Stereotypy and the effects of temperature on some spatio-temporal subcomponents of the ‘courtship wave’ in the fiddler crabs Uca minax (Le Conte) and Uca pugnax (Smith) (Brachyura,Ocypodidae)

The effects of air temperature on some spatial and temporal subcomponents of the male wave display were investigated under laboratory conditions in the fiddler crabs, Uca minax and Uca pugnax. Also investigated were the stereotypies of these subcomponents. In general, wave duration, ascending and descending wave times decreased significantly with increasing air temperatures between 13 and 26 C. These decreases were generally not significant at air temperatures greater than 26 to 30 C. Wave frequency and the number of cheliped jerks (U. minax) increased with temperature, whereas the interwave time showed no systematic change with temperature. Uca pugnax had waves with discrete cheliped jerks at temperatures less than 20 C, whereas the wave appeared relatively smooth at higher temperatures. Temperature effects on waving varied between species and among behavioural contexts. Stereotypies of the wave subcomponents, as measured by the coefficient of variation, underwent no systematic change with temperature. However, stereotypies were different within and between species. Behavioural context also had an effect on subcomponent stereotypy. Comparisons of stereotypies yielded predictions concerning communication functions of waving subcomponents.     

Burrowing,root mat density,and the distribution of fiddler crabs in the eastern United States

This paper discusses the distribution of two species of fiddler crabs across an estuary. The possibility that sediment size, physical factors (or micro-climate), or food limitation could lead to the observed distributions is discussed, and evidence is presented to show that these factors probably play a minor rôle in controlling the distribution of Uca pugnax (Smith) and U. minax (Le Conte) across an estuary. The results show that the best correlate of the dominant species of Uca in a particular habitat is the density of the root mat of that area. A series of experiments showed that variations in the abundance of large U. pugnax at two tide levels in a marsh very closely parallel the ability of U. pugnax to burrow in substrata taken from those same two areas of the marsh. These two areas have significant differences in density of the root mat.     

Osmoregulation by six species of fiddler crabs (Uca) from the Mississippi delta area in the northern Gulf of Mexico

Six species of fiddler crabs (Ocypodidae: Uca) were collected for osmoregulation studies from 25 locations near the delta of the Mississippi River in the northern Gulf of Mexico. Three of the species are classified as members of the Celuca subgenus, Uca spinicarpa, Uca panacea and Uca pugilator, while the remaining three are in the Minuca subgenus, Uca minax, Uca longisignalis and Uca rapax. In the field, U. minax, U. spinicarpa and, occasionally, U. longisignalis are found in freshwater habitats (FW; 0-299 mosM). Two Minuca species, U. longisignalis and U. rapax, are typically collected in brackish water habitats (BW; 300-629 mosM). On the other hand, U. panacea and U. pugilator are most abundant in eurysaline habitats (EH; >630 mosM). In the laboratory, populations of each species were challenged with media ranging from 30 to 3450 mosM (1-110‰). The FW species, U. spinicarpa and U. minax, did not tolerate osmotic concentrations >2100 mosM. The EH species, U. panacea and U. pugilator, however, tolerate concentrations >2800 mosM. The BW species, U. longisignalis and U. rapax, succumb to osmolalities between 2100 and 2800 mosM. Each species keeps its hemolymph concentration fairly constant in 30-1400 mosM solutions. The [ISO], isosmotic medium concentration (in mosM), is calculated for each taxon: U. minax, 659; U. spinicarpa, 682; U. longisignalis, 693; U. rapax, 769; U. pugilator, 816; and U. panacea, 822. In media with >1600 mosM, each species expresses different osmoregulating capabilities. The FW species, U. spinicarpa and U. minax, cannot control hemolymph osmolality above 1500 mosM while the BW-EH species, U. panacea, U. pugilator and U. rapax, regulate hemolymph values in media up to 2300 mosM. Within the FW/BW species U. longisignalis, the ability to osmoregulate corresponds with site of collection. Specimens from FW populations do not regulate as well as those from BW if challenged with hypertonic media. If adapted to a 1800 mosM in the laboratory, survivorship for U. longisignalis shifts to the right and the [ISO] increases to 832 mosM. This suggests that this species adapts to acute hypertonic conditions by tolerating elevated internal osmolality. Generally, these observations extend our knowledge about the physiological capabilities of fiddler crabs from different salinity populations across the northern Gulf of Mexico.     

A calibration method to generate seasonal hatching profiles for the fiddler crabs Uca pugnax (Smith, 1870) and Uca minax (LeConte, 1855) (Decapoda : Brachyura)

A statistical method for generating seasonal egg hatching profiles is applied to the brachyuran crabs Uca pugnax (Smith) and U. minax (LeConte) in New Jersey. Calibration experiments quantified the time course of egg development, using serial egg sampling at 1- to 2- day intervals from marked ovigerous females maintained in cages in the field. Egg stage was ranked from 1 to 10, based on morphological changes during development. Equations for predicting the number of days remaining until hatching from egg developmental stage were obtained from the calibration experiments, using stepwise polynomial regression. To cover the reproductive season, three consecutive calibration experiments using 15 or more females were run for U. pugnax; two for U. minax. Significant seasonal differences in the time course of egg development were detected. Weekly collections of females for each species were made; the date of larval release for each ovigerous female was predicted from the proximate calibration equation, yielding weekly hatching profiles. Weekly hatching profiles were summed to obtain seasonal hatching profiles. The average number of broods produced per female over the reproductive season was 1.9 for U. pugnax and 1.6 for U. minax. Hatching peaks for both species were associated with spring tides. The merits of this and other methods used to estimate daily variation in egg hatching of crabs are discussed.     

Experimental infections of Orchitophrya stellarum (Scuticociliata) in American blue crabs (Callinectes sapidus) and fiddler crabs (Uca minax)

Outbreaks of an unidentified ciliate have occurred on several occasions in blue crabs from Chesapeake Bay held during winter months in flow-through systems. The parasite was initially thought to be Mesanophrys chesapeakensis, but molecular analysis identified it as Orchitophyra stellarum, a facultative parasite of sea stars (Asteroidea). We investigated the host-parasite association of O. stellarum in the blue crab host. Crabs were inoculated with the ciliate, or they were held in bath exposures after experimentally induced autotomy of limbs in order to determine potential mechanisms for infection. Crabs inoculated with the ciliate, or exposed to it after experimental autotomy, rapidly developed fatal infections. Crabs that were not experimentally injured, but were exposed to the ciliate, rarely developed infections; thus, indicating that the parasite requires a wound or break in the cuticle as a portal of entry. For comparative purposes, fiddler crabs, Uca minax, were inoculated with the ciliate in a dose-titration experiment. Low doses of the ciliate (10 per crab) were sometimes able to establish infections, but high intensity infections developed quickly at doses over 500 ciliates per crab. Chemotaxis studies were initiated to determine if the ciliate preferentially selected blue crab serum (BCS) over other nutrient sources. Cultures grown on medium with BCS or fetal bovine serum showed some conditioning in their selection for different media, but the outcome in choice experiments indicated that the ciliate was attracted to BCS and not seawater. Our findings indicate that O. stellarum is a facultative parasite of blue crabs. It can cause infections in exposed crabs at 10–15 °C, but it requires a portal of entry for successful host invasion, and it may find injured hosts using chemotaxis.     

Feeding ecology and trophic impact of the hydroid Obelia dichotoma in the Kongsfjorden (Spitsbergen, Arctic)

Obelia dichotoma is a thecate hydroid with a worldwide distribution, occurring mainly on shallow water hard substrates. Since the trophic ecology of hydroids in polar waters is badly understood, the aim of the present work was to study qualitatively and quantitatively the diet of these organisms in an Arctic environment and to determine their trophic significance. For this purpose, the density of the hydroid population was documented, and simultaneously, zooplankton was sampled in two different years (1997 and 1998). Prey capture rates were estimated by analysing the gastrovascular content of the polyps in a diurnal cycle. Additionally, the digestion time of O. dichotoma was measured by laboratory feeding experiments using diatoms as food items. The analyses of the gastrovascular cavities of the polyps sampled during the diurnal cycles showed that O. dichotoma fed mainly on faecal pellets, organic matter and microalgae. Zooplankton prey was also observed, but gastrovascular contents and zooplankton abundance did not show any correlation in both years. The consumption rates of the hydroid populations differed between the 2 years. It was almost double (8.9 mg Carbon m^?2) in 1998 compared to 1997 (5.5 mg Carbon m^?2). The significance of the environmental variability in the feeding ecology and population dynamics of hydroids under Arctic conditions is discussed.     

Limited gene flow in Uca minax (LeConte 1855) along a tidally influenced river system

For crab larvae, swimming behaviors coupled with the movement of tides suggests that larvae can normally move upstream within estuaries by avoiding ebb tides and actively swimming during flood tides (i.e., flood-tide transport [FTT]). Recently, a 1-D transport model incorporating larval behavior predicted that opposing forces of river discharge and tidal amplitude in the Pee Dee River/Winyah Bay system of South Carolina, USA, could limit dispersal within a single estuary for downstream transport as well as become a dispersal barrier to recruitment of late stage larvae to the freshwater adult habitats of Uca minax (LeConte 1855). We sequenced 394-bp of the mitochondrial cytochrome apoenzyme b for 226 adult U. minax, from four locales along a 49-km stretch of the Pee Dee River/Winyah Bay estuary, above and below the boundary of salt intrusion. Results of an analysis of molecular variance (AMOVA) and an exact test of population differentiation showed a small, but statistically significant (α=0.05) population subdivision among adults of the 4 subpopulations, as well as all subpopulations being significantly differentiated (α=0.05). This pattern fitted with model predictions, which implies that larval transport within the tidally influenced river system is limited.     

Fleas of the Moiynkum Desert and their epizootic significance

The species composition of fleas in the Moiynkum Desert (Chu-Talas interfluve) and indices of their domination on different hosts was established on the basis of long-term observations (1973–2000). The most significant and prolonged changes in the population density of mass flea species (Xenopsylla gerbilli minax and Coptopsylla lamellifer, parasites of the great gerbil) occur in association with changes of the population density of rodents. Changes in the population density of rodents are usually observed year after similar changes in the population density of hosts. The leading role in the transmission of the plague vector belongs to the flea X. gerbilli minax.     

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.     

Burrowing behavior and ecology of the crab-eating Indian snake eel Pisoodonophis boro

Synopsis The snake eel Pisoodonophis boro burrows, causing leaks in the embankments and damaging the paddy fields and salt pans near estuaries. Field observations and laboratory experiments were made to study this behavior. P. boro was burrowing to eat the fiddler crab Uca annulipes in the mud flats. The eel showed a patchy distribution within the Uca zone. Salinity and the physical nature of the deposits controlled the distribution of the eel. Eel population density was low when the estuary was completely filled with neritic waters during the summer and fresh water during the monsoon period. The region of greatest abundance contained a good mixture of sand, silt and clay. Eels were not found where medium and fine sand formed the bulk of the substratum. The laboratory experiments showed that P. boro preferred loam soil although it could invariably burrow into hard substratum like sand for protection. The eel adapted itself to the experimental substrates ranging from sand to fine clay. However, their natural distribution was determined by Uca distribution. As U. annulipes is not found either in salt pans or in paddy fields P. boro rarely occurs in these habitats.     

Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contestsover breeding burrows and is waved to attract females. We recentlydiscovered that males of the red-jointed fiddler crab, Uca minax,also use the claw to kill smaller-sized fiddler crabs, U. pugnaxand U. pugilator, with which they co-occur in Atlantic coastsalt marshes. Large U. minax males use walking legs or the enlargedclaw to capture prey feeding on moist sand flats. On sand flats,small U. minax males and females are much less common than largemales, suggesting that large males move onto sand flats to seekprey. Males of prey species use the major claw against attackingpredators and, consequently, are more likely than females toescape. In laboratory experiments, large U. minax males weremore likely to attack and kill small-clawed males and femalesthan large-clawed males, consistent with a preference for morevulnerable, less threatening prey. The size of the major clawis a positive allometric function of body size. The allometricfunction varies little among species. Also, the mechanical advantageand indices of closing speed and closing force of the majorclaw, when corrected for body size, are not consistently greaterin U. minax relative to prey species. Thus, predation by U.minax males may reflect the opportunity afforded by larger bodysize and positive allometric growth, which result in a majorclaw that is more massive than the prey it is directed against.     

The importance of Planolites in the Cambrian substrate revolution

Early Cambrian subtidal shelf substrates were characterized by low water content and steep chemical gradients, conditions likely facilitated by the presence of microbial mats as reflected by an abundance of microbially-mediated sedimentary structures in Lower Cambrian strata. Such substrate conditions would have been unfavourable for burrowing by benthic metazoans. A combination of environmental restrictions and a lack of adaptations to vertical burrowing likely prevented most benthic metazoans from burrowing infaunally in Early Cambrian subtidal shelf substrates. The eventual acquisition of burrowing adaptations by benthic metazoans later in the Cambrian promoted an increase in the depth and intensity of bioturbation and initiated a transition toward well-hydrated substrates in which extensive infaunal activity was possible.Siliciclastic units of the Lower Cambrian succession in the White–Inyo Mountains, eastern California, contain abundant horizontal bioturbation on bedding planes, as documented by bedding plane bioturbation indices, but little vertical bioturbation, as shown by ichnofabric indices and x-radiography. Planolites, a simple horizontal trace fossil, represents the dominant type of bioturbation in these units. Planolites is found in a range of diameters, indicating that more than one species of tracemaker likely produced this type of trace. Although these Planolites do not have a vertical component, their abundance on bedding planes indicates that the activities of Planolites tracemakers had a significant impact on subtidal shelf substrates, represented by Lower Cambrian units in the White–Inyo Mountains, early in the Cambrian substrate revolution.     

Developmental shift in the selective tidal-stream transport behavior of larvae of the fiddler crab Uca minax (LeConte)

Following hatching, larvae of the fiddler crab Uca minax (La Conte) are exported from the adult habitat in estuaries to coastal and shelf waters where they undergo development prior to re-entering estuaries as postlarvae (megalopae). Studies of the spatial distribution of both newly hatched zoeae (Stage I) and megalopae indicate they undergo rhythmic vertical migrations associated with the tides for dispersal and unidirectional transport (selective tidal-stream transport) both within estuaries and between estuaries and the nearshore coastal ocean. We tested the hypothesis that U. minax zoeae possess a circatidal rhythm in vertical migration that facilitates offshore transport in ebb tidal flows, while postlarvae (megalopae) return to estuaries using a similar flood-phased endogenous rhythm. We also determined if the expression of the rhythm was influenced by the salinity conditions zoeae and megalopae experience as they transition between low-salinity regions of estuaries and high-salinity coastal waters. Stage I zoeae were collected by holding ovigerous female crabs in the lab until hatching. Megalopae were collected from the plankton and identified to species using molecular techniques (PCR-RFLP). Under constant laboratory conditions, both zoeae and megalopae exhibited endogenous circatidal rhythms in swimming that matched the principal harmonic constituent of the local tides (12.39 ± 0.07 h; X¯ ± SE). Upward swimming in Stage I zoeae occurred 2.5-4 h after high tide near the time of expected maximum ebb currents in the field. Rhythmic swimming of megalopae occurred slightly earlier in the tide (2.5 ± 0.09 h after high tide; X¯ ± SE) but was not entirely synchronized with flood currents, as expected. Salinity conditions had no apparent effect on the expression or pattern of the rhythms. Results indicate that this circatidal rhythm forms the behavioral basis of selective tidal-stream transport (STST) in early stage U. minax zoeae, but does not undergo a sufficient phase shift to account for vertical distribution patterns exhibited by megalopae in the field.     

Microgeographic genetic structure of the fidller crab,Uca arcuata De Haan (Ocypodidae) in Taiwan

The genetic structure of Uca arcuata in Tanshui mangrove swamp of northern Taiwan was examined. Using as genetic markers, isozymes identified through starch gel electrophoresis indicate that there was moderate genetic differentiation among subpopulations within the population (FST = .085). Gene flow appeared high when estimated indirectly (Nm = 2.69). The results suggest that the patterns of genetic structure of Uca arcuata were influenced by the interaction of local selection due to microhabitat differences and gene flow among fiddler crab colonies in the mangrove swamp.     

Burrowing behaviour of the European eel (Anguilla anguilla): Effects of life stage

The European eel (Anguilla anguilla) is a fascinating species, exhibiting a complex life cycle. The species is, however, listed as critically endangered on the IUCN Red List due to an amalgam of factors, including habitat loss. This study investigated the burrowing behaviour and substrate preference of glass, elver and yellow stages of A. anguilla. Preference was determined by introducing eels in aquaria with different substrates and evaluating the chosen substrate for burrowing. In addition, burrowing was recorded using a camera in all substrate types and analysed for kinematics. The experiments showed that all of these life stages sought refuge in the sediments with particle sizes ranging from sand to coarse gravel. Starting from a resting position, they shook their head horizontally in combination with rapid body undulations until half of their body was within the substrate. High-speed X-ray videography revealed that once partly in the sediment, eels used only horizontal head sweeps to penetrate further, without the use of their tail. Of the substrates tested, burrowing performance was highest in fine gravel (diameter 1–2 mm; lower burrowing duration, less body movements and/or lower frequency of movements), and all eels readily selected this substrate for burrowing. However, glass eels and elvers were able to use coarse gravel (diameter >8 mm) because their smaller size allowed manoeuvring through the spaces between the grains. Further, burrowing performance increased with body size: glass eels required more body undulations compared to yellow eels. Interestingly, the urge to hide within the sediment was highest for glass eels and elvers. Documentation of substrate preference and burrowing behaviour of A. anguilla provides new information about their potential habitat use. Considering that habitat alterations and deteriorations are partly responsible for the decline of the eel, this information can contribute to the development of more effective conservation measures.     

Arboreal gaits in three sympatric rodents Apodemus agrarius,Apodemus flavicollis (Rodentia,Muridae) and Myodes glareolus (Rodentia,Cricetidae)

Vertical stratification of the arboreal habitat allows the coexistence of several species in a given area, because the complex arboreal strata can be used in different ways by arboreal and scansorial mammals. The present report experimentally investigated the gait metrics on different arboreal substrates, of three sympatric rodents living in a deciduous forest in Poznań, Poland. Arboreal locomotion was compared between the burrowing striped field mouse, Apodemus agrarius, the scansorial bank vole, Myodes glareolus, and the more arboreal yellow-necked mouse, Apodemus flavicollis. We filmed two wild-caught individuals from each species walking on four different substrate diameters (2 mm, 5 mm, 10 mm, 25 mm) and three different inclinations (45° descending, horizontal, 45° ascending) at 240 fps and collected a set of gait parameters from a total of 273 complete cycles. Our results did not demonstrate clear relationships between arboreal locomotion and the ecology of the three species. Only A. flavicollis exhibited locomotor features partly associated with arboreal competence, including lower velocity and diagonality on narrow substrates and asymmetrical gaits on wider ones. On the other hand, the two Apodemus species, despite their different ecologies, shared a few locomotor similarities, such as velocity regulation primarily by stride frequency, and similar effects of substrate size and inclination on diagonality, duty factor, and duty factor index indicating the possibility of a phylogenetic signal. Because the selected gait parameters provided limited insight into the ability of small mammals to move competently through an arboreal habitat, these findings indicate that the relationship between behaviour and ecology is complex.     

Delayed courtship in the fiddler crab Uca musica terpsichores

Courting male fiddler crabs of the species Uca musica terpsichores congregate in the upper central portion of the colony, while receptive females leave their burrows located at the colony's periphery and wander among the communally displaying males prior to choosing a mate. I observed that courting males in a newly-established population were significantly smaller than courting males in large high-density colonies. This observation led to a series of translocation experiments designed to ascertain whether high population density influences the size (=age) at which males begin to court. Smaller courting males from a low-density population failed to court after being placed among larger courting males in a high-density population. The reciprocal translocation revealed that smaller noncourting males from the high-density population would start courting shortly after being placed in a low-density population. Smaller males placed in the high-density population were subsequently observed significantly further away from where they were initially placed than were larger males similarly translocated. The results suggest that smaller males delay courtship activities once they are forced, via encounters with larger males, to the periphery of the colony. I believe that both intrasexual selection (competition from larger males) and intersexual selection (female choice of large males) are responsible for the delay in male courtship activities.     

Effect of Substrate Type and Moisture on Pupation and Emergence of <Emphasis Type="Italic">Heortia vitessoides</Emphasis> (Lepidoptera: Crambidae): Choice and No-Choice Studies

Heortia vitessoides Moore is the most destructive defoliator of Aquilaria sinensis (Loureiro) Sprenger, an endangered and economically important plant that produces the high-prized agarwood. After maturing, H. vitessoides larvae will leave the A. sinensis and pupate in soil, however, factors affecting this behavior have not been previously addressed. In the present study, choice and no-choice tests were conducted to study the effects of substrate type (sand, sandy loam A, sandy loam B, and silt loam) and moisture condition (0 %, 20 %, 40 %, 60 %, 80 %, and 100 % water saturation) on burrowing, pupation and emergence behaviors of H. vitessoides under laboratory conditions. Six-choice tests showed that significantly fewer larvae burrowed and pupated in each substrate under extremely dry (0 % saturation) or wet (80 % and 100 % saturation) conditions. Four-choice tests showed that at 20 % saturation a significantly lower percentage of larvae burrowed and pupated in silt loam as compared to other substrates, whereas at 40 % saturation sand was least preferred. At 60 % saturation, however, there was no significant difference in burrowing and pupation among the four substrates. No-choice test showed that either or both 0 % and 80 % water saturation suppressed burrowing behavior and emergence success compared with the intermediate saturation levels in sandy loam (A and B) and silt loam, but not in sand. In addition, the effect of substrate type varied with saturation level. Our study enhance the understanding of pupation ecology of H. vitessoides, and applied implications are discussed.     

Colonization and substratum preference of an introduced burrowing crustacean in a temperate estuary

Habitat use in marine invertebrates is often influenced by multiple abiotic and biotic factors. Substratum composition is one factor known to have a dramatic effect on habitat selection. The Australasian burrowing isopod (Sphaeroma quoianum, H. Milne Edwards 1840) is a common introduced species in many estuaries on the Pacific coast of North America. S. quoianum burrows into a variety of firm substrata including marsh banks (composed of peat, clay, and/or mud), wood, friable rock, and Styrofoam floats. In some areas, isopods achieve high densities and may accelerate the rate of shoreline erosion and damage marine structures; thus, understanding the substratum preference of this species may be important for conservation and management efforts. Field experiments were conducted in Coos Bay, Oregon to examine substratum preference, burrowing rates, and the life stage of colonizers. In three experimental trials (Fall 2005, Spring 2006, Fall 2006), replicates of four intertidal substrata (marsh banks, decayed wood, sandstone, Styrofoam) were deployed near intertidal populations of S. quoianum. The numbers of burrows created in each substratum were enumerated weekly or daily (depending on trial). After the trials were completed, the total numbers of isopods inhabiting each substratum were counted. In weeks, S. quoianum extensively burrowed the substrata but exhibited a distinct preference for decayed wood. Significantly more isopods were present in wood than the other substrata at the end of the experiments and rates of burrowing were greatest in wood, although significance varied across time in one trial. Nearly 90% of colonizing isopods were under 5 mm in length suggesting that juvenile isopods primarily colonize intertidal substrata. Differences between burrow densities measured in the field and the results from these preference trials may indicate other factors, such as relative availability of substrata, recruitment and dispersal limitations, and possible gregarious behavior also influence local isopod densities.     

Transport of juvenile clams: effects of species and sediment grain size

Erosion and transport of juvenile benthic invertebrates, including bivalves, have the potential to alter patterns of distribution and abundance during the early post-settlement period. However, the factors influencing rates of postlarval dispersal are not well understood. Both hydrodynamics and behaviour (e.g. burrowing) are likely to play a role in determining patterns of transport of juvenile bivalves. To determine the relationship between sediment transport and bivalve dispersal, experiments were conducted in a racetrack flume to examine the effect of grain size, flow, and clam size on rates of erosion of two species of juvenile clams (Mya arenaria and Mercenaria mercenaria). Results of the experiments were compared to predictions of erosion thresholds based on the physical characteristics of the sediment and clams. Erosion of Mercenaria was greater than Mya, the opposite of predictions based on Mercenaria's greater density, indicating the importance of burrowing behaviour. In most cases, erosion also was greater in the finer sand, in contrast to the predicted similarity of erosion thresholds of the two sediments. However, clam erosion did increase with increasing shear velocity and decrease with clam size, as expected. The results of this study indicate that both hydrodynamics and behaviour play roles in the transport of these two species of juvenile bivalves and that their vulnerability to passive erosion cannot be predicted solely from knowledge of sediment transport.