burrowing behaviour of the softshell clam (mya arenaria) following erosion and transport

y and Ecology 340 (2007) 103–111www.elsevier.com/locate/jembe

Journal of Experimental Marine Biolog

Burrowing behaviour of the softshell clam (Mya arenaria)following erosion and transport

Philippe St-Onge, Gilles Miron ⁎, Gaétan Moreau

Département de biologie, Université de Moncton, Moncton, New Brunswick, Canada E1A 3E9

Received 2 June 2006; received in revised form 26 July 2006; accepted 26 August 2006

Abstract

Erosion and transport of juvenile individuals may alter the distribution pattern of intertidal bivalves. The burrowing success ofrecently transported juvenile softshell clams (Mya arenaria) was studied in a laboratory flume under a wide range ofhydrosedimentary environments. Juvenile individuals (5–20 mm) were observed under a simulated 30 min slack tide beforeinitiating the flow for a period of 60 min. Five different free-stream velocities (0, 3, 5, 10 and 24 cm s−1) and four sediment types(mud, sandy-mud, sand and gravel) were used. The mean proportion of juvenile clams that initiated (MPI) or completed (MPC) aburial decreased with increasing shell length. Erosion from the sediment was more important in large juveniles suggesting that largejuveniles may have more difficulty successfully relocating once transported. The MPI increased with increasing flow speed inexperimental runs held at speed b24 cm s−1. This was observed in all sediment types. Most individuals were unable to burrow at24 cm s−1 because they got eroded. The MPC also increased with increasing flow speed in mud, sandy-mud and sand. The MPC'sresponse to flow was more complex in gravel because of a shell length×flow speed interaction effect. Our observations suggestthat water movement may induce the burrowing behaviour of recently eroded juvenile clams. Results are discussed in an ecologicaland aquacultural context.© 2006 Elsevier B.V. All rights reserved.

Keywords: Burrowing behaviour; Mya arenaria; Post-larval dispersal; Recruitment; Softshell clams; Tidal currents

1. Introduction

Soft-bottom environments are home to complex anddynamic post-settlement events that constantly reshapethe structure of benthic communities. Active and passivedispersal of post-settlers and young juveniles, forinstance, gradually transport individuals on flats from apoint (initial settlement) to another (final establishment)over an indefinite number of tidal cycles (e.g., Bagger-

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E-mail address: [email protected] (G. Miron).

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man, 1953; Sigurdsson et al., 1976; Lane et al., 1985;Beukema and de Vlas, 1989; Emerson, 1991; Emersonand Grant, 1991; Martel and Chia, 1991; Armonies andHellwig-Armonies, 1992; Armonies, 1994; Commitoet al., 1995a; Coffen-Smout and Rees, 1999; Norkkoet al., 2001; Hunt and Mullineaux, 2002).

Bivalves are capable of moving actively on flats (e.g.,Sigurdsson et al., 1976; Lane et al., 1985; Beukema and deVlas, 1989; Armonies and Hellwig-Armonies, 1992;Armonies, 1996; Wang and Xu, 1997; Mullineaux et al.,1999; Lundquist et al., 2004). Some species extend theirbyssus to regulate their buoyancy and thus take advantageof favorable boundary shear velocities that will transport

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104 P. St-Onge et al. / Journal of Experimental Marine Biology and Ecology 340 (2007) 103–111

them on the flat (Sigurdsson et al., 1976; Lane et al., 1985).This behaviour is observed in the first life stages and seemsless frequent as individuals get older (Armonies andHellwig-Armonies, 1992). Passive transport also regulatespopulations of intertidal invertebrates (e.g., Emerson,1991; Emerson and Grant, 1991; Commito et al., 1995a,b; Turner et al., 1997; Hunt, 2004b, 2005). Matthiessen(1960) showed, for instance, that the location of juvenileMya arenaria constantly varied in the intertidal zone dueto bedload transport. This was confirmed by various fieldand flume studies (e.g., Emerson and Grant, 1991;Roegner et al., 1995; Hunt, 2004b).

Individuals can often take advantage of an erosionevent. They can reach a more suitable habitat or escapedense patches of conspecifics and/or predators (Armoniesand Hellwig-Armonies, 1992; Mullineaux et al., 1999;Commito and Tita, 2002; Hunt, 2004a). However, erodedindividuals are not yet safely established and becomemore vulnerable. They can, for instance, succumb todessication (Emerson et al., 1990) and predation (e.g.,Glude, 1954; Pihl and Rosenberg, 1984; Kube, 1996; vander Veer et al., 1998; Mackenzie and McLaughlin, 2000;Huxham and Richards, 2003; Hunt, 2004a) or getentangled inmacroalgae (e.g., Olafsson, 1988;Mackenzieand McLaughlin, 2000; Auffrey et al., 2004).

Erosion and transport of M. arenaria post-larvae isfrequent (Roegner et al., 1995; Norkko et al., 2001; Hunt,2004a,b, 2005). Post-larvae are not able to burythemselves deeply enough to avoid erosion (Medcof,1950; Zwarts andWanink, 1989;Mullineaux et al., 1999).A high percentage of early settlers are thus redistributedon the flat with the advection of sediments (Armonies andHellwig-Armonies, 1992; Roegner et al., 1995). Largejuveniles (5–20mm)may also get eroded in high numbersduring tidal events (Matthiessen, 1960; Emerson andGrant, 1991; Norkko et al., 2001). We, however, havelittle knowledge of how juveniles recuperate after beingeroded and transported and how many of them willrelocate themselves successfully on the flat and survive.

The burrowing behaviour of bivalves is affected bythe size of the individual as well as the type of sediment(Medcof, 1950; Pfitzenmeyer and Droebeck, 1967;Eldon and Kristoffersson, 1978; Robert and Smith,1980; Emerson et al., 1990; Coffen-Smout and Rees,1999; Pariseau et al., 2005). Studies carried out inlaboratory also showed that hydrodynamic disturbancesin the immediate proximity of bivalves could lead themto bury faster and deeper (Ansell and Trevallion, 1969).However, the extent to which different intensities oftidal currents can influence the burrowing behaviour oferoded juveniles is still poorly understood. To the best ofour knowledge, the plausible interaction between tidal

current and shell length on the burrowing behaviour ofrecently eroded bivalves in variable sediment types hasnot been investigated.

The aim of this study was to document how juvenilesof the softshell clam M. arenaria are able to recoverfrom erosion and transportation. The study was carriedout in a flume to evaluate the effects of sediment type,shell length and flow speed on the burrowing behaviourof individuals ranging from 5 to 20 mm during asimulated slack water period followed by a tidal current.We predict that juvenile clams will initiate or complete aburial less frequently with increasing flow speed andshell length. We also predict that their burrowingbehaviour will vary in relation to the sediment type.

2. Methods

2.1. Flume characteristics

The study was carried out in a custom madePlexiglas© racetrack flume. It had a maximum capacityof 250 L and a width of 20 cm at all points. A series ofnine motorized 3 mm wide parallel Plexiglas© wheelsgenerated flows by friction and could reach speeds up to40 cm s−1. Wheels were controlled by a 12 V DC shuntmotor and a 4A gear box with a pulse width modulationspeed control. These wheels were located in one of two75 cm long straight channels connected by two semi-circular sections, with an inner core diameter of 44 cmand an outer core diameter of 89 cm. The second straightchannel was occupied by a built-in working section. Thissection allowed the insertion and removal of a sedimentbox (40×20×10 cm). Once the box was inserted, thesediment surface was flush with the flume bottom. Aseries of parallel walls were installed in the curvedsection upstream of the working section to minimize aside-wall effect observed during preliminary trials due tothe (width of flume)/(depth of boundary layer thickness)ratio (Jumars and Nowell, 1984; Muschenheim et al.,1986). To minimize an existing inner-wall effect, afraction of the sediment box (19×12×10 cm) wasdelimited towards the outer wall of the flume. Thissection was filled with a given sediment type on whichthe burrowing behaviour of juvenile M. arenaria wasstudied. Salinity during the flume experiments was21 ppt while seawater temperature was 17 °C.

2.2. Collection of softshell clams (M. arenaria) andsediments

All juvenile softshell clams were collected fromCôte-à-Fabien (Kouchibouguac National Park, NB)

Table 1Grain-size composition (%) of each sediment type used in the study

Particle size Sediment type

(mm) Mud Sandy-mud Sand Gravel

b0.063 15.08 0.82 0.09 0.050.063–0.125 9.55 6.50 0.15 0.070.125–0.250 14.48 38.82 1.28 0.440.250–0.500 17.20 43.18 68.45 7.950.500–1.000 16.01 8.51 29.70 16.111.000–2.000 18.68 1.23 0.33 20.732.000–4.000 8.76 0.94 0.00 28.62N4.000 0.23 0.00 0.00 26.04

Fig. 1. Speed velocity profiles (mean±SE, n=60) taken at thirteendifferent heights above the sediment for four different free-streamvelocities (u⁎ represents the mean shear velocities).

105P. St-Onge et al. / Journal of Experimental Marine Biology and Ecology 340 (2007) 103–111

over a period of 6 days. Once collected, individuals weresorted into three different size classes (5–10, 10–15 and15–20 mm) and transported to the laboratory where theywere maintained in 4 °C aerated natural seawater. Theseawater was renewed each week. This provided freshnatural phytoplankton to the clams. Clams were kept inthese conditions for a period of 4 weeks during which allthe experimental runs were carried out. The amount offood provided with the renewed seawater was sufficientsince individuals were kept at 4 °C which slowed downtheir metabolism and their feeding rates. This holdingmethod did not seem to impede the clams' generalhealth or behaviour. Mortality was very low during thisperiod. Individuals were conditioned in 17 °C seawaterfor a period of 24 h before inclusion in the flumeexperiments. Clams were remeasured before theirintroduction in the flume. Dead clams of the same sizeclasses were also used to investigate active and passiveprocesses of erosion and transportation. Individualswere killed and preserved in a formalin-based solution(4%) prior to their use in the flume.

Softshell clams are found in various types ofsediments (Robert and Smith, 1980; LeBlanc andMiron, 2005, 2006). Four representative types wereused in the present study: mud, sandy-mud, sand andgravel. Sediments were collected from three differentsites in south New Brunswick, Canada: mud and gravelfrom Anthony's Cove (St. John); sandy-mud froml'Aboiteau (Cap Pelé); sand from Sandy Beach (CapPelé). Each type of sediment was sieved to eliminate thepresence of the macrofauna. Sediments were kept at 4 °Cin the laboratory, stirred biweekly and closely monitoredto prevent any build-up of hydrogen sulfide. Sedimentsamples from the refrigerated stock were homogenizedin texture prior to the flume experiments. Grain-sizecomposition of the sediments is presented in Table 1. Themud samples were heterogeneous and mostly composedof silt particles (b63 μm). Particles between 0.125 and

0.5 mm characterized the sandy-mud samples, whileparticles between 0.25 and 1 mm characterized the sandsamples. The majority of particles from the gravelsamples were larger than 0.5 mm.

2.3. Current speed velocities

Five free-stream velocities (Uf) were selected toinvestigate the burrowing behaviour of clams: 0, 3, 5, 10and 24 cm s−1. The 0 cm s−1 was used as a controlspeed. The 3, 5 and 10 cm s−1 are typical tidal flowsobserved along the coast of the Northumberland Strait,NB. The 24 cm s−1 is representative of the Bay ofFundy, NB. These free-stream velocities were measuredabove the working section of the flume at a depth of2.5 cm below the surface of a 21 cm water column.Current velocities were measured with a Marsh–McBirney current meter at 13 different heights abovethe sediment to create vertical velocity profiles for eachcurrent speed (Fig. 1). Only the six depths that wereclosest to the bottom of the flume (2 to 7 cm) were usedto determine the shear velocity estimates (u⁎) (Fig. 1).The latter were estimated using Jumars and Nowell's(1984) “law of the wall”.

2.4. Experimental setup

Five clams of each size class were deposited on oneof four sediment types in the working section of thesediment box. A single type of sediment was used foreach experimental run. The clams were randomly placedin rows (left, middle or right) of similar size, parallel to

Fig. 2. Mean proportion of recently transported juvenile clams (M. arenaria) that initiated (MPI) and completed (MPC) a burial during the simulationof a 30 min slack tide and subsequent 60 min tidal current in relation to flow speed (all shell sizes confounded, n=15) (left) and shell length (all flowspeeds confounded, n=25) (right). Proportions are presented with a single error bar provided by the standard model error (SEM) for (A) mud, (B)sandy-mud, (C) sand and (D) gravel. Bars having dissimilar letters above them (lower cases: initiation; upper cases: completion) differ significantlyfrom each other (Bonferroni multiple comparison test).


the flow direction. This design was used to minimizepotential small-scale eddies or any other type ofboundary layer turbulences that a large individualmight have produced around a smaller clam positionedin the same row. Once all the clams were deposited ontothe sediment, the water was kept motionless for 30 minto simulate the slack water period that occurs betweenflood and ebb tides. One of five previously chosen free-stream velocities was initiated after the simulated slackwater period and kept constant for 60 min. All clams thatinitiated (individual upright with its foot anchored andstable in the sediment) and/or completed a burial (shell

entirely under the sediment surface) during the total90 min experimental run were noted. Any occurrence oferosion from the working section of the sediment boxwas also recorded.

A total of 20 treatment combinations were produced(5 current speeds×4 sediment types) with all sizeclasses being used within any single run. Eachcombination was replicated four times for a total of 80runs, which were randomly carried out. The sameexperimental procedure was used for the dead clamtrials but only one flume run was carried out for each ofthe 20 possible treatments (n=1).

Table 2Summary of a two-way ANOVA for a split-plot design carried out onthe mean proportion of recently transported juvenile clams (M.arenaria) that initiated a burial (MPI) during the simulation of a 30 minslack tide and subsequent 60 min tidal current in mud, sandy-mud,sand and gravel

Sediment Source of variation SS df MS F P

Mud Flow (F) 0.72 4 0.18 3.26 0.041Main plot error 0.83 15 0.06 – –Size (S) 2.53 2 1.26 44.81 b0.001S×F 0.38 8 0.05 1.70 0.139Subplot error 0.85 30 0.03 – –

Total 5.32 59 – – –Sandy-mud Flow (F) 0.68 4 0.17 1.56 0.236

Main plot error 1.64 15 0.11 – –Size (S) 1.59 2 0.79 32.48 b0.001S×F 0.13 8 0.02 0.67 0.709Subplot error 0.73 30 0.02 – –

Total 4.77 59 – – –Sand Flow (F) 1.38 4 0.35 4.47 0.014


Total 6.30 59 – – –Gravel Flow (F) 0.72 4 0.18 3.28 0.041


Total 5.65 59 – – –


2.5. Statistical analyses

The main effects of flow speed and shell length aswell as their interaction were examined using twovariables collected during the 90 min slack and tidalcurrent simulation: (1) mean proportion of juvenileclams that initiated a burial (MPI) and; (2) mean pro-portion of juvenile clams that completed a burial (MPC).

To account for the split-plot design caused by the useof a single flow speed per experimental run, the responsevariables were analyzed with the GLM procedure (SASInstitute, 1982). The experimental run was included in themodel as a random factor. This analysis was carried outseparately for all four sediment types, a categorical factor,because (1) there was evidence that sediment type wasacting as a confounding factor, and (2) only one sedimenttypewas present in each experimental run. Residuals wereexamined to ensure that postulates for normality andhomogeneity of variances were met. Bonferroni multiplecomparison tests were used to detect significant differ-ences between different classes of shell length. Significantdifferences due to flow speed were identified using thestandard errors of the general statistical models (SEM).Because all data points for initiation or completion that areincluded in a single graph belonged to a single plannedexperiment, SEM values were presented using a singleerror bar (Wehner and Shaw, 1994). Simple descriptivestatistics were used to discriminate differences betweenthe burying rates in accordance to the sediment types andthe evidence from dead and live clam data set. For allanalyses, the level of significance used was α=0.05.

3. Results

3.1. Mean proportion of juvenile clams that initiated aburial (MPI)

Experiments yielded similar results in relation to theMPI regardless of sediment types (Fig. 2). No significantshell length×flow speed interaction effect was observedon MPI whatever the sediment type (Table 2). Asignificant flow speed effect was detected on the MPIfor mud, sand and gravel (Table 2). Individuals started tobury more frequently at 10 cm s−1 than at velocities of 0and 24 cm s−1 (Fig. 2A,C,D). The numbers of clams thatinitiated a burrow increased from 3 to 5 cm s−1 towards apeak value observed at 10 cm s−1 (Fig. 2A,C,D). Over-all, flow speed did not have a significant effect on theMPI in sandy-mud because of a considerable variabilityin the behaviour of clams within distinct experimentalruns. However, the SEM indicates that clams tended toburymore frequently at speeds of 5 and 10 cm s−1 than at

speeds of 3 cm s−1 and 24 cm s−1 (Fig. 2B). Shell lengthinfluenced the MPI in all four sediment types (Table 2).Clams had the tendency to initiate burrowing lessfrequently as their shell length increased (Fig. 2).Bonferroni comparison tests indicated that clamsbelonging to the small and medium size classes startedto bury at similar rates in mud, sandy-mud and gravel(Fig. 2A,B,D). However, small juveniles initiatedburrowing more frequently than medium-size juvenileswhen put onto a sandy substrate (Fig. 2C). As forjuveniles from the large size class, Bonferroni compar-isons indicated that they exhibited a lower MPI (allP≤0.01) than the other two size classes in all sedimenttypes (Fig. 2).

3.2. Mean proportion of juvenile clams that completeda burrow (MPC)

Although theMPCwas approximately 10% lower thanthe MPI in each treatment, both variables showed similartendencies in mud, sandy-mud and gravel (Fig. 2A,B,C).The MPC rates were lower in gravel (Fig. 3) than in theother three sediment types (Fig. 2A,B,C). There wasevidence for a flow speed×shell length interaction effect

Table 3Summary of a two-way ANOVA for a split-plot design carried out onthe mean proportion of recently transported juvenile clams (M.arenaria) that completed a burial (MPC) during the simulation of a30 min slack tide and subsequent 60 min tidal current in mud, sandy-mud, sand and gravel

Sediment Source of variation SS df MS F P

Mud Flow (F) 0.36 4 0.09 1.70 0.202Main plot error 0.79 15 0.05 – –Size (S) 3.22 2 1.61 36.19 b0.001S×F 0.30 8 0.04 0.85 0.567Subplot error 1.33 30 0.04 – –

Total 6.00 59 – – –Sandy-mud Flow (F) 0.72 4 0.18 1.72 0.197


Total 5.24 59 – – –Sand Flow (F) 1.52 4 0.38 4.66 0.012


Total 6.85 59 – – –Gravel Flow (F) 0.66 4 0.17 4.61 0.013


Total 7.25 59 – – –


on the MPC in gravel but not in mud, sandy-mud or sand(Table 3). The MPC reached a peak at 24 cm s−1 in thesmall and large size classes and at 10 cm s−1 for themedium size class (Fig. 3). Flow speed significantlyaffected the MPC in sand but due to a considerable va-riability in the response of clams within distinct ex-perimental runs, no overall effect of flow speed wasdetected in mud or sandy-mud (Table 3). Nevertheless,based on the SEM, clams initiated burrowing more fre-quently at 10 cm s−1 in both sandy-mud and sand, and at5 cm s−1 in mud (Fig. 2A,B,C). Clams also exhibited thelowest rates of completion at 24 cm s−1 inmud and sandy-mud (Fig. 2A,B). As observed with the MPI, shell lengthinfluenced the MPC in all sediment types (Table 3).Bonferroni comparison indicated that individuals fromthe large size class completed their burrows less fre-quently than individuals from the small and medium sizeclasses in mud, sandy-mud and sand (Fig. 2A,B,C).Small-size juveniles exhibited greaterMPC thanmedium-size juveniles in sand but had similar MPC to medium-size juveniles in mud and sandy-mud.

3.3. Live versus dead juvenile clams

The erosion of clams was almost exclusivelyobserved at the highest flow speed. About 48% of liveclams (n=240) and 57% of dead clams (n=60) wereeroded at 24 cm s−1. The sand trials showed the highesterosion rates for both live and dead clams (n=60). In thelive clam trials, the next highest rates were found, indecreasing order, in mud, sandy-mud and gravel (alln=60). Similarly, the remaining highest erosion rates

Fig. 3. Interaction effect between flow speed and shell length on themean proportion (n=5) of recently transported juvenile clams (M.arenaria) that completed a burial (MPC) in gravel during thesimulation of a 30 min slack tide and subsequent 60 min tidal current.Proportions are presented with a single error bar (⋆) provided by thestandard model error (SEM).

in dead clam trials were observed in sandy-mud, mudand gravel (all n=15). No erosion occurred at the threelowest flow speeds (0, 3 and 5 cm s−1) regardless ofsediment type for any of the live or dead clams. Of allthe live and dead clam trials carried out at 10 cm s−1,only one large live juvenile was eroded during the mudtrials. Large live (all n=80) and dead clams (all n=20)were eroded in higher numbers than the small andmedium-size juveniles, regardless of sediment types.

4. Discussion

4.1. Shell length

Our results showed that shell length has a strongeffect on the burrowing behaviour of juveniles of thesoftshell clam M. arenaria. The proportion of juvenileclams that initiated (MPI) or completed (MPC) a burrowdecreased as shell length increased. Shell length,depending on the sediment type, explained 27% to48% of the variation observed in the MPI and MPC.Other studies have reached similar conclusions (e.g.,Pfitzenmeyer and Droebeck, 1967; Emerson et al., 1990;Coffen-Smout and Rees, 1999; Pariseau et al., 2005).


Our results also showed that the effect of shell length onthe burrowing behaviour of juvenile clams was alwayspresent regardless of current speed, hence the generalabsence of an interaction between both factors. Smalljuveniles are usually limited to the top layer of thesediments which increases their chances of being erodeddue to bedload transport. Small juveniles will howeverinitiate and complete a burial more frequently than largerones and consequently avoid transportation.

Moreover, the present study showed that largejuveniles may be eroded under strong tidal currentswhen they are not already buried. In general, the ma-jority of clams that got eroded at 24 cm s−1 were large-size juveniles. Our results demonstrate that largejuveniles have a harder time than small ones to burrowafter being eroded from the sediments, thus increasingtheir probability of being relocated again. This may bedue to the relative size of the foot which decreases as theindividual grows (Pfitzenmeyer and Droebeck, 1967;Emerson et al., 1990). Therefore, the foot becomes lesspowerful, making it more difficult to achieve a verticaltraction (Pfitzenmeyer and Droebeck, 1967; Emersonet al., 1990). Friction against the sediment bed duringburial should also contribute to a decrease in burrowingperformance as the animal increases in size. However,the greater vulnerability to erosion of large clams, whichis associated with the total surface of the individualfacing the flow, may also contribute to the greatertransport of large clams at a flow speed of 24 cm s−1.Large clams also have longer distances to burrow inorder to completely bury themselves in the sediment.This could explain why large clams had low burialfrequencies and high erosion rates. Such effects wouldresult in a size threshold beyond which successfulrelocation decreases significantly.

4.2. Flow speed

Our results showed that flow speed influenced theburrowing activities of juvenile clams. Behavioural res-ponseswere observed throughout the experiments inmosttypes of sediment. For instance, juvenile clams initiatedburrowing more frequently in currents ≤10 cm s−1. Thisincrease in burrowing frequency with increasing speedmay underline the need to bury rapidly in the sediment toescape erosion. Most of the clams that did not initiate aburrowing activity during the simulated slack tide didinitiate burrowing in the first minutes once water flowwas initiated. This was particularly true for trials at10 cm s−1.

Foot activation is known to be the first step of theburrowing sequence (Trueman, 1967, 1968; Trueman

and Ansell, 1969). Our observations showed that clamsbecame more active with increasing flow velocity, withburrowing starting in the first minutes following watermovement. This suggests that the intensity of the currentmight activate the burrowing process. This is similarto observations made by Ansell and Trevallion (1969)in laboratory. These authors showed that the bivalveMactra olorina could bury more rapidly when smallcurrents of water were produced around their siphontips. The present study tends to confirm that flowspeed may act as a physical cue to initiate burrowing.

Our results also showed that the burrowing behaviourof juvenile clams was highly variable. Variations wereobserved in relation to the experimental runs when usingthe same hydrosedimentary environment. The burrowingrates in sandy-mud, for instance, were high in certain trialswhile they were low in others. Preliminary flume analysesshowed that flows over the working section of thesediment box were similar in each replicate for a givenhydrosedimentary environment. We, however, have tokeep in mind that each juvenile clam was depositedpassively on the sediment surface and that their initialposition may have modified their burrowing response.This is probably similar to what an individual encountersin the field after being eroded and transported as theymaynot always fall onto the sediment floor in a favorableposition for burrowing.

4.3. Sediments

Our results showed that theMPI andMPC respectivelydisplayed a similar response to flow regardless of thesediment type. Overall, these results are consistent withthe fact that softshell clams may be observed in varioussediment types in the wild (Trueman and Ansell, 1969;Robert and Smith, 1980; LeBlanc and Miron, 2005,2006). Our results, however, showed that the interactionbetween shell length and flow speed affected the MPC ingravel. Part of this interaction can be attributed to differentburrowing behaviours in relation to shell length. In ourtrials, juvenile clams were often smaller than the meanparticle size of gravel which ranged between 0.5 and15.0 mm. The mean particle size was similar to gravelpatches where clams may be observed in the Bay ofFundy (Robert and Smith, 1980; LeBlanc and Miron,2005, 2006).Gravel particles can affect the hydrodynamicconditions a few centimeters above the sediment bed(Eckman, 1990). Pockets of reduced turbulence wereoften observed in the flume behind larger gravel particles.Apparently, smaller clams took advantage of these refugesat 24 cm s−1 to complete their burrowing. This waspossible because the reduced turbulence made them less


vulnerable to erosion, which was less likely to occur in theother sediment trials. Conversely, most gravel particleswere not large enough to protect medium-size individuals.The MPC for mid-size juveniles followed the sametendency observed in the other sediment trials anddropped considerably at 24 cm s−1. Increasing flowspeed did not substantially affect the MPC in the case oflarge juveniles.

4.4. Implications for benthic ecology and aquaculture

Benthic ecologists that study the recruitment ofmarine invertebrates in soft-bottom environments haveto deal with a series of challenges imposed by bedloadtransport and burrowing depth. Conservation biologistsmust therefore have sufficient fundamental knowledgerelated to these factors in order to create valid and robustpredictive models of recruitment (Carpenter et al., 1995;Constable, 1999). Our results underline the importanceof shell length and flow speed in the burrowing successof eroded and transported juveniles clams, with clam sizeacting as a natural marker of burrowing performances,and water movement as physical burrowing cue.

Aquaculture stakeholders may benefit from our studyas it may help improve the retention rates of hatcheryclams seeded in the wild. For instance, our resultssuggest that seeding should be carried under low tidalcurrents (≤10 cm s−1) and that seeds should be between5 and 10 mm. These recommendations are likely toaffect positively the burrowing success of seeded clams.However, one should be careful in seeding in sandysediments because the clams might be more vulnerableto bedload transport than if they were seeded on mud.Rough settlement mats offering refuges from turbulencecould also be placed over sandy sediments in order tohelp juvenile clams to complete burrowing.

Field studies on the burrowing success of clams du-ring seeding activities are needed to validate these sug-gestions. These studies should integrate current speed,sediment type and body size, as well as the importance ofbedload transport on the relocating success of smalljuvenile M. arenaria.

Acknowledgements

Wewish to thank P. St-Onge for the construction of theracetrack flume. We also wish to thank É. Tremblay andthe staff of Kouchibouguac National Park for their helpand dedication. A. Chiasson provided the current meter.Helpwith the collection of juvenile clams byA.Mallet, R.Sonier, S. LeBlanc, B. Pavey, A. Léger, M. Verret, K.Dufour, F. LeBlanc and K. Burke was much appreciated.

P. Archambault and H. Hunt reviewed earlier drafts of thismanuscript. Comments from two anonymous reviewershelp increased the clarity of the text. Funding for thisresearch has been provided by NSERC, DFO and CFIgrants to Gilles Miron. The Université de Monctonprovided a scholarship to Philippe St.-Onge. [SS]

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Further reading

Sutherland, W.J., 1998. The importance of behavioural studies inconservation biology. Anim. Behav. 56, 806–809.

burrowing behaviour of the softshell clam (mya arenaria) following erosion and transport

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