Using short-term growth of enclosed 0-group European flounder, Platichthys flesus, to assess habitat quality in a Danish bay

Instantaneous growth rates of 0‐group European flounder, Platichthys flesus, obtained from two 9‐day enclosure experiments were used to assess the relative quality of three eelgrass and three bare sand habitats in the Bay of Aarhus, Denmark. Measurements of temperature, dissolved oxygen, salinity, sediment structure and biomass of benthic macrofauna were made during the experiments. Gut content analysis was performed after retrieval of the fish. Growth rates of flounder at two bare sand sites were higher or equal to growth rates at the three eelgrass sites, and at a muddy bare sand site in both experiments. The variation in gut fullness of flounder between vegetated and bare sand sites followed the pattern observed in growth rates, but diverged from the spatial variation in prey abundance, which indicates that there were differences in the feeding efficiency between sites. Overall, habitat complexity, i.e. the presence or absence of vegetation seems to be the most important factor determining the growth rates and, by corollary, habitat quality for juvenile European flounder in the Bay of Aarhus.     

Habitat choice by juvenile cod (Gadus morhua L.) on sandy soft bottoms with different vegetation types

Habitat choice by juvenile cod (Gadus morhua L.) on sandy bottoms with different vegetation types was studied in laboratory. The experiment was conducted day and night in flow-through tanks on two different size-classes of cod (7–13 and 17–28 cm TL). Four habitats, typical of shallow soft bottoms on the Swedish west coast:Fucus vesiculosus, Zostera marina, Cladophora sp. and bare sand, were set up pair-wise in six combinations. The main difference between habitats in this study was vegetation structure, since all parameters except vegetation type was considered equal for both sides of the experimental tanks and natural prey was eliminated. The results showed a difference in habitat utilization by juvenile cod between day (light) and night (dark). During day time the fishes showed a significant preference for vegetation, while nocturnally no significant choice of habitat was made. Both size-classes preferredFucus, considered the most complex habitat in this study, when this was available. The smaller size-class seemed to be able to utilize the other vegetation types as well, always preferring vegetation over sand. Larger juvenile cod, on the other hand, appeared to be restricted toFucus. This difference in habitat choice by the two size-classes might be due to a greater dependence on shelter from predation by the smaller juveniles, causing them to associate more strongly with vegetation. The larger juveniles avoidedCladophora, since they might have difficulties in entering the compact structure of this filamentous algae. Availability of vegetation at day time, as a predation refuge, as well as of open sandy areas for feeding during night, thus seems to be important for juvenile cod. It is concluded that eutrophication-induced changes in habitat structure, such as increased dominance by filamentous algae, could alter the availability of predation refuges and foraging habitats for juvenile cod.     

Sex‐specific distribution and diet of Platichthys flesus at the end of spawning in the northern Baltic Sea

This study examined the relationship of seascape structure, prey availability and sex on the post‐spawning distribution and diet of European flounder Platichthys flesus in the northern Baltic Sea. The objectives were to determine whether: (1) wave exposure and substratum affect abundance and distribution of P. flesus, (2) diet reflects the benthic prey composition and (3) sex affects the distribution or diet of P. flesus. The results showed that P. flesus was evenly spread in the archipelago with no correlation to wave exposure. The distribution was, however, sex specific; reproductive males dominated the exposed zone and mainly post‐reproductive females dominated the intermediate and sheltered zones. Platichthys flesus fed mainly on two bivalve prey species: blue mussels Mytilus edulis and Baltic tellins Macoma balthica. Hard substratum invertebrates dominated the diet in all habitats and apart from some typical soft substratum species, there was no clear link between fish feeding and the dominance structure of benthic prey. Diet was further sex specific, with females showing a broader range of diet than males. Results suggest that P. flesus is a specialist molluscivore found commonly and equally in soft‐ and hard‐substratum habitats throughout the archipelago area. Previous studies on P. flesus in the Baltic Sea have yielded inconsistent results regarding diet and it has commonly been believed that the distribution of Baltic Sea P. flesus is linked to sand and soft substrata. The present findings emphasize the importance of including the entire range of habitats when diet and regional species distributions are assessed.     

Vulnerability of newly settled plaice (Pleuronectes platessa L.) to predation: effects of habitat structure and predator functional response

Plaice (Pleuronectes platessa) nursery grounds on the Swedish west coast have been subject to increasing cover of annual green macroalgae during recent years, with growth of algae starting at the time of plaice settlement in April to May. A laboratory experiment was performed to investigate how the vulnerability to predation of metamorphosing plaice was affected by the presence of filamentous algae. Predation by shrimps (Crangon crangon) on settling plaice larvae was higher on sand than among algae, whereas predation by crabs (Carcinus maenas) was unaffected by habitat type, suggesting a lower overall mortality of plaice in the vegetated habitat. When predators and prey were presented with a combination of the two habitats, predation by shrimps was as high as that in the sand treatment alone, whereas predation by crabs was lower than that in the two treatments with one habitat. Based on these results, an additional experiment was performed, investigating the functional response of shrimps to six densities of juvenile plaice in a sand habitat with alternative prey present. The proportional mortality of juvenile plaice (12-16 mm total length (TL)) was density-dependent and was best described by a type III (sigmoid) functional response of the predatory shrimps. The results suggested that the combined predation pressure from shrimps and crabs was lower among algae than on sand, but settling plaice and predatory shrimps chose the sand habitat. Plaice densities in the sigmoid part of the obtained functional response curve represented normal to high field densities of plaice on the Swedish west coast, suggesting that shrimp predation could have a stabilising effect on plaice recruitment. The formation of macroalgae mats could therefore lead to a concentration of plaice juveniles in the remaining sand habitat and increased mortality through density-dependent predation by shrimps.     

Drifting Algae as a means of Re-Colonizing Defaunated Sediments in the Baltic Sea. A Short-Term Microcosm Study

We conducted a microcosm experiment to evaluate the capability of fauna inhabiting or being transported by drifting filamentous algae to colonize defaunated sediment. We expected meiofauna would perform a quicker and more effective re-colonization of disturbed areas by means of the algal mats than their macrofaunal counterparts. Similarly, within meiofauna, we expected more mobile taxa such as ostracods and harpacticoids to colonize the sediment more readily than other more sedentary ones such as nematodes. Naturally drifting algae were collected from the field and placed in 1 l aquaria on top of 5 cm of defaunated sediment. After 3 and 6 days, one core sample (5 cm deep) was taken from each aquarium; the first 2 cm were sliced into 2 mm layers, and the remaining fraction into 1 cm layers. The sediment remaining in the aquaria was sieved through a 0.5 mm sieve to collect the re-colonizing macrofauna. The dominant macrofaunal taxa inhabiting the algae were juvenile bivalves and gastropods, with Cerastoderma glaucum accounting for the majority of the bivalves and Hydrobia sp. for most of the gastropods. After 3 and 6 days, the most abundant macrofaunal taxa colonizing the sediment were Cerastoderma glaucum, Hydrobia sp. and gammarid amphipods. Higher abundances were found after 6 days than after 3, though differences were not significant for any of the major taxa. Meiofauna inhabiting the algae were dominated by rotifers, nematodes, ostracods, chironomid larvae and harpacticoid copepods. Contrary to our predictions, nematode and harpacticoid species inhabiting the drifting algae were not driven to sediment re-colonization but remained in the algae. Our results indicate that some benthic animals may indeed benefit from drifting algal mats as a means of dispersal and re-colonization of previously defaunated sediments in relatively short periods of time. Also, they may contribute to explain some of the trends found in other studies, regarding species increase under drifting algae and the recovery patterns found in areas often exposed to algal conglomerates.     

Feeding ecology and growth of age 0 year Platichthys flesus(L.) in a vegetated and a bare sand habitat in a nutrient rich fjord

A study was carried out to investigate the diet and feeding strategies of age 0 year juvenile flounder Platichthys flesus in two different micro‐tidal habitats in the nutrient enriched Mariager Fjord on the Danish east coast. Juvenile flounder and benthic macrofauna were sampled monthly from June to October 1999 in a bare sand habitat and a habitat covered by filamentous and mat forming macroalgae. The presence of the 'opportunistic' macroalgae created a shift in the dominance of surface dwelling prey such as epifaunal amphipods to more infaunal groups such as oligochaetes and polychaetes. The diet of the flounder varied considerably between the two habitats mainly reflecting prey availability relative to their abundance, prey spatial distribution, habitat structure and ontogenetic prey shifts as a function of total length. Flounder in the vegetated site fed on a diverse diet of copepods, polychaetes and oligochaetes, whereas those caught in the bare sand site fed primarily on the amphipod Corophium volutator which was numerically dominant at this site. During the growth season, two diet shifts were observed: from copepods early in the season to macrofauna organisms and, later in the season, the inclusion of more hyperbenthic prey such as Mysidea spp; Idotea spp. and the common goby Pomatoschistus microps .     

Consumer versus resource control and the importance of habitat heterogeneity for estuarine bivalves

The relative influence of consumers (top down) and resources (bottom up) on the distribution and abundance of organisms remains a key question in ecology. We examined the relationships between consumer and resource variables along a productivity gradient for a dominant predator–prey interaction in a marine soft‐sediment system. We 1) quantified density and size of the clam Macoma balthica (prey species) in six replicate sites at each of four habitat types (shallow mud, deep mud, muddy sand and detrital mud) in the Rhode River, Chesapeake Bay. We selected one habitat type of high food availability and clam density (shallow mud) and another of low food availability and clam density (muddy sand) for manipulative experiments. Then, we 2) measured M. balthica survival and growth through transplants, 3) measured food availability as sedimentary organic carbon content, 4) quantified predator density, and 5) calculated predator foraging efficiency in the two habitat types. Clam density in the four habitat types differed and was related to sedimentary carbon availability and predator density. One of the habitats, detrital mud, appeared to be a population sink because it only held juvenile Macoma that never survived to reproductive age. Macoma size and growth, and predator (mainly blue crab Callinectes sapidus) densities were positively correlated with productivity and were higher in shallow mud than muddy sand. In contrast, Macoma mortality, local ‘interaction strength’, and predator foraging efficiency were lower in the productive habitat (shallow mud). Thus, predation intensity was inversely correlated with productivity (food availability); consumer and resource effects differed by habitat type; and, at a relatively small spatial scale, consumer and resource forces jointly determined population dynamics in this soft‐sediment marine system.     

The effects of seafloor habitat complexity on survival of juvenile fishes: Species-specific interactions with structural refuge

Marine fishes are often associated with structurally complex microhabitats that are believed to provide a refuge from predation. However, the effects of habitat complexity on predator foraging success can be strongly modified by predator and prey behaviors. We conducted a series of laboratory experiments to evaluate the effects of sea floor habitat complexity on juvenile fish survivorship using multiple predator (striped searobin and summer flounder) and prey (winter flounder, scup, and black sea bass) species to identify potentially important species-habitat interactions. Three habitats of varying complexity (bare sand, shell, and sponge) common to coastal marine environments were simulated in large aquaria (2.4 m diameter, 2400 L volume). Prey survivorship increased significantly with greater habitat complexity for each species combination tested. However, examination of multiple prey and predator species across habitats revealed important effects of predator × habitat and prey × habitat interactions on prey survival, which appeared to be related to species-specific predator and prey behavior in complex habitats. Significant species × habitat interactions imply that the impact of reduced seafloor habitat complexity may be more severe for some species than others. Our results indicate that the general effects of seafloor habitat complexity on juvenile fish survivorship may be broadly applicable, but that the interaction of particular habitats with search tactics of predators as well as habitat affinities and avoidance responses of prey can produce differences among species that contribute to variable mortality.     

Fish predation on epiphytic microcrustacea in Tivoli South Bay,a Hudson River tidal freshwater wetland

The littoral microcrustacean fauna of Tivoli South Bay was studied from July to September, 1989. The effects of fish predation on microcrustacean densities were tested in a short-term predator exclusion experiment. Fish were excluded from water chestnut (Trapa natans) plots in four screened exclosures. An equal number of open cages allowed foraging. Fish predation did not have a significant effect on densities of ostracods or other microcrustaceans. Ostracod densities increased in cages throughout the experimental period, whereas cladoceran and copepod densities decreased in both treatments. Gut contents analysis of banded killifish (Fundulus diaphanus) revealed that ostracods and other microcrustaceans were commonly ingested by larval, juvenile, and adult killifish utilizing T. natans as habitat. These results suggest that ostracods and other microcrustacean epifauna associated with T. natans may represent an important trophic link in the tidal freshwater wetlands of the Hudson River Estuary. Deceased     

Benthic macroalgae as a dispersal mechanism for fauna: influence of a marine tumbleweed

Effective dispersal is problematic for benthic organisms without planktonic larvae; rafting and vertical migrations are mechanisms that can potentially be employed by such fauna, but these strategies entail considerable predation risk as well as other disadvantages. Unattached, but non-floating, “drift” algae harbor large numbers of fauna and may serve as an alternative dispersal mechanism in some systems. This paper reports field manipulations in Florida Bay, Florida, USA designed to determine (1) if such algae can disperse benthic animals, and (2) if dispersal efficiency varies as a function of two common substrata types: seagrass and bare sediment. A live immersion stain was used to mark faunal associates of Laurencia spp. algal clumps in situ. The fidelity of molluscs, decapods, ophiuroids, and fishes to stationary algal clumps was then compared with the fidelity of these animals to clumps that were forced to tumble over a given distance with a blower apparatus; these experiments were performed over both sand and seagrass substrata. Measurements of frequency, spatial extent, and rate of algal drift were made to aid in assessing the potential importance of benthic algae as a dispersal mechanism.

Algal clumps often rolled in a manner similar to that of terrestrial tumbleweeds; mark-recapture work showed that algal clumps can move up to 0.5 km/day and that algal drift is a frequent phenomenon. The algal masses were effective transporters of benthic fauna, including mobile shrimps and fishes; dispersal was more efficient over sand than over seagrass. Dispersal of fauna via this mobile habitat should entail lower risk than other adult dispersal stratagems such as vertical migration or rafting; this mechanism would be most advantageous for brooding species or those with limited planktonic phases. Differential fidelity to clumps tumbling across seagrass versus sand suggests that the algae could facilitate exchange of fauna between isolated seagrass patches.     

Relationship between body shape and trophic niche segregation in two closely related sympatric fishes

Diet, shape and their covariation were analysed in juvenile flounder Platichthys flesus and plaice Pleuronectes platessa specimens from a tidal inshore habitat (Dublin Bay) in the Irish Sea. Stomach content analysis revealed differences in the diet, suggesting a substantial degree of resource partitioning between the species. Geometric morphometrics detected significant and discriminating differences in body shape, particularly in the head region. Most interestingly, a high degree of correlation was detected between diet and shape. The ontogenetic and ecological implications of the diet and shape covariation pattern were discussed, suggesting that morphological differences, in particular in the head region, may represent the functional platform which allows for the existence of ecologically significant differences in prey search and capture between P. platessa and P. flesus.     

The occurrence and ecology of a marine hydrobiid mudsnail in the southern hemisphere: the Knysna Estuary, South Africa

Earlier published records of the gastropod Hydrobia from the warm temperate Knysna Lagoon on the Indian Ocean coast of South Africa are in error. Nevertheless, an animal hitherto identified as an Assiminea (Assimineidae) or as a Tomichia (Pomatiopsidae) is in fact a Hydrobia s.l., and it does occur intertidally within the Knysna system. The same species also occurs in salt pans in the cool temperate Great Berg Estuary on the Atlantic coast of South Africa. This extends the distribution of confirmed species of the dominant coastal hydrobiid mudsnails of the northern hemisphere into the southern hemisphere. H. knysnaensis (Krauss) occurs patchily at low density near the head of the Knysna Estuary in waters of low salinity. This appears to be a suboptimal habitat, and it is suggested that Hydrobia is restricted to this zone as a result of interference competition from potamidid mudwhelks and ocypodid crabs. The prevalence of potamidids and ocypodids in the tropics and in the temperate southern hemisphere may account for the rareness or absence of hydrobiids in these areas. Unlike its equally intertidal relative, the widespread North Atlantic/Arctic H. ulvae, H. knysnaensis develops directly like other Hydrobia.     

The effects of predation on the survival and size-distribution of estuarine fishes: an experimental approach

Synopsis The effect of southern flounder,Paralichthys lethostigma, predation on the survival and size-distribution of spot,Leiostomus xanthurus, was investigated using feeding experiments in outdoor tanks and estuarine ponds. In the outdoor tank experiments, southern flounder, 143 mm average standard length (SL), fed for 48 h on spot (25–30 mm SL), over a range of densities (4–20 m^–2). In these trials, flounder showed a Type II functional response to spot density. In the estuarine pond experiment, flounder density (2 or 4 flounders 70 m^–2) contributed significantly to spot mortality relative to predator-free controls. The effect of flounder density on spot mortality was non-linear; in the 4-flounder treatment the daily instantaneous mortality due to flounder (0.0177) was 3.5 times that in the 2-flounder treatment (0.0051). The total spot mortality rate in the 4-flounder treatment (0.028) was similar to that observed from published field observations. Flounder also altered the size distribution of spot relative to predator-free controls. Predation by flounder resulted in fewer intermediate-sized spot at the end of the experiment when compared to predator-free controls. Flounder also significantly reduced survival and mean length of two killifishes,Cyprinodon variegatus andFundulus heteroclitus, that invaded the pond. Flounder predation was a significant size-structuring force on the prey fish assemblage in the pond. Results from both the outdoor tank experiment and the pond experiment indicate that flounder may represent a significant source of mortality for juvenile estuarine fishes.     

Microhabitat use by a post-settlement stage estuarine fish: evidence from relative abundance and predation among habitats

Seagrass beds provide food and shelter for many fish species. However, the manner in which fishes use seagrass bed habitats often varies with life stage. Juvenile fishes can be especially dependent on seagrass beds because seagrass and associated habitats (drift macroalgae) may provide an effective tradeoff between shelter from predation and availability of prey. This study addressed aspects of habitat use by post-settlement pinfish, Lagodon rhomboides (Linneaus), an abundant and trophically important species in seagrass beds in the western North Atlantic and Gulf of Mexico. Abundance of post-settlement fish in seagrass beds was positively related to volume of drift macroalgae, but not to percent cover of seagrass, indicating a possible shelter advantage of the spatially complex algae. Tethering experiments indicated higher rates of predation in seagrass without drift macroalgae than in seagrass with drift macroalgae. Aquarium experiments showed lower predation with higher habitat complexity, but differences were only significant for the most extreme cases (unvegetated bottom, highest macrophyte cover). Levels of dissolved oxygen did not differ between vegetated and unvegetated habitats, indicating no physiological advantage for any habitat. Seagrass beds with drift macroalgae provide the most advantageous tradeoff between foraging and protection from predation for post-settlement L. rhomboides. The complex three-dimensional shelter of drift macroalgae provides an effective shelter that is embedded in the foraging habitat provided by seagrass. Drift macroalgae in seagrass beds is a beneficial habitat for post-settlement L. rhomboides by reducing the risk of predation, and by providing post-settlement habitat within the mosaic (seagrass beds) of adult habitat, thus reducing risks associated with ontogenetic habitat shifts.     

Microhabitat selection of ostracods in relation to predation and food

Experiments with the cyprinid fishVimba vimba as predator and the ostracodsCypridopsis vidua, Darwinula stevensoni andCytherissa lacustris as prey show that conspicuous coloration enhances predation risk for the ostracods. When the ostracods are allowed to retreat into sediment, risk is markedly reduced. ostracods show clear microhabitat preferences which are influenced by habitat structure and food supply. Exposed plant surfaces are visited only if they bear food and if the ostracods are not satiated.     

Exploitation of a deep-water algal maximum by Daphnia: a stable-isotope tracer study

The exploitation of a deep algal maximum by Daphnia in the absence of fish predation was studied in large indoor mesocosms. Facing the dilemma of low food but high temperature in the epilimnion vs. high food but low temperature in the hypolimnion, Daphnia distribute above and below the thermocline in order to optimise their fitness. Labelling hypolimnetic algae with ¹⁵N revealed that the vertical distribution of Daphnia is dynamic, i.e., all individuals traverse the thermocline and allocate a certain proportion of their time to feeding in the cold water. The overall energy gain from the deep-water algal maximum is lower than from the same algal concentration in the epilimnion due to the low temperature and the limited time an individual spends in the hypolimnion. The results provide mechanistic support for the hypothesis that Daphnia chose their habitat according to an Ideal Free Distribution with Costs model.     

Effects of green algal mats on bivalves in a New England mud flat

Concurrent with the spread of green algal mats on tidal flats, reports of macrofauna dieoffs under dense algal mats have increased in numbers. Bivalves seem to be particularly affected by persistent dense algal mats. Bivalve species with a long extendible siphon seem to be less affected underneath algal mats, but no distinction has been made in the past between species with short and those with long siphons,Mya arenaria andMacoma balthica, on an intertidal mudflat in New England. Abundances ofM. arenaria declined substantially during the study period when a thick green algal mat covered the mudflat for several months. Numbers of the small bivalveGemma gemma also decreased substantially, whereas abundances ofM. balthica showed minimal variation during the time of algal coverage. In algae removal/addition experiments numbers ofM. arenaria decreased, but effects were only significant in an algal addition to previously algal-free mudflat areas. Abundance ofM. balthica did not change significantly in the algal removal/additition experiments. Over the time period of the experiment (9 weeks),M. arenaria showed measurable size increase in uncovered mudflat areas, but not underneath algal mats. Similarly,M. balthica only increased in size in the uncovered mudflat area. From these results it is concluded thatM. balthica can survive time periods of dense algal coverage because it is able to penetrate through the algal mat with its long extendible siphon, and thus can reach well-oxygenated water layers above the mat.M. arenaria with its thick, less extendible, siphon cannot push through dense algal mats and therefore is more likely to die underneath persistent algal mats.     

Effect of habitat shifts on feeding behaviour and growth of 0 year-group flounder Platichthys flesus (L.) transferred between macroalgae and bare sand habitats

In this study, short-term growth and feeding behaviour were compared among juvenile European flounder Platichthys flesus reared in enclosures in either their native habitat (bare sand or vegetation) or transferred to the opposite habitat. Growth was poorest in the vegetated habitat regardless of origin of the fish. The effect of the habitat shift differed between years. In 2000, the relatively small fish used grew fastest in their native habitat. In contrast, in 2001 when larger fish were used, growth was similar between native and introduced fish in the vegetated habitat, and introduced fish grew faster than native fish in the bare sand habitat. Diet composition and feeding intensity within a habitat were also similar among native and introduced fish in 2001, suggesting that the habitat switch had a minor influence on foraging efficiency. The different results obtained from the experiments in 2000 and 2001 suggest that fish size may determine the extent to which short-term habitat shifts influence feeding and growth in juvenile flounder, and, importantly, that the negative effects of habitat fragmentation are more severe for small compared to larger juvenile flounder.     

Distribution and life cycle of the North American Spionid polychaeteMarenzelleria viridis (Verrill, 1873) in the Ems estuary

In 1983 the first specimens of the North American spionid polychaeteMarenzelleria viridis were found along the European mainland shore in the Ems estuary. Since then, this polychaete has spread over several estuaries around the North Sea and the Baltic. In the inner part of the Ems estuary juveniles were predominantly present in muddy sediments high in the intertidal zone; in more sandy sediments at higher salinities juveniles and adults co-occured. Detailed information was obtained at a muddy and at a sandy station. Gametes were present in the coelomic fluid from November through March. In May new recruits were found in the sediment samples, reaching densities of over 10⁵ M⁻² at the muddy station, andc. 2000 m⁻² at the sandy station. During summer, densities decreased at the muddy station, coinciding with a density increase at the sandy station, suggesting migration of juveniles from a nursery to the adult habitat. Cage experiments showed that the decrease of juveniles at the muddy station could be attributed to migration and not to mortality due to predation. At the sandy habitat palps and anterior parts ofM. viridis made up 4–11% of the stomach content of juvenile plaice (Pleuronectes platessa). In juvenile flounder (Platichthys flesus) only in Aprilc. 10% of the stomach content consisted ofM. viridis. During 1983–1990 increasing densities ofM. viridis at the sandy habitat coincided with a reduced abundance ofNereis diversicolor, however, this inverse relationship was not found to be statistically significant. Density fluctuations ofM. viridis andCorophium volutator showed a significant positive relationship, the cause of which is not yet understood.     

The relative importance of habitat-specific settlement, predation and juvenile dispersal for distribution and abundance of young juvenile shore crabs Carcinus maenas L.

Young juveniles of many motile benthic species are concentrated in structurally complex habitats, but the proximate causes of this distribution are usually not clear. In the present study, I assessed three potentially important processes affecting distribution and abundance of early benthic stages in the shore crab (Carcinus maenas): (1) selection of habitat by megalopae (postlarvae); (2) habitat-specific predation; and (3) post-settlement movements by juveniles. These processes were assessed concurrently over 3-9 days at two spatial scales: at the scale of square meters using cage techniques within nursery areas, and at the scale of hectares using isolated populations of juvenile shore crabs in small nursery areas as mesocosms. The results were compared to habitat-specific distribution in the field.Shore crab megalopae and first instar juveniles (settlers) were distributed non-randomly among micro-habitats in the assessed nursery areas, with great densities in both mussel beds, eelgrass and filamentous algal patches (on average 114-232 settlers m⁻²), and significantly smaller densities on open sand habitats at all times (on average 4 settlers m⁻²). The same habitat-specific settlement pattern was found in cages where predators were excluded, suggesting that active habitat selection at settlement was responsible for the initial distribution. Older juveniles (second to ninth instar crabs) were also sparse on sand, but in contrast to settlers, were concentrated in mussel beds, which showed significantly greater densities than eelgrass and algal habitats. The cage experiment demonstrated a dynamic distribution of juvenile crabs. Young juveniles constantly migrated over open sand habitats (20 m or further) and colonized the experimental plots in a habitat-specific pattern that reflected the distribution in the field. This pattern was also found for very small crabs colonizing predator-exclusion cages, suggesting that selection of habitat by migrating juveniles caused the ontogenetic change in habitat use. Although post-settlement movements were great within nursery areas, juvenile dispersal at a regional scale appeared to be small, and the recruitment of juvenile shore crabs to the shallow bays occurred mainly through pelagic megalopae.Conservative estimates at the scale of whole nursery areas, based on migration trap data and field samples, indicated great mortality of settlers and early benthic stages of shore crabs. Results from the cage experiment suggest that predation by crabs and shrimp were responsible for the high settlement mortality. Both enclosed cannibalistic juvenile crabs and local predators on uncaged habitat plots caused significant losses of settlers in all habitats (on average 22% and 64% 3 day⁻¹, respectively). The effect of predators was highly variable between trials, but differed little between habitat types, and predation had no detectable proximate effect on juvenile distribution, despite the great losses. Small settlement densities on sand habitats in combination with a refuge at low prey numbers, and an aggregation of cannibalistic juvenile crabs in nursery habitats appear to decrease the effect of habitat-specific predation rates on the distribution of juvenile shore crabs. This study demonstrates that active habitat selection at settlement followed by a dynamic redistribution of young juveniles can be the proximate processes responsible for habitat-specific distribution of epibenthic juveniles, and indicate that predation represents a major evolutionary process reinforcing this behavior.