Sh) 2 10.02 1.64 0.199

Zo�Sp 1 114.06 46.24 0.093

Sh�Zo�Sp 1 2.47 5.41 0.146

Sp�Zo�Pa (Sh) 2 0.46 0.07 0.928

Residual 128 6.13

Transformation Ln (X +0.01)

Cochran’s C 0.172

P NS

Sampling was carried out at two patches nested in each of two shores

in SW Ireland, n =9. SNK=Student–Newman–Keuls. Abbreviations

in terms: Sh=shore, Pa=patch, Zo=zone, Sp=species.

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165158

3.2.1. C. stellatus

There was no difference in abundance of 0–3 month

C. stellatus recruits into different adult zones (non-

significant zone effect, df =1, F =1.47, P=0.292,

Table 4

Analysis of variance of recruits (N1 month b11 months old) in Chthamalu

Source of variation df MS

Zo 1 6.1

Sh (Zo) 2 119.3

Da 3 4.3

Sp 1 30.1

Zo�Da 3 23.2

Zo�Sp 1 190.7

Da�Sh (Zo) 6 46.7

Sp�Sh (Zo) 2 1.8

Da�Sp 3 7.4

Zo�Da�Sp 3 4.2

Sp�Da�Sh (Zo) 6 12.8

Residual 256 2.2

Transformation Sqrt (X +1)

Cochran’s C 0.090

P NS

SNK multiple comparisons:

Zo within levels of Sp

C. montagui C. mo

C. stellatus C. ste

Sp within levels of Zo

C. montagui zone C. mo

C. stellatus zone C. ste

Analysis was carried out on two shores nested in each of two zones over fou

Zo=zone, Sh=shore, Da=date, Sp=species.

Table 2 and Fig. 3). There was significant variation

in this age group of C. stellatus between shores in

C. montagui adult zones, i.e., GARR had signifi-

cantly higher recruitment of C. stellatus than the

other two shores. However, there was no such var-

iation among shores at C. stellatus adult zones.

Significant levels of variation between sites within

shores occurred at all shores except BBW and CPK

(in which C. montagui zones were sampled). The

amount of variation associated with shore was large

relative to sites and residual (replicates within sites)

(Table 2).

3.2.2. C. montagui

C. montagui recruits of 0–3 months old did not vary

between different adult zones (non-significant zone

effect, df =1, F =0.08, P=0.797, Table 2 and Fig. 3).

Although Cochran’s C was still significant following

transformation, as the relevant term in the analysis is

non-significant, we can be confident in the interpreta-

tion of no adult zone effect on C. montagui recruitment

(see Methods). Shore to shore variation occurred at C.

montagui adult zones, with 0–3 month recruitment at

GARR being significantly higher than either BBW or

CPK. However there was also variation in the C. stel-

latus adult zone, both NOH and BBE had significantly

s stellatus and C. montagui in SW Ireland

F P

8 0.05 0.841

4 53.70 0.000

3 0.09 0.961

9 16.31 0.056

4 0.50 0.697

4 103.03 0.009

0 21.01 0.000

5 0.83 00.436

9 0.59 0.646

4 0.33 0.804

0 5.76 0.000

2

ntagui zoneNC. stellatus zone

llatus zoneNC. montagui zone

ntagui NC. stellatus

llatus NC. montagui

r dates, n =9. SNK=Student–Newman–Keuls. Abbreviations in terms:

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 159

more recruitment than OHK. Variation between sites

within shores only occurred at GARR. Shores and

replicates within site (residual variation) contributed

greatest to the variation in recruitment of 0–3 month

C. montagui and variation associated with sites was

much smaller (Table 2).

3.3. Abundance of cyprids+b17 day old metamorphs

between adult zones (within shores)

Recruitment (cyprids+b17 day metamorphs) of C.

montagui was extremely variable between shores and

patches. Abundance of C. montagui was low in both

zones except in patch 1 in the upper level at one of the

shores (BBW). However, there was no difference be-

tween adult zones in either species’ abundance (non-

significant zone� species interaction, df =1, F =46.24,

P=0.093, Table 3). This confirmed that recruitment to

the b17 day age group was not different between the

two adult zones at the same shore.

0

10

20

30

40

50

60

70

80

90

100

bbe noh bbe noh

Nov '96 Dec '96

C. stellatus

C. montagui

Mea

n A

bund

ance

/dm

2M

ean

Abu

ndan

ce/d

m2

C. montagui zone

C. stellatus zone

0

10

20

30

40

50

60

70

80

90

100

garr bbw garr bbw

Nov '96 Dec '96

C. stellatus

C. montagui

NS

NS NS

**

NS

**

NS

*

Fig. 4. Mean abundance (FS.E.) of Chthamalus stellatus and C. montag

C. montagui in SW Ireland. Significant differences between the species ar

names as in Fig. 1.

3.4. Recruit abundance (N1 month and b11 months

old) between adult zones

Both C. stellatus and C. montagui recruits in this

age group, combined across all four sampling dates

analysed, were more abundant within conspecific

zones than in congeneric zones. Within C. stellatus

adult zones, C. stellatus recruits were more abundant

than C. montagui and vice versa for C. montagui

recruits being more abundant than C. stellatus in the

C. montagui zone (significant zone� species interac-

tion, df =1, F =103.03, P=0.009, Table 4 and Fig. 4).

The sampling date at which recruits dominated conspe-

cific zones varied, e.g., at NOH (the C. stellatus zone),

patterns were significant in December 1996 but were

not significant again one month later in January 1997.

At C. montagui zones recruits of this species were

significantly higher than C. stellatus at one of the two

shores on every sampling date. At all shores there were

significantly more abundant recruits in conspecific

noh bbe ohk noh

Jan '97 June '97

garr cpk garr bbw

Jan '97 June '97

NS

NS * *

**

NS

**

**

ui N1 month b11 month recruits in adult zones of C. stellatus and

e indicated at *P b0.05 and **P b0.01 level. Abbreviations of shore

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165160

zones by June 1997, at which time recruits were N1

month old with a maximum potential age of 11 months.

3.5. Cyprids as a proportion (of cyprids+b1 month old

metamorphs) between adult zones, shores and sampling

dates

Proportions of cyprid to metamorph did not vary in

the two species according to whether we sampled in the

C. stellatus or C. montagui adult zone (non-significant

zone� species interaction, df =1, F =0.15, P=0.714,

Table 5). There was a consistently higher proportion

of cyprids relative to metamorphs in C. stellatus than in

C. montagui, irrespective of shore or zone sampled

(significant species� shore (zone) interaction, df =4,

F =3.29, P=0.011, Table 5). With respect to the four

dates analysed, C. stellatus always had a higher relative

abundance of cyprids compared to metamorphs than C.

montagui. This proportion was significantly higher in

C. stellatus than in C. montagui on 18 /24 sampling

occasions (significant species�date� shore (zone) in-

teraction, df =12, F =2.30, P=0.007, Table 5 and Fig.

Table 5

Analysis of variance of cyprids as a proportion (of cyprids+b1 month old m

Source of variation df M

Zo 1 76

Sh (Zo) 4 71

Da 3 66

Sp 1 36

Zo�Da 3 47

Zo�Sp 1 0.

Da�Sh (Zo) 12 26

Sp�Sh (Zo) 4 5.

Da�Sp 3 13

Zo�Da�Sp 3 15

Sp�Da�Sh (Zo) 12 4.

Residual 816 1.

Transformation ArcSin

Cochran’s C 0.049

P NS

SNK Multiple comparisons:

Sp within Sh (Zo)

C. montagui zone GA

BB

CP

C. stellatus zone BB

OH

NO

Sh within (Zo� Sp)

C. montagui zone C.

C.

C. stellatus zone C.

C.

Sampling was carried out at three shores nested in each adult zone over four

terms: Zo=zone, Sh=shore, Da=date, Sp=species. Abbreviations of shore

5). There was some variability between the shores in

the proportion of cyprids and metamorphs for both

species but only at the C. montagui adult zones. Pro-

portions were similar for both species at C. stellatus

zones.

4. Discussion

The regulation of population distribution and abun-

dance, through density-dependent relationships be-

tween settlers and adults, may hold true at some shore

heights but not at others (Minchinton and Scheibling,

1991; Menge, 2000). The present study examines ver-

tical distributions and confirms that factors acting on

recruitment and not on settlement were important in

explaining distributions of the barnacles C. stellatus

and C. montagui. A second major objective was to

determine when distinctive adult distributions were

established, by examining the spatial variation in re-

cruitment of each species at local scales in SW Ireland.

Related but smaller-scale studies have previously

shown that neither attached cyprids nor metamorphs

etamorphs) in Chthamalus stellatus and C. montagui in SW Ireland

S F P

.88 1.07 0.359

.65 40.14 0.000

.62 2.53 0.107

9.99 63.03 0.001

.92 1.82 0.197

91 0.15 0.714

.33 14.75 0.000

87 3.29 0.011

.68 3.33 0.057

.82 3.85 0.039

11 2.30 0.007

79

RR C. stellatus NC.montagui

W C. stellatus NC.montagui

K C. stellatus NC.montagui

E C. stellatus NC.montagui

K C. stellatus NC.montagui

H C. stellatus NC.montagui

montagui BBW=GARRNCPK

stellatus BBWNGARRNCPK

montagui NS

stellatus NS

dates, n =18. SNK=Student–Newman–Keuls. Abbreviations of model

s as in Table 1.

_________________________________

C. stellatus adult zone C. montagui adult zone

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

earlyAug

lateAug

Sept Oct earlyAug

lateAug

Sept OctearlyAug

lateAug

Sept Oct earlyAug

lateAug

Sept Oct earlyAug

lateAug

Sept OctearlyAug

lateAug

Sept Oct

bbe ohk noh garr bbw cpk

Mea

n P

ropo

rtio

n/dm

2

C. montagui

C. stellatus

Fig. 5. Cyprids as a proportion (of cyprids+b1 month metamorphs) in Chthamalus stellatus and C. montagui at six shores in SW Ireland.

Abbreviations of shore names as in Fig. 1.

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 161

of either species were more abundant within species’

adult zones compared to outside adult zones (Power et

al., 1999a; Delany et al., 2003). The present study,

conducted at a greater number of shores than the

earlier work, demonstrates that distinctive distribution

patterns observed in adults were not apparent in total

recruitment after the main settlement season had ended

and, in fact, did not emerge until a considerable time

afterwards.

Although adults of both species occurred over a

broad range of vertical heights, characteristic patterns

of C. stellatus and C. montagui were evident at six

shores in SW Ireland. Of the two species, C. stellatus

was numerically dominant at the mid tide levels of three

more wave-exposed shores and vice versa for C. mon-

tagui, which was dominant at the upper shore levels at

more sheltered shores. These findings are in agreement

with previous reports of adult distributions of each

species in north west Atlantic populations (Crisp et

al., 1981; O’Riordan, 1992; Healy and McGrath,

1998; Power, 2000; Delany et al., 2003). The vertical

zonation of populations at northern parts of the range

along the Atlantic coast differs from that of populations

at lower latitudes, where C. montagui is dominant at

most levels and exposures (Sousa et al., 2000; Range

and Paula, 2001).

In order to assess the relative abundance of different

age groups recruiting into adult zones, densities of

settlement and recruitment stages of C. stellatus and

C. montagui were enumerated over one year in natural

barnacle beds. The general pattern of settlement was

consistent across all six shores with a single peak of

cyprid density of both species in autumn, between

August and October 1996. This peak in cyprids was

augmented by a persistent trickle of recently metamor-

phosed individuals in subsequent months, even when

cyprid densities were negligible. A few cyprids of both

species were recorded in April 1997, several months

after the main settlement season had ended, though this

was recorded at only a single shore. One explanation of

why cyprids occurred in only a subset of the months in

which b1 month old metamorphs were found could

have been due to an underestimate of the age of those

metamorphs collected in winter and spring. This is

unlikely however: a study one year later at the same

shores found that recruitment of C. montagui and C.

stellatus into monthly-cleared quadrats occurred over a

7 month period, even though cyprids were only found

in a subset of these months (O’Riordan et al., 2004).

Many chthamalid barnacles can breed over long peri-

ods, (Wethey, 1983; Raimondi, 1990; Sutherland, 1990;

Pineda, 1994; Range and Paula, 2001) and both C.

stellatus and C. montagui have been reported to be

capable of producing broods at times outside the main

breeding season in SW Ireland (O’Riordan et al., 1995).

Since in the present study, recruitment must have oc-

curred in both C. stellatus and C. montagui even when

cyprids were not present, it can be concluded that

attached cyprids are a poor indicator of recruitment

and that a census measuring settlement as defined by

attached cyprids, is only effective when settlement is

high (see also Power et al., 1999a).

A different proportion of cyprids relative to meta-

morphs between C. stellatus and C. montagui has

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165162

already been shown by comparing total cyprids and

metamorphs collected over a year (Power et al.,

1999a; O’Riordan et al., 2004). This has been shown

in the present study to be a consistent pattern, there are

more cyprids relative to metamorphs in C. stellatus

than in C. montagui, irrespective of the adult zone,

shore or date sampled. This has previously been attrib-

uted to 1) a faster rate of metamorphosis in C. montagui

(see Cruz, 1999; Cruz et al., 2005), 2) temporal sepa-

ration of settlement of cyprids of the two species, e.g.,

during neap tides or at night time in C. montagui and

vice versa during spring tides or in day time in C.

stellatus (e.g., Raimondi, 1990 in C. anisopoma;

Cruz, 1999): the latter would have been overlooked in

the present study by sampling during daytime on spring

tides, see Power et al. (1999a) or 3) a higher metamor-

phic success in C. montagui or higher post-settlement

mortality in C. stellatus (e.g., Pineda, 1994 in Chtha-

malus spp.). If any of these three possibilities are true,

the measurement and interpretation of settlement rates

between the species are confounded since a cyprid is

not an equivalent unit between C. stellatus and C.

montagui (see Power, 2000 for further discussion).

Thus in the present study we have not compared the

species directly, instead we analysed settlement and

recruitment in each species separately (or summed

cyprids and metamorphs) so that potentially different

metamorphic rates will not affect the interpretation of

our results.

At a subset of the shores examined in the present

study, Delany et al. (2003) reported that adult zone had

no effect on total input of either cyprids or b1 month-

old metamorphs over the main settlement season be-

tween August and October 1996. By contrast, in Ply-

mouth dnewly settledT C. stellatus were reported to be

more abundant on the low shore at the most exposed

site, where adults of that species were also relatively

abundant (Burrows, 1988). A more recent study, also

from Plymouth, found that larval choice at settlement

was an important determinant of adult distributions

(Jenkins, 2005). In the present study, recruitment of

animals up to a maximum age of 3 months did not

vary between adult zones in either C. stellatus or C.

montagui. Comparisons between adult zones in the

present study did not depend on accurate ageing of

the individual barnacles because a comparison of re-

cruitment between species was made using a single

point estimate in October, after the majority of settle-

ment had taken place.

In both Delany et al. (2003) and the present study,

upper and mid shore zones were always sampled on

different shores, to maintain statistical independence of

adult zones. This is relevant because of evidence of

reduced recruitment at upper shore levels compared to

lower down on the shore in C. montagui (Kendall and

Bedford, 1987; Cruz, 1999). The opposite pattern has

been documented however, with fewer settlers below

compared to above mid-tide in C. stellatus (probably

actually C. montagui) (Connell, 1961a; Moyse and

Knight-Jones, 1967) and Chthamalus fissus (Menge,

1991). To examine how patterns of recruitment varied

vertically at the same shores, we carried out compar-

isons in 1998 between adult zones within shores for

cyprids and early metamorphs (in this case metamorphs

were b17 days old). Results of this experiment were

consistent with the hypothesis that adult zones were

unimportant in determining early species composition,

i.e., there was no difference in the abundance of either

species between mid and upper shore zones at the two

study shores. There was a large degree of variation in

settlers between shores and patches however, therefore

this result requires confirmation at more shores and

over a longer sampling period. Settlers/recruits of the

related species Chthamalus anisopoma (Raimondi,

1990) and Chthamalus spp. (Pineda, 1994) also did

not show any difference in abundance between shore

heights.

By contrast with early recruitment, a comparison of

predominantly older recruits up to a potential maximum

age of 11 months, did display the same distribution

patterns as adults. There was a significant effect of

adult zone for the entire group considered together

but significant effects were also seen on several dates

within the time period analysed. Particularly at upper-

shore C. montagui adult zones, a significant numerical

dominance of this species was observed 4, 5 or 6

months after the initiation of settlement. However this

pattern was not observed at all shores on any of those

dates and, once observed, was not necessarily main-

tained in subsequent months. Assessing the month

when zonation came about at individual shores was

complicated by the fact that only two of the three shores

in each zone were analysed for each of the sampling

dates (this was done to keep the experimental design

balanced by omitting shores for which there were miss-

ing data). By June 1997 when recruits were potentially

11 months old, all shores displayed significantly higher

recruit abundance within conspecific adult zones for

both species.

Variability in early life stages has been shown to be

very large in intertidal studies (e.g., Caffey, 1985;

Raimondi, 1990; Underwood and Chapman, 1996;

Benedetti-Cecchi et al., 2000; Jenkins et al., 2000;

Jeffery, 2003; O’Riordan et al., 2004). The spatial

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165 163

scales at which variation occurs may indicate which

factors are important in the population dynamics of a

species. For example, if recruitment varies more at large

spatial scales, this is more likely to be due to larval

supply which also varies at large spatial scales, whereas

factors such as physical stress are more likely to cause

variability at smaller spatial scales (e.g., Caley et al.,

1996; Underwood and Chapman, 1996). There is evi-

dence to support these general predictions, for example,

at large spatial scales regulation of a species can occur

at settlement in some parts of the geographic range

(where input rates are low) but comes about due to

mortality at other parts of the range (where input is

higher) (Connell, 1985; Menge, 1991, 2000). On a

smaller scale within shores however, Denley and Un-

derwood (1979) found that although settlement was

important in regulating the upper and lower vertical

limits of barnacle species, post-settlement mortality

was more important in governing which species colo-

nised microhabitats within the barnacle zone (shade

patches). Specifying the spatial scale at which variation

has been examined is, at any rate, crucial for making

comparisons between studies and drawing general con-

clusions about the relative importance of various factors

(Underwood and Petraitis, 1993).

Despite homogeneous 0–3 month recruitment into

adult zones in the present study, significant variation

did occur in this age group for both species at all spatial

scales examined. In C. stellatus, more than twice as

much of this variation was attributed to the larger scale

of shore (i.e., 1000s of metres) than was attributed to

sites within shores (10s of metres) or residual error

(within 4 m2). This implies that factors at the scale of

shores were more important than those related to smal-

ler spatial scales (within shores) for 0–3 month C.

stellatus recruits. Similar recruits of C. montagui, var-

ied mainly at the scale of shores but residual variation

within sites (i.e., within 4 m2) was also high, indicating

that C. montagui displayed patchiness at both larger

and smaller spatial scales. Variation at the scale of

shores occurred in both species of recruits inside C.

montagui zones because of a consistently higher re-

cruitment of both species at GARR than at the other

two C. montagui zone shores. GARR, in common with

other C. montagui zones, was sampled at upper shore

levels, which are all subjected to fewer hours of tidal

inundation. Variation between GARR and the other

upper zones may have arisen due to the fact that of

the three shores where C. montagui zones were sam-

pled, this was the most exposed to wave action. There is

an increasing body of evidence to suggest that recruit-

ment is higher at sites with a greater exchange of water

(Hatton, 1938; Caffey, 1985; Raimondi, 1990; Bertness

et al., 1992; Burrows, 1988; Menge, 2000; Hancock

and Petraitis, 2001). At exposed shores where C. stel-

latus zones (mid-shore) were sampled, only C. monta-

gui recruit abundance varied between shores, as all

shores had similar abundances of C. stellatus.

Post-settlement mortality has long been described in

chthamalid barnacles, for example, classic studies into

the emergence of intertidal distributions of dC.stellatusT before the recognition of C. montagui as a

separate species, report that adult vertical patterns at

settlement were unimportant because spat were abun-

dant at middle (Hatton, 1938) and low (Connell,

1961a; Moyse and Knight-Jones, 1967) shore levels

but did not survive there. In the present study, vertical

zonation in both C. stellatus and C. montagui was

indistinct until a minimum of 5 months and a maxi-

mum of 11 months after the settlement season had

begun (when settlement rates were negligible). There-

fore zonation does not occur at settlement and mortal-

ity acting on populations several months after

settlement is more important in structuring vertical

zones in these barnacle species in SW Ireland. There

is evidence of significant differential mortality in indi-

viduals between 6–12 months of age outside conspe-

cific adult zones in C. stellatus and C. montagui

(Delany et al., 2003; Burrows, 1988). The process

which brings about differences in abundance at C.

stellatus and C. montagui zones has been suggested

to be associated with desiccation (Power et al., 2001).

C. stellatus was found in higher densities in wetter

areas than in adjacent dry areas on the upper shore.

Conversely, a higher abundance of C. montagui was

found in drier areas of the upper shore compared to

adjacent wet areas (Power et al., 2001). In that study

the pattern was significant in C. montagui (maximum

age of 5 months) before it was significant in C. stella-

tus (maximum age of 10 months). A laboratory inves-

tigation into species-specific mortality under different

submersion regimes has shown that C. montagui suf-

fered less mortality in the shortest immersion regime

but C. stellatus showed most mortality in the same

treatment (Burrows, 1988). Future studies which link

wetness of the habitat to differential survival of C.

stellatus and C. montagui would confirm this as a

causative factor in determining vertical patterns in

both of these barnacle species.

Acknowledgements

We are grateful to the European Community for

funding the work under the MAST-3 programme, con-

A.M. Power et al. / Journal of Experimental Marine Biology and Ecology 332 (2006) 151–165164

tract MAS3-CT95-0012. The manuscript was prepared

while AMP was in receipt of a post doctoral fellowship

(PDOC/01/006) funded by the Marine Institute and

National Development Plan (Ireland) under the Marine

RTDI Measure. Comments from an anonymous referee

and A.J. Underwood made a valuable contribution to

the manuscript. [AU]

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