assessment of a trap for measuring larval supply of intertidal barnacles on wave-swept, semi-exposed...

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Journal of Experimental Marine Biology and Ecology

290 (2003) 247–269

Assessment of a trap for measuring larval supply

of intertidal barnacles on wave-swept,

semi-exposed shores

Christopher D. Todd*

Gatty Marine Laboratory, University of St. Andrews, St. Andrews, Fife, Scotland, KY16 8LB, UK

Received 15 February 2002; received in revised form 6 December 2002; accepted 7 February 2003

Abstract

Larval supply and settlement of Semibalanus balanoides (L.) on a semi-exposed wave-swept

shore in E Scotland were assessed by means of a new larval trap design. Two midshore sites were

utilised throughout the peak of the 2001 settlement season (May 11–June 3). Larval supply was

quantified by 176 ml baffled cylinder traps filled with a 4-M killing solution of urea in seawater.

Larval trap washout of urea typically ranged from f 25% per day under minimal wave action to a

maximum of 39% under heavy wave action during an onshore gale. The maximum capture for a

single trap was 334 cyprids over one tide. Significant and consistent positional effects, both on urea

retention and larval capture, were found for replicate traps separated by 9 cm. These indicated fine-

scale variations in hydrodynamic flow and larval supply over the substratum which will have

implications for the quantification of spatial heterogeneity of larval input to the benthos. The

retention of urea and larval capture efficiency generally were not compromised by traps being

serviced daily, as opposed to tidally, except perhaps for captures at very low larval densities. Larval

supply measured on a tidal basis was strongly correlated between sites (r = 0.975), but supply at Site

C was f 5� that at Site T: the reduction at Site T could not be explained by its slightly shorter

immersion time and hence ‘availability’ of cyprids alone. At Site C settlement was quantified for

5� 5 cm quadrats of natural substratum cleared daily. Settlement on these clearances (subject to

possible grazing by limpets) was quantified only from May 20–June 3, after the peak of larval

supply on May 17, and the variation in larval supply explained 65% of the variance in settlement.

The larval supply/settlement relationship at Site T was quantified using grooved acrylic panels. A

high correlation coefficient (0.961) was obtained for the daily supply/settlement relationship for

these panels over the period May 15–June 3, which included the peak of supply. Larval supply

varied up to 15-fold within a site on consecutive tides and up to 100-fold between sites on the same

0022-0981/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-0981(03)00093-5

* Tel.: +44-1334-463454; fax: +44-1334-463443.

E-mail address: [email protected] (C.D. Todd).

C.D. Todd / J. Exp. Mar. Biol. Ecol. 290 (2003) 247–269248

tide. Although further improvements to both the larval trap and the settlement panels can be made,

this larval trap does appear capable of providing high-resolution data on tidal or daily larval supply

over a wide range of wave conditions and larval concentrations.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Cyprid; Larval supply; Panel; Semibalanus balanoides; Settlement; Trap

1. Introduction

The concept of larval supply—as a measure of propagule input—and the relative

importance of pre- and post-settlement processes are pivotal elements of current theory on

population regulation/limitation and community dynamics in marine benthic systems (e.g.

Booth and Brosnan, 1995; Caley et al., 1996; Connolly and Roughgarden, 1998; Menge,

2000; Bradbury and Snelgrove, 2001; Connolly et al., 2001). The pelagic larval stages of

most benthic or demersal species generally are presumed to be passively dispersed in the

water column prior to their acquiring competence to metamorphose (Booth and Brosnan,

1995; Caley et al., 1996). For communities characterised by species with long-lived larvae

the likelihood is, therefore, that local adult reproduction (output) and larval colonisation

(input) will be de-coupled and the populations demographically open. Nonetheless, at

large spatial scales (e.g. 102 km) there may yet be significant stock-recruitment relation-

ships in, for example, coral assemblages (Hughes et al., 2000).

Intertidal barnacles have featured prominently in studies of settlement, recruitment and

larval supply. More than most taxa, barnacles do provide a realistic opportunity of

providing a clear understanding of the interplay between larval supply, larval habitat

selection (in relation to resource [ = space] availability), and juvenile/adult mortality

schedules in determining local population abundances. There are well-documented

negative or detrimental influences of limpet grazers on the recruitment and establishment

of intertidal barnacles (e.g. Branch, 1981; Hawkins, 1983; Hunt and Scheibling, 1997).

Accordingly, and although they may well be representative of successful realised recruit-

ment, counts only of settled barnacle cyprids on otherwise unmanipulated natural substrata

can in some geographic localities (such as the British Isles) be markedly error-prone as

measures of potential input over a given tide. For these reasons, the approach here to

quantifying the larval supply/settlement relationship has involved a small larval trap for

deployment adjacent to artificial panels that are assured to be free of the grazing impact of

limpets. The broader aim of future studies will be that patterns of settlement on panels can

then be appraised together with counts for patches of natural substratum accessible to

grazers, and from which grazers have been excluded.

Bertness et al. (1996) reported wind-driven settlement patterns at Rhode Island, USA.

On a larger spatial and temporal scale Gaines and Bertness (1992) showed that most of the

variation in settlement over a 9-year period in Narragansett Bay, RI, USA was explained

by measures of flushing, and therefore the likely export of larvae from the bay. Such

studies encourage confidence that the supply/settlement relationship is quantitatively

tractable. But in order to ascertain the importance of larval input in determining adult

abundances it is essential to be able to quantify the actual delivery of larvae to the benthos

C.D. Todd / J. Exp. Mar. Biol. Ecol. 290 (2003) 247–269 249

(e.g. Setran, 1992; Gaines and Bertness, 1993; Castilla and Varas, 1998; Noda et al., 1998;

Miron et al., 1999) and on wave swept shores. Larval concentrations in the water column

often have been used as proxies for larval supply (Gaines et al., 1985; Miron et al., 1999;

Snelgrove et al., 1999; Jeffery and Underwood, 2000; Olivier et al., 2000; Ross, 2001;

Pineda et al., 2002) but this approach is incomplete. Larval supply is a flux and depends

not only larval concentration in the water column but also upon water flow over the

substratum (Yund et al., 1991; Gaines and Bertness, 1993) and, for intertidal substrata,

immersion time (Minchinton and Scheibling, 1991): assuming no interfering influence of

larval depletion by settlement (e.g. Satumanatpan and Keough, 2001), a low concentration

can still comprise a high supply under high flow or heavy wave action (Todd, 1998).

Early attempts to quantify barnacle larval supply with passive cylinder traps showed

these to be operationally limited to calm localities with little or no vertical component to

water movement. Moreover they had to be deployed in the sublittoral at several metres

depth and tens of metres distant from the intertidal (Yund et al., 1991; Bertness et al.,

1992; Gaines and Bertness, 1993). This clearly is disadvantageous if consistent and

significant variation in settlement densities is detected on small spatial scales within

shores. Most previously reported larval barnacle trapping rates have been very low (e.g.

V 10 larvae trap� 1 day� 1; Yund et al., 1991; Bertness et al., 1992) in relation to variation

in the densities of settlement. A disadvantage of some traps has been their reliance on

formaldehyde to retain trapped cyprids. Other traps incorporating nets within valved pipes

attached to the shore (Setran, 1992) are effective but may be unreliable under conditions of

heavier wave crash or when subject to clogging. Yet other designs involve some

specialised construction and are bulky (Castilla and Varas, 1998; Jeffery and Underwood,

2000): the latter feature may be especially important to local hydrodynamic flow, though

Jeffery and Underwood (2000) reduced this problem by deploying their traps in crevices.

Suitable crevices may not always be available on a given shore, but considerations of trap

size also raise difficulties of replication over appropriately small spatial scales.

Here a new larval trap design was assessed for use in parallel with adjacent settlement

panels and counts of settlers on natural substrata, as a means of quantifying the supply/

settlement relationship in barnacles on wave-swept shores. The ideal larval trap should be

simple, small, efficient, durable, effective over a wide range of environmental conditions,

cheap, easy to make (and to repair in situ), and easily serviced (perhaps on a tidal basis and

at night) in large replicate numbers. The described trap is a modification of previous

passive cylinder designs and appears to satisfy the above criteria. Most important, this trap

is capable of high-resolution spatial and temporal data on larval supply for intertidal

barnacles over a wide range of larval concentrations and wave conditions on semi-exposed

rocky shores. Wave action, energy dissipation and associated water velocities are physi-

cally complex and extremely difficult to measure (e.g. Elgar et al., 2001), but a simple

integrated index of wave action was required to confirm that the larval traps will operate

over a wide range of wave conditions. An indirect measure of overall wave action each

day was obtained from trapped suspended sand.

The primary objectives were to confirm the reliability of the trap—as a means of

measuring Semibalanus balanoides larval supply within and between sites—and to assess

the utility of acrylic panels for quantifying the supply–settlement relationship. For

practical and logistic reasons most barnacle settlement studies are undertaken on a daily


basis, but such data may conceal ecologically important tidal variation both in larval

supply and settlement. A priority was the comparison of the tidal vs. daily performance

and reliability of the trap. Differences in larval supply among days during the settlement

season were expected, but for the comparisons of tidal vs. daily emptying regime the null

hypothesis was that there would be no significant difference in trap performance. A clear

alternative was that capture efficiency of daily traps might fall systematically for the

second tide, perhaps due to washout of the urea killing solution during the first tide. For

larval settlement on acrylic panels, the null hypothesis was that plane panels and grooved

panels would be equally attractive settlement substrata: a likely alternative was that

settlement would be significantly enhanced on grooved panels. Moreover, settlement on

horizontally grooved panels might plausibly exceed that on vertically grooved panels. The

latter alternative hypothesis derives from field observations (Hills and Thomason, 1998,

personal observation) of cyprids showing a general net upward movement during

exploration of substrata lacking significant texture.

2. Materials and methods

2.1. Study sites

Larval traps, settlement panels and sediment traps (to provide an indirect measure of

wave action) were placed at two intertidal sites in the Semibalanus balanoides (L.) zone of

the rocky shore at St. Andrews, Scotland (56j21VN 02j45VW; Ordnance Survey grid

reference no. 529 160). Mean and maximum spring tide amplitudes in this locality are 4.8

and 6.0 m, respectively. The 15 km of coastline between St. Andrews and Fife Ness

headland has a NE aspect and is exposed to wind-generated waves from the NNW to ESE:

winds from SSE to WNW are offshore and generate minimal wave action. S. balanoides

are abundant, especially on steep or vertical surfaces, and there are extensive lower

midshore beds of Mytilus edulis (L.). Fucoid macroalgae are common on broken, or more

horizontal, bedrock substrata and the predominant infralittoral kelp is Laminaria digitata

(Huds.) Lamour.

Site C was a vertical rock face projecting f 1.5 m in height from the bedrock and

which faced SW (i.e. upshore). The rock face was emersed on every tide. There were no

major reefs to seaward and although the outcrop was exposed to waves the trap arrays

were protected from direct upshore wave crash by their SW aspect. The rock face was

populated by Littorina spp. and limpets (Patella vulgata L.) and cover of adult S.

balanoides approached 100% at the upper margins of the rock face. Site T also was

populated by limpets and littorinids but was a smaller vertical face of a reef outcrop f 30

m from Site C. The Site T traps and panels were located f 30 cm higher up the vertical

shore profile than at Site C, and faced directly downshore with an unprotected N aspect.

2.2. Larval traps

The trap was a development of the basic cylinder of Yund et al. (1991) but differed in

its size and the inclusion of internal conical baffles. Traps were fabricated from proprietary


polypropylene laboratory tissue culture tubes (Cellstark, Product no. 210270; Greiner

Labortechnik, Germany; http://www.greiner-lab.com) of 57 ml capacity and with a conical

bottom surrounded by a skirt extension of the tube wall. Traps of 176 ml volume (Fig. 1)

were constructed from sections cut from four tubes to provide (i) a 57-ml entry chamber,

(ii) an intermediate three-cone baffled section (volume f 39 ml, each baffle separated by

3 cm) and (iii) a basal reception chamber (80 ml) which incorporated the screw-cap for

emptying/resealing the trap in situ. Holes of either 1 cm diameter (uppermost two baffles)

or 0.6 cm diameter (lowermost) were made in the centres of the cones by means of a

heated steel rod. To prevent the trapping of air on refilling, two holes were melted with a

needle through each of the baffles where they adjoin the interior cylinder wall. The cut

edges of the cylinder sections were abraded to a clean right angle finish with a sanding

wheel. This allowed the sections to be tightly abutted flush to one another. All sections

were firmly held together and sealed by their insertion into sections of 2.5 cm inside

diameter, 0.3 cm thick, clear PVC piping which had been softened in hot water. Placement

of the finished article in a freezer at � 20 jC for 20 min tightened and hardened the PVC

sections to provide a rigid leak-proof cylinder.

The trap was of overall dimensions 29 cm height and internal diameter 2.8 cm, resulting

in an aspect ratio of 10.4. Although less than the ratio of 12 for the plain cylinders of Yund

et al. (1991), the inclusion of baffles specifically aimed to (1) stabilize the volume of the

dead space, and (2) to minimize advection of larvae out of the trap by wave washout,

whilst not hindering advection of larvae into the trap. To further minimize advection out of

Fig. 1. Larval trap design (not to scale; see text for measurements and dimensions). Traps were emptied and

refilled in situ by means of the bottom screw cap.

http:\\www.greiner-lab.com


the trap, a dense killing solution was required. Because the traps were specifically intended

for intertidal deployment, and would be accessible to the public, it was expedient to avoid

formaldehyde. A suitable alternative natural product was provided by a 4-M solution of

urea in seawater. Trap washout was quantified by adding 1 ppt bromophenol blue (BPB)

solution to the 4-M urea stock to provide a working concentration of 10 ppm BPB in the

traps. The concentration of BPB (and thence urea retention) at each emptying was

determined by spectrophotometry (Shimadzu UV-1601; absorbance at 594 nm; seawater

blank).

At Site C larval traps were cable-tied to a clear acrylic mounting plate which itself was

cable-tied to 0.6 cm nylon cords against the barnacle-covered substratum; the cords were

very tightly tied, and applied to, the rock face by means of 5 cm nylon tension rings. The

mounting plate was effectively held firmly against the vertical rock face, without the need

to drill the substratum and to cement securing bolts, by cable-ties to a cord lattice. Bolts

would confer rigidity and place excessive focused stress on the acrylic in heavy wave

action. Movement or flexing of the mounting plate was possible, but minimized. No

mounting plates, traps or settlement panels (Section 2.3 below) were lost or damaged by

wave action.

The Site C trap mounting plate had eight slot positions, each 9 cm apart and each being

occupied by a trap throughout the season. From May 7 to June 3 2001 four replicates were

deployed for continuous data of larval supply to Site C: there were 32 consecutive tidal

observations (May 7–23), followed by 11 daily observations (May 23–June 3) when

larval numbers became low. Although the first settlement of S. balanoides was on April

23, and there was a small but distinct peak of settlement input on May 1, the period May

11–June 3 accounted for 85% of the 2001 season’s settlement at Site C (data not shown).

Within-mounting plate positional effects on the larval capture efficiencies of the replicate

traps were assessed by altering the slot position randomisations. The slot positions

allocated were nos. 2, 4, 7, 8 (May 7–11), 1, 2, 3, 7 (May 11–14) and 1, 3, 5, 7 (May

14–June 3) with the remainder occupied by dummy traps. Because significant and

consistent positional effects were observed, the detailed analyses of the supply/settlement

relationship for Site C were confined to the three replicate traps occupying slot positions 1,

3 and 7 throughout the period May 11–June 3.

Larval capture efficiency was assessed by emptying traps either on a tidal (n = 4

replicate traps) or a daily (n = 4) basis, for five consecutive days (May 17–21) at Site C.

Comparisons were drawn between the daily summed captures for ‘tidal’ traps emptied and

refilled after each tide and ‘daily’ traps emptied and refilled only after two tides.

Spectrophotometry also enabled a comparison of urea retention for the ‘tidal’ vs. ‘daily’

traps in relation to varying levels of wave action.

At Site T lateral space on the vertical rock face was restricted, partly due to the need for

a large backplate to accommodate a mounting plate bearing triplicate settlement panels for

each of three panel treatments (see below). From May 9, duplicate larval traps were cable-

tied to the cord lattice, one at the left and right ends of the backplate. A further pair of

larval traps (separated by 9 cm), providing duplicates at each end of the settlement panel

array, was deployed from May 15 to June 3. Between-site comparisons of urea retention

were undertaken for traps serviced both tidally (13 consecutive tides, May 17–23) and

daily (8 days, between May 24 and June 3). Detailed analyses of the supply/settlement


relationship for Site T were confined to cyprid counts for traps (n = 4) deployed for the 17

tidal observations (May 15–23) + 11 daily observations (May 23–June 3).

2.3. Placement of traps and panels, and settlement counts

Settlement panels at Site T were of black acrylic (8� 13� 0.6 cm). Although S.

balanoides will settle on plane acrylic substrata, settlement densities might be expected to

be much enhanced—perhaps up to levels typical of natural substrata—by the inclusion of

grooves on the panels (see also Pineda et al., 2002). Triplicate panels of two grooved

treatments (vertical or horizontal grooves; 0.5 cm wide and 0.1 cm deep) and triplicate

plane (non grooved) panels were allocated randomly to the nine positions on the 84 cm

mounting plate as a horizontal series in portrait orientation, with each separated by a 1-cm

gap. Panel randomisations were changed daily. Between each deployment, panels were

brush-scrubbed in hot freshwater, air-dried, abraded with a manual rotary sander, re-

washed and re-dried. All panels were then painted with an aqueous extract of adult

barnacles and air-dried before deployment. Panels were attached (by a nylon screw and

wing-nut) to a removable clear acrylic mounting plate (84� 24� 0.6 cm), which was itself

cable-tied to a clear acrylic backplate.

As a general protocol the aim was to undertake daily retrieval, counting and redeploy-

ment of panels, but also to obtain in situ counts of cyprids on the panels after the first tide.

From prior experience of likely settlement densities and numbers of cyprids to be counted

after a single tide just 12 cm2 area of groove was included for each grooved panel. For the

horizontal groove treatment, three 8-cm long grooves (2.75, 5.75 and 9.75 cm from the

top edge of the panel) were milled across the width of each panel. To maintain the same

groove length/area as for the horizontal groove treatment, the vertical groove treatment

comprised two grooves each of 12 cm length (2.5 cm in from each long side of the panel),

starting at the upper free edge of each panel but stopping 1 cm from the lower edge of the

panel.

Settlement on the natural rock substratum at Site C was quantified for triplicate quadrats

that straddled the backplate accommodating the traps. Clearances of approximately 7� 7

cm (leaving intact the peripheral barnacles) were made and a 5� 5 cm square was marked

with a fine permanent marker pen. Limpets were cleared to distances of at least 1 m from the

quadrats. Quadrats were scrubbed clean with a toothbrush and clean seawater after each

count. Counts were made tidally between May 20 and May 23 (7 tides) and thereafter daily

until June 3 (11 days). There were no data for May 28. Limpets were found to be occupying

one or more of the quadrats on 4 days and these were removed on each occasion.

2.4. Sediment traps

Sediment traps to provide an integrated index of wave action were constructed from

two Cellstar tubes (total volume 120 ml) with a single cone baffle, including a 0.6 cm

aperture, midway down the trap. Duplicate sediment traps were cable-tied adjacent to the

larval traps at Site C from April 20 to June 3. Sediment traps were refilled with clean

seawater following daily emptying. Retrieved sediment samples were washed through a

50-Am gauze with a soft paintbrush. Drift macroalgal fragments were removed and the


sand fraction retained on the sieve screen was oven-dried (90 jC, overnight), weighed,ashed (550 jC, 4.5 h) and re-weighed.

Analysis of the effects of variation in the daily wave crash index ( = trapped sediment) on

daily retention of urea by the Site C larval traps (below) was possible for eight days in the

periodMay 23–June 3, when contemporaneous daily sediment and larval trap samples were

obtained. The end of this period included an onshore northerly gale with heavy wave action.

2.5. Wind data

Wind strength (ms� 1; Beaufort scaled) and direction were recorded with a 1-WirekWeather Instrument Kit V3.0 (Tecnologıa Aplicada, Mexico) mounted on the roof of the

laboratory, approximately 500 m from the field sites. From the datalogs the prevailing

wind direction and its median and maximum strength (10 min averages) over the course of

each tidal period were derived.

2.6. Data analyses

All cyprid trap and quadrat counts were log x + 1 transformed prior to regression

analysis or ANOVA. The data for percent urea retention by the traps required angular

transformation. Factorial ANOVA involving multiple observations of, for example, cyprid

counts or urea retention by traps over time dictated that ‘Day’ be included as a random

factor. For comparisons of sediment trap/larval trap emptying frequency (i.e. daily [2 tides]

vs. tidally [1 + 1 tides]), the sampling frequency factor ‘Tides’ was considered fixed.

Comparisons between Sites C and T included Site as a random factor. GLM analyses of

positional effects on trap urea retention and larval captures within the mounting plate

included repeated measures observations for slot positions that are necessarily unreplicated

on any one tide or day. GLM (Slot position, fixed; Day, random) was applied to these data,

but the analysis proceeded to unplanned Tukey multiple comparisons only following

confirmation (by Tukey’s test for nonadditivity; Sokal and Rohlf, 1981; Underwood,

1981) that the differences among means were non-multiplicative and attributable to the

independent effects of the two factors.

Two factor ANOVA revealed, in certain instances, a significant interaction and hence

logically uninterpretable main effects. In these cases the means for each factor were

compared separately for the various levels of the second factor by unplanned comparisons

(SNK tests; Underwood, 1997). This procedure allowed the identification and explanation

of the source of the interaction and thereby the interpretation of the main effects.

3. Results

3.1. Sediment traps: daily vs. tidal emptying regime, Site C

Wave conditions varied markedly over the 5 days of comparison of daily (2 tides) vs.

tidal (1 + 1 tide) sediment trap samples (Fig. 2). Day 1 (April 20/21) followed an extended

period of Beaufort Force 8 N gales and heavy wave action. By Day 2 the wind had backed

Fig. 2. Sediment traps, Site C. Mean ashed weight ( + S.E.) of the sand fractions (particles z 50 Am) taken in traps

emptied tidally and daily over consecutive days, April 20–25 inclusive. Data for the tidally emptied traps on each

day were summed for comparison with those emptied just once daily.


to the SW (offshore) and wave action fell quickly. Day 3 had SE/ESE winds at a maximum

of Force 5, with the wind veering to the SW on Day 4. By Day 5 the wind had again

veered to onshore, and from the NE, at a maximum of Force 6. Two-factor ANOVA

confirmed the significant Day effect (F4,34 = 1214.28; p < 0.001) and also that there was no

significant difference (F1,4 = 2.41; p = 0.195) between sediment traps emptied daily, and

those emptied tidally and summed each day. There was no significant interaction between

emptying regime and days (F4,34 = 0.59; p = 0.670). Accordingly, only daily data for

sediment traps were obtained throughout the subsequent settlement season.

3.2. Larval traps: daily vs. tidal emptying regime, Sites C and T

Larval catches at Site C for traps emptied daily were compared with those emptied

tidally, but with their cyprid catches summed, for each of the five analytical days (Fig. 3).

Larval captures ranged over three orders of magnitude. ANOVA (Table 1) showed a

significant interaction between emptying regime and time. The unplanned comparisons

confirmed that there were no significant differences between emptying regimes for Days

1–3 inclusive when larval supply was moderate to high. But for the lowest larval densities

on Days 4 and 5 (Fig. 3) there were significant differences (SNK test) in both directions for

emptying regime, leading to the interaction. Efficiency of the traps generally was not

compromised by their being serviced daily, except perhaps at very low larval densities, but

even then there was a lack of consistency of effect of the emptying regime. The

disproportionate influence of the log transformation on Days 4 and 5 (leading to the

Fig. 3. Larval traps, Site C. Comparison of larval catches ( + S.E.) for traps (n= 4) emptied daily vs. traps (n= 4)

emptied tidally, and for which the two tides’ catches were summed for each day.


significant interaction), when larval catches were very low, was confirmed by ANOVA of

the untransformed data: the main effect for Day remained significant (F4,30 = 87.72;

p < 0.001) but both emptying regime (F1,4 = 1.78; p= 0.253) and the interaction (F4,30 =

0.07; p = 0.991) were non-significant.

3.3. Larval traps: within-site heterogeneity in larval supply and between-site comparisons

of urea retention, Sites C and T

Positional effects for the eight trap slots on the mounting plate at Site C were suspected.

Fig. 4 shows the percent urea retention and larval captures of three replicate traps deployed

in the same slots (1, 3, 7) over 14 tides, with a fourth trap which was moved from slot 2 to

slot 5 after eight tides. Urea retention by traps here generally was high, varying only from

68% to 77%. Larval captures varied considerably and ranged from means of 41 to 112

Table 1

Larval traps: emptying regime, Site C

Source df SS MS F p

Tides 1 0.008 0.008 0.12 0.748

Day 4 22.145 5.536 317.13 < 0.001

Tides�Day 4 0.260 0.065 3.72 0.014

Error 30 0.524 0.018

Total 39 22.937

ANOVA for cyprid larva captures (log x+ 1) in traps (n= 4) emptied tidally vs. traps emptied daily. The factors

were emptying frequency, i.e. Tides [1 + 1 vs. 2 tides](fixed), and Day (random).

Fig. 4. Larval traps, Site C. Positional effects of percent (F S.E.) urea retention (the inverse of ‘‘washout’’) and

cyprid capture (F S.E.) for replicate traps (n= 4) within the eight-slot mounting plate. Data were collected tidally;

after eight tides the position of one of the traps (in slot 2) was randomly reassigned (to slot 5). Slots 1, 3 and 7

were occupied by traps throughout the period May 12–19, but individual traps were randomly reassigned to those

slots at the May 16 changeover. Upper case letters represent the Tukey groupings from GLM unplanned

comparisons (Tables 2 and 3).


larvae trap� 1 tide� 1. There was a clear and consistent overall positional effect on trap

capture performance and a correlation between larval capture rates and urea retention

amongst the traps; on a comparative basis, trap slots 7, 3 and 1 performed very similarly

across both the eight-and six-tide periods. GLM (Tables 2 and 3) showed the very

Table 2

Larval traps: positional effects on trap performance, Site C

Source df Adj SS Adj MS F p

(A) Eight tides, May 12–16; Trap slot positions 1, 2, 3 and 7

Slot position 3 128.189 42.730 35.62 < 0.001

Tide 7 732.589 104.656 87.24 < 0.001

Error 21 25.191 1.200

Nonadditivity 1 3.635 3.635 3.372 > 0.05

Remainder 20 21.556 1.078

Total 31 885.969

(B) Six tides, May 17–19; Trap slot positions 1, 3, 5 and 7

Slot position 3 66.526 22.175 27.10 < 0.001

Tide 5 136.749 27.350 33.43 < 0.001

Error 15 12.273 0.818

Nonadditivity 1 2.328 2.328 3.279 > 0.05

Remainder 14 9.945 0.710

Total 23 215.547

GLM (including Tukey’s test for nonadditivity) for urea retention (arcsine percent) over eight and six tides (Slot

position, fixed; Tide, random). There was no significant nonadditivity, therefore the analysis proceeded to assess

the factor Slot position by Tukey multiple comparison tests. The multiple comparison groupings are included in

upper case on Fig. 4.

Table 3

Larval traps: positional effects on trap performance, Site C

Source df Adj SS Adj MS F p

(A) Eight tides, May 12–16; Trap slot positions 1, 2, 3 and 7

Slot position 3 0.4745 0.1582 20.42 < 0.001

Tide 7 8.9430 1.2776 164.96 < 0.001

Error 21 0.1626 0.0077

Nonadditivity 1 0.0103 0.0103 1.353 > 0.05

Remainder 20 0.1523 0.0076

Total 31 9.5801

(B) Six tides, May 17–19; Trap slot positions 1, 3, 5 and 7

Slot position 3 0.1279 0.0426 6.79 0.004

Tide 5 4.0052 0.8010 127.52 < 0.001

Error 15 0.0942 0.0063

Nonadditivity 1 0.0098 0.0098 1.633 > 0.05

Remainder 14 0.0844 0.0060

Total 23 4.2273

GLM (including Tukey’s test for nonadditivity) for cyprid catches (log x+ 1) over eight and six tides (Slot

position, fixed; Tide, random). Details as for Table 2.


significant Slot position and Tide main effects both for urea retention and for larval

capture. The inverse relationship between trap urea retention and the index of wave action

for Site C is shown in Fig. 5 for 12 daily larval trappings over the period May 23–June 3.

More comprehensive data (and especially additional observations at moderate to high

wave action) are needed to resolve the fuller detail of the true shape of this relationship.

The highest sediment values here were attributable to Force 8 onshore N gales (April 20,

Fig. 5. Larval traps, Site C. Scatterplot of mean percent urea retention for replicate daily larval traps (n= 4) against

the index of wave action (ashed sand fraction weight; n= 2 traps) at Site C during the period May 23–June 3. The

larval traps are differentiated into two subsets (n= 4 days, closed circles; n= 8 days, open circles) according to the

mounting plate slot positions that they occupied. The heaviest wave action and lowest percent urea retention were

recorded following the Force 8 onshore northerly gale on June 1/2. The bulk of the data were for days of offshore

winds and minimal wave action.

Fig. 6. Larval supply (Sites C, T), index of wave action (ashed sand fraction, Site C) and wind data (maximum

Beaufort strength and predominant direction over each tide from 10 min averages) for May 7–June 3. All error

terms are + S.E. Sediment traps were emptied daily throughout, whereas the larval traps initially were emptied on

a tidal basis, switching to a daily basis from May 23.


Table 4

Larval traps: between-site comparison of performance, Sites C and T, over 13 consecutive tides

Source df SS MS F p

Site 1 18.935 18.935 2.10 0.173

Tide 12 953.812 79.484 8.79 < 0.001

Site�Tide 12 108.450 9.037 1.17 0.318

Error 78 601.756 7.715

Total 103 1682.953

ANOVA for urea retention (arcsine percent) of traps (n= 4) emptied tidally. The factors were Site (random) and

Tide (random).


June 2; Figs. 2 and 6) and levels of wave action in excess of this are uncommon during the

S. balanoides settlement season.

3.4. Larval traps: tidal and daily between-site comparisons of urea retention, Sites C and T

Although there was the expected time (Tide/Day-related) variation in urea retention for

the larval traps deployed at both Sites C and T, there was no significant Site effect and no

significant interaction for traps serviced tidally (Table 4). For the subsequent eight

samples, over which traps were emptied daily (i.e. after two tides), there was, however,

a significant interaction between Site and Day (Table 5). Six of the eight daily comparisons

between sites showed no significant difference (SNK tests). Day 5 (May 28/29) showed a

significantly higher retention of urea at Site T, whereas on Day 7 (June 1/2) the Site C

traps showed significantly higher urea retention. Fig. 6 shows the index of wave action

over this period of sampling. Day 5 was during an extended calm period with offshore

winds. For both Days 7 (significant SNK for Site) and 8 (non-significant SNK) wave

action was maximal over the observed settlement season, due to the onshore Force 8 gale

on June 2. The explanation of the interaction is therefore one of a non-systematic variation

in trap performance at the two sites on two days of minimal and maximal wave action.

Notwithstanding the significant daily interaction, there was overall comparability of

larval trap urea retention at the two sites, despite Site C traps being protected from direct

wave crash and Site T traps facing directly into oncoming waves. Any gross tidal or daily

differences in larval supply recorded for these two sites will not be simply attributable to

wave crash-related differences in urea retention by traps, and hence their larval capture

efficiency.

Table 5

Larval traps: between-site comparison, Sites C and T, over 8 days

Source df SS MS F p

Site 1 21.842 21.842 1.26 0.298

Day 7 653.618 93.374 5.40 0.020

Site�Day 7 120.981 17.283 3.97 0.002

Error 48 209.188 4.358

Total 63 1005.628

ANOVA for urea retention (arcsine percent) of traps (n= 4) emptied daily. The factors were Site (random) and

Day (random).


3.5. Larval traps: within and between-site patterns in larval supply, Sites C and T

Larval supply for S. balanoides at Sites C and T, together with the wind observations

and sediment trap data (Site C), are presented in Fig. 6. Larval supply was consistently

approximately five times higher at Site C. This difference cannot be explained by the

slightly shorter immersion periods (and hence reduced larval ‘availability’, sensu Min-

chinton and Scheibling, 1991) for traps at Site T. For the final 13 tides (May 17–23) of the

tidal data set, three of the four larval traps at Site C occupied consistent positions in the

eight slot trap array. For those tides, the correlation in larval supply between Sites C and T

was 0.975 ( p < 0.001), but for the final 11 days (May 23–June 3), when the traps were

emptied daily, this fell to 0.193 (ns). This latter period was, however, one of a generally

very low larval supply, with only the 1 day (at Site C) when the mean exceeded 10 cyprids.

Pooling of the initial tidal data (n = 13) with the later daily data (n = 11) (see Fig. 6)

provided an overall correlation coefficient for supply at Sites C and T over the period May

17–June 3 of 0.969 ( p < 0.001). The consistently lower larval supply at Site T was a real

ecological difference and was not attributable to reduced urea retention, or reduced

efficiency, of the traps at Site T (Tables 4 and 5).

3.6. Larval supply/settlement relationships on natural substrata, Site C

Fig. 7 shows the larval supply/settlement relationship for the traps and 5� 5 cm

quadrats of natural substratum at Site C over the final fortnight of the observed settlement

season. Settlement here was maximally 12.8 cyprids cm� 2 day� 1. Given that the larval

Fig. 7. Mean tidal settlement densities (Site C; n= 3 quadrats; each of 25 cm2 area) of cleared natural substratum

adjacent to triplicate larval traps in relation to larval supply. Small open circles denote tidal data (May 20–23

inclusive), large open circles denote daily data (May 24–June 3) and the large filled circles denote days upon

which limpets were found resident in one or more quadrats. The regression equation ( y=� 2.35 + 21.556x;

r = 0.827, p< 0.001, n= 13) excludes the filled circles.

Fig. 8. Daily mean settlement (F S.E.) of cyprids on triplicate plane, horizontal grooved and vertical grooved

panels at Site T, May 5–June 3 inclusive.


peak had been passed by May 18, and that larval supply at this late stage was consistently

low, detailed analysis would be inappropriate but in terms of the utility of the trap a clear

supply/settlement relationship was apparent. At least some of these data (four low supply

occasions, Fig. 7 black fill) were affected by limpet grazers, which remained within one or

more quadrats at low tide. The correlation coefficient for the regressions either including

(0.822, p< 0.001; n = 17) or excluding (0.827, p < 0.001; n = 13) the limpet-affected data

were closely similar.

3.7. Larval supply/settlement relationships on panels, Site T

Both the horizontal and vertical grooved panels provided clear and comparable

settlement signals throughout the season. About 97% of the larvae settling on both the

vertical and horizontal grooved treatments settled in the grooves (area 12 cm2), rather than

on the plane areas of the panel surfaces (area 92 cm2), but settlement densities on the

horizontal groove treatment generally exceeded the vertical treatment (Fig. 8). Daily

Table 6

Settlement panels: between-treatment comparisons, Site T, May 5–June 3

Source df SS MS F p

Panel type 2 23.852 11.926 64.00 < 0.001

Day 29 46.012 1.587 48.94 < 0.001

Panel type�Day 58 10.809 0.186 5.75 < 0.001

Error 180 5.835 0.032

Total 269 86.508

ANOVA for cyprids (log x+ 1) settling on triplicate panels of three treatments (plane, horizontal grooved, vertical

grooved). The factors were Panel type (fixed) and Day (random).

Fig. 9. Scatterplot of mean tidal settlement densities per horizontal grooved panel (area 104 cm2) per tide (n= 17)

at Site T, May 15–23, in relation to larval supply. The supply data, as cyprid numbers per trap (n= 4), were for

traps deployed in pairs at either end of the settlement panel array. Regression equation: y=� 1.128 + 2.167x;

r = 0.919, p< 0.001; r2 = 0.845.


settlement on all three panel treatments over the 30-day period showed the expected highly

significant main effects and interaction (Table 6), the latter being attributable to the

consistently poor or zero settlement on the plane panels almost irrespective of the variation

in larval supply. Exclusion of the uninformative plane panels from ANOVA confirmed the

superior settlement performance of the horizontally grooved panels over the vertically

grooved panels (F1,29 = 10.63, p = 0.003) and the lack of interaction (F29,120 = 0.98,

p = 0.503) for these two panel types.

Fig. 9 shows the tidal supply/settlement relationship for S. balanoides at Site T

(horizontal grooved panels only) over the peak of the settlement season (May 15–23

inclusive) and the high correlation coefficient. No striking instances of settlement failure

were apparent and a high overall proportion (85%) of the variance in settlement on the

horizontal groove panels was explained by the variation in larval supply over this

period.

4. Discussion

4.1. Tidal vs. daily performance of traps

Although an objective of any analysis of the larval supply/recruitment relationship

for littoral species should be the acquisition of data on a tidal basis, it has to be

acknowledged that for logistic reasons, or for species with extended seasonality of

settlement, it may prove that only daily data are practicable. A priority of the present


study was confirmation that the traps would provide comparable and reliable data over

either one or two tides deployment (Fig. 3) and under conditions of varying wave

action. Peak larval supply occurred over May 12–19, when the comparative assess-

ment of the performance of the traps on a tidal vs. daily basis was made. Larval

catches fluctuated considerably (Fig. 6) and this period also was one of a wide range

of wave action due initially to sustained onshore (NE) Force 4–5 winds, followed by

an extended run of strong offshore (SW) Force 5–6 winds (Fig. 6). Although more

severe gale force onshore winds and heavy wave action occurred in early June, the

May 12–19 tidal vs. daily data showed generally comparable larval captures over a

wide range of ambient conditions and larval densities. Capture efficiency of the traps

generally was not compromised by their being serviced daily. Although tidal servicing

of traps would be optimal for achieving maximum precision of the data on the supply/

settlement relationship, if only daily servicing were expedient this still will provide a

reliable, integrated measure of larval supply over the two tides in any one day. None

the less, a lack of consistency of performance was shown at very low larval densities

(Fig. 3, Table 1).

4.2. Within and between site tidal variation in larval supply

Consistent differences in season-long larval supply and settlement between sites

separated by tens of metres at the same tidal height probably are typical on broken rocky

shores (e.g. Gaines et al., 1985), but site rankings might switch under particular levels of

wave action, or when waves break from particular directions. The actual causes of the

between-site differences observed here were not of central interest in the present context of

assessing the trap design. Although qualitatively strikingly similar (Fig. 6), quantitatively

there were marked contrasts in supply to these two sites of closely similar tidal height and

separated linearly by only f 30 m. Larval supply varied up to 15-fold between

consecutive tides at Site C (May 16) and such variation will be reduced by almost an

order of magnitude if data are available only from daily trapping of larvae. The

consecutive tides of May 16 p.m./17 a.m. showed an exceptional (30-fold) level of

variation within Site T, due to a virtual absence of larvae trapped (mean 1.8 per trap) on the

p.m. tide (when a mean of 183 cyprids were trapped at Site C; Fig. 6). The May 16 p.m.

tide showed a 100-fold difference in larval supply between sites: that one tide’s larval

supply at Site C accounted for 15% of the recorded season’s input and hence was not

trivial. The ecological relevance of such tidal variations in supply will be particularly

manifest if there are significant tide to tide variations in early survivorship of recruits (e.g.

Hunt and Scheibling, 1997). Density dependent effects, larva–larva interactions and the

ameliorating influences of perhaps excessive levels of wave action, or seasonal variations

in the timing of acquisition of larval metamorphic competence and metamorphic delay

behaviour all will, to some extent, be implicated here. Extreme failures in settlement, and

within-shore spatial variations in larval supply such as the above, are the crux of the

unexplained variation in the supply/settlement relationship. Given the marked between-

cohort differences in early mortality of settled postlarvae noted by Wethey (1986), such

variation also can have clear and immediate implications on the ultimate determination of

local abundances of adults.


4.3. The supply/settlement relationship and tidal vs. daily data

Access to only daily (rather than tidal) supply/settlement data will tend to inflate the

coefficient of determination for the supply/settlement relationship and lead to an over-

estimation of the strength of the relationship by concealing marked—and arguably

ecologically important—levels of tide to tide variation. At Site T, for acrylic panels, a

linear regression model gave a high correlation between tidal larval supply and settlement

(May 11–23, r = 0.919; Fig. 9). Collapsing of the initial tidal data set at Site T into daily

measures, and their pooling with the May 23–June 3 daily data yielded an even higher

correlation ( y =� 0.315 + 2.128x; r = 0.961, p < 0.001, n = 20). Whilst an ability to explain

so much of the variance in settlement by variation in larval supply engenders confidence in

the larval trapping protocol, daily data do limit the possibilities of achieving the rather

more important objective of accounting for the unexplained variation, much of which

occurs on a tidal basis (e.g. the p.m. tide of May 16; Fig. 6).

4.4. Trap positional effects on larval captures, Sites C and T

Positional effects on trap retention of urea (Table 2) and larval capture (Table 3; Fig. 4)

arose from unexpectedly small-scale patterns, driven presumably by local variations in

hydrodynamic flow over the vertical substratum. The observed variation may be

artefactual, in being generated by the replicate traps themselves (here separated by 9

cm). More likely, however, these positional effects are due to real, fine-scale hydro-

dynamically driven variations in larval supply to the heterogeneous substratum, although

this remains to be confirmed by further experimental manipulation of the trap array profile

and of trap spacing within a mounting plate. Notwithstanding this requirement, a

comparison of the relative larval capture performances of the duplicate traps (1 and 2)

deployed at Site T, before vs. after the deployment of the additional replicate traps (3 and

4), showed no change in trap performance. There were 12 occasions when traps 1 or 2

caught five or more cyprids; four when only the two traps were present, and eight when

four traps were deployed. There was no significant difference in arcsine larval capture

proportionality for traps 1 and 2 before vs. after deployment of the additional two traps

(t = 1.05, p = 0.298). Assuming that the small-scale variation in larval supply to the

substratum noted at Site C (Fig. 4) is indeed real, this level of heterogeneity will demand

especial consideration in future experimental design in terms of trap replication and their

spatial separation.

4.5. Trap washout and larval capture rates

Although the sediment traps provided a useful index of wave conditions in assessing

trap performance over a range of wave action, these data are limited in their utility and of

comparative value only within a given site locality. Future quantification of wave action

will be by means of autonomous data-logging pressure transducers. An inverse relation-

ship between larval capture and urea retention revealed by the positional analyses (Fig. 4)

indicated that over short periods of heavy wave action a greater proportion of larvae

entering the trap will be advected out before they pass irretrievably to the reception


chamber. Under such circumstances larval supply will be slightly under-estimated.

Approximately 25% of the urea content of the present design of larval trap is washed

out over one or two tides under minimal levels of wave action (Fig. 5), be those under

calm atmospheric conditions or during periods of strong offshore winds. At the highest

levels of wave action observed over the settlement season (during an onshore Force 8 gale,

June 2; Fig. 6), trap washout of urea peaked at a mean of just 39% (Fig. 5). This latter level

of washout is likely to approximate to the typical maximum during the settlement season

of S. balanoides in E Scotland, although periods of similar strength N/NE winds (and

greater wave action) are possible. The overall pattern was one of increased wave action

leading to increased trap washout, which also correlated with a decreased larval capture

rate (Fig. 4). Clearly, further improvements to the trap design to minimize larval advection

out of the trap can be incorporated, but any alterations to reduce advection losses must also

minimally compromise the entry of larvae into the trap.

4.6. Improvements to panel designs with respect to settlement on natural substrata

Yund et al. (1991) and Bertness et al. (1992) deployed their cylinder traps at 4–5 m

depth and up to 40 m offshore and serviced these fortnightly. Their captures of larvae were

low (V 10 trap� 1 day� 1), although scatterplots of settlement of S. balanoides on trapped

larval supply showed very high correlations and apparent linearity of the supply/settlement

relationship; but the predictive value of a regression of settlement on supply was weak due

to the numbers of cyprids trapped. From Bertness et al. (1992), for example, an increase in

cyprid captures from, say, 4 to 5 larvae per trap per day yielded a predicted increase of

settlement from f 16 to 21 cyprids/cm2 of quadrat/day. The converse problem was

encountered in the present study. Although larval captures commonly were high (peaking

at a tidal mean of 267 cyprids trap� 1; Site C, 17 May), settlement densities on the panels

(but not the rock substratum) generally were low (e.g. Fig. 8).

The grooved panels at Site T were considerably more effective than plane panels as

settlement substrata. May 17 was the highest settlement on the horizontal groove panels at

Site T, with a mean density of 1.2 cyprids cm� 2 of panel tide� 1, or f 10 cyprids cm� 2 of

groove tide� 1. Cyprids exploring the plane surface of these grooved panels evidently can

move considerable distances on their antennules (e.g. Hills et al., 1998), and can migrate

vertically into the grooves, but probably many exploring cyprids relinquished the plane

surface before encountering a groove. The number of grooves here had been restricted to

three to facilitate in situ scoring at night, but evidently will have to be increased in future

studies to ensure ecologically more realistic settlement rates of exploring cyprids.

Settlement data on natural substrata (Site C) and parallel trapped estimates of larval

supply were obtained only after the peak of larval supply (May 17) had passed for the

season. Although of limited extent, the data (Fig. 7) do corroborate the supply/settlement

patterns revealed by the panels at Site T, but it is clear that the present panels provide a

quantitatively inadequate reflection of the densities of natural settlement. The precision of

the data for the supply/settlement relationship on natural substrata potentially obtainable

from the larval traps is none the less apparent (Fig. 7): the expectation is that f 5

cyprids will settle per cm2 of rock for each cyprid trapped per cm2 of the trap entrance

(area 6.15 cm2).


4.7. Effects of grazers on quantification of the supply/settlement relationship

Minchinton and Scheibling (1991) showed high correlations between settler density

and recruit density for S. balanoides in Nova Scotia. With the exception of Littorina spp.,

which are not a major source of mortality to post-settlement S. balanoides (Hancock and

Petraitis, 2001), and a low shore acmaeid limpet, settling barnacles in that locality are not

subject to intense post-settlement mortality from molluscan grazers. In NE England, over

the first 5 days post-settlement, Wethey (1986) recorded extremely high early mortality

rates, ranging from 60% (midshore) to 90% (high shore), for S. balanoides cyprids. The

sources of mortality in Wethey’s cleared patches were not specified but it is likely that their

removal by limpets and desiccation effects were major factors.

Other than manual clearances of limpets within f 1 m, no attempt was made here to

prevent access of limpets to the three settlement quadrats. On four occasions limpets were

found residing in one or more of the quadrats (Fig. 7). The possibility remains that limpets

grazed some, or all, of the quadrats to varying extents on each of the other occasions (at

high and/or night time low tide). Settlement rates greater than the foregoing 5:1 ratio might

therefore be expected on ungrazed quadrats. Notwithstanding the potentially severe

detrimental influence of limpet grazing on early recruitment of barnacles in the British

Isles, and the quantitative problems of under-estimation or over-estimation of settlement

input according to the time intervals over which census counts of settlers are made

(Minchinton and Scheibling, 1993), the greater challenge always has lain in the reliable

estimation of larval supply to the substratum. For example, Hawkins and Hartnoll (1982)

recorded a correlation between onshore winds and S. balanoides settlement on two shores

on the Isle of Man. In the absence of independent measures of larval supply one cannot

distinguish between different causations for such pattern. Equally plausible, but funda-

mentally different, explanations of the patterns they noted might include, for example, (i)

onshore winds concentrating larvae over the intertidal substrata, or (ii) higher wave action

( = greater larval encounter of the intertidal substratum) alone leading to an increased larval

supply whilst larval concentration remains unchanged.

Even if realistic, ‘‘grazer-free’’, and appropriately detailed comparative measurements

of natural barnacle settlement are available, there are likely to be important density

dependent interactions between the actual flux of pelagic larvae to the substratum and the

numbers that successfully settle and metamorphose. Final delivery of larvae to the

substratum (or to traps) during heavy wave action may be considered a largely passive

process, but actual choice of settlement site and attachment by barnacle cyprids is far from

passive (e.g. Hills et al., 1998). Variations in larval behaviour, together with environ-

mentally driven physical limitations on settlement (e.g. in extremely high flow), also will

contribute to the typically observed patterns of high spatial and temporal variation of

settlement on the shore (Gaines et al., 1985). The numbers of cyprids settling over a given

tide will not be a simple proportion of those available in the water column, and

superimposed upon this there may be seasonal variations in benthic–pelagic coupling

and specificity of larval responses to settlement cues (e.g. Jarrett, 1997). Ecologically

realistic predictive models (e.g. Roughgarden et al., 1985; Hyder et al., 2001), and a better

understanding of the seasonal, regional and local variations in settlement (e.g. Jeffery,

2000; Menge, 2000)—and thereby population and community dynamics—at particular


locations perhaps can be best achieved by integrating representative data for larval supply,

rather than larval concentration, with actual observations of settlement.

Acknowledgements

Helen Gurney–Smith and Bob Wilson assisted with aspects of the fieldwork and I am

grateful for their efforts. Two referees made helpful and constructive criticisms of an

earlier draft. [AU]

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assessment of a trap for measuring larval supply of intertidal barnacles on wave-swept, semi-exposed...

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