Intercohort Cannibalism Research Articles

Optimal reproductive phenology under size-dependent cannibalism.

Intra‐cohort cannibalism is an example of a size‐mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade‐off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game‐theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age‐structured game‐theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size‐dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter‐cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions.

Cannibalism among green sea urchins Strongylocentrotus droebachiensis in the laboratory and field

The green sea urchin Strongylocentrotus droebachiensis plays an important ecological role as a consumer of kelp in temperate subtidal ecosystems, but is known to be omnivorous, and recent studies suggest it is potentially cannibalistic. This study ex- amined cannibalism of small juvenile S. droebachien- sis by larger conspecifics in the laboratory, and esti- mated rates of inter-cohort cannibalism in the field based on gut contents of adult sea urchins collected from subtidal sites in the Mingan Archipelago, St. Lawrence Estuary, Canada. Initial 48 h trials investi- gated factors (substratum, presence of food) that may influence cannibalism, and a subsequent 4 wk laboratory experiment found that adults and larger juveniles consumed 21 and 24% of small juveniles (3.5�6 mm test diameter), respectively, on cobble substratum when kelp was provided ad libitum. Fre- quencies of cannibalism from gut contents of urchins in the field were combined with an estimate of gut passage time determined in the laboratory and the densities of sea urchins at each site to estimate rates of cannibalism. The proportion of adults that had can- nibalized ranged from (mean ± SD) 0 ± 0 to 0.31 ± 0.12 across sites in 2013 and 2014, resulting in estimated rates of cannibalism from 0 ± 0 to 68.3 ± 0.60 small juveniles cannibalized m �2 wk �1 . This is the first study to report size-structured cannibalism among S. droe- bachiensis, and suggests that cannibalism can be a major source of mortality for small juvenile sea urchins in subtidal barren grounds.

No Evidence for Intercohort Cannibalism in Mixed‐Size Cultures of Food‐Size and Fingerling Channel Catfish × Blue Catfish Hybrids Grown in Ponds

AbstractFemale Channel Catfish Ictalurus punctatus × male Blue Catfish I. furcatus hybrids are normally harvested by seining single‐batch pond or split‐pond production systems in fall or winter. Ponds are typically restocked with fingerlings without draining. There is concern that without completely draining the pond after harvest, food‐size hybrids that escape seining may cannibalize restocked fingerlings. The purpose of this study was to determine if such hybrid fingerlings stocked after the fall harvest would be cannibalized by the larger carryover fish during winter or the following growing season. Three 0.04‐ha ponds were each stocked on November 25, 2013, with 100 food‐size hybrid catfish averaging 1.07 kg. Those three ponds and an additional three ponds were then stocked on November 27, 2013, with 300 hybrid catfish fingerlings averaging 14.8 g. The fish were not fed over winter. Mean survival of the fingerlings after 4 months at 91.9% and 92.9% was not significantly different between treatments. Survival of the food‐size fish was 99.7%. Four ponds stocked with fingerling and food‐size hybrids and an additional four ponds stocked with just fingerlings in April 2014 were harvested in October. Survival of the fingerlings was not significantly different between ponds with (88.8%) and without (90.3%) food‐size fish. We found no significant intercohort cannibalism by food‐size hybrid catfish on understocked fingerlings either overwinter or during the following growing season.

Top-down vs. bottom-up drivers of recruitment in a key marine invertebrate: investigating early life stages of snow crab

AbstractMany snow crab fisheries have fluctuated widely over time in a quasi-cyclic way due to highly variable recruitment. The causes of this variability are still debated. Bottom-up processes related to climate variability may strongly affect growth and survival during early life, whereas top-down predator effects may be a major source of juvenile mortality. Moreover, intrinsic density-dependent processes, which have received much less attention, are hypothetically responsible for the cycles in recruitment. This study explored how climate, larval production, intercohort cannibalism and groundfish predation may have affected recruitment of early juvenile snow crab in the northwest Gulf of St Lawrence (eastern Canada) over a period of 23 years. Abundance of early juvenile snow crabs (2.5–22.9 mm in carapace width), representing the first 3 years of benthic life, came from an annual trawl survey and was used to determine cohort strength. Analyses revealed a cyclic pattern in abundance of 0+ crabs that may arise from cohort resonant effects. This pattern consisted of three recruitment pulses but was reduced to two pulses by age 2+, while the interannual variability of cohort strength was dampened. This reconfiguration of the earliest recruitment pattern was dictated primarily by bottom water temperature and cannibalism, which progressively overruled the pre-settlement factors of larval production and surface water temperature that best explained abundance of 0+ crabs. The results strongly suggest that bottom-up and density-dependent processes prevail over top-down control in setting the long-term trends and higher-frequency oscillations of snow crab early recruitment patterns.

Emigration-corrected seasonal survival of a size‑structured fish population in a nursery habitat

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 514:191-205 (2014) - DOI: https://doi.org/10.3354/meps10982 Emigration-corrected seasonal survival of a size‑structured fish population in a nursery habitat Andrew B. Barbour1,*, Aaron J. Adams2, Kai Lorenzen1 1School of Forest Resources and Conservation, Program of Fisheries and Aquatic Sciences, 7922 NW 71st Street, University of Florida, Gainesville, Florida 32653, USA 2Florida Institute of Technology, Biological Sciences, Vero Beach Marine Laboratory, 150 W University Boulevard, Melbourne, Florida 32901, USA *Corresponding author: ufsnook@gmail.com ABSTRACT: We studied how emigration and seasonal dynamics affect apparent survival estimates in a nursery habitat, thereby altering habitat valuation. During a 3 yr study, we marked 1917 juvenile common snook Centropomus undecimalis and resighted 85.7% with a telemetry array in 4 mangrove creeks. We grouped individuals by size class, marking year, and creek, and estimated survival (φ) using the Barker joint-data model. Using telemetry data, we estimated seasonal emigration probabilities and simulated the effect of emigration on φ estimates. We found a minimal effect of emigration on φ estimates, the magnitude of which may be explained by emigration underestimates due to telemetry coverage or high resighting rates reducing bias. We found seasonal and size-based variation in φ estimates. Our approach allowed survival estimation during a severe thermal-disturbance event that reduced survival in 2 creeks and may have had its impact mitigated in a third creek by an anthropogenic habitat alteration serving as a thermal refuge. Significant fine-scale spatial segregation of size classes likely acted to reduce intercohort cannibalism, thereby maintaining the high survival rates. We recommend caution when estimating apparent survival, since the lack of estimated survival bias was likely due to our extensive telemetry data, which buffered against the effects of emigration. We conclude that temporal variability in juvenile habitat use and survival requires seasonal classification of nursery habitat value, the consideration of multi-habitat nursery mosaics, and an exploration of the role of predation on habitat use. KEY WORDS: Mark-resighting · PIT tags · PIT tag antenna · Charlotte Harbor Full text in pdf format PreviousNextCite this article as: Barbour AB, Adams AJ, Lorenzen K (2014) Emigration-corrected seasonal survival of a size‑structured fish population in a nursery habitat. Mar Ecol Prog Ser 514:191-205. https://doi.org/10.3354/meps10982 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 514. Online publication date: November 06, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Cannibalistic behavior of octopus (Octopus vulgaris) in the wild.

The first description of cannibalism in wild adult Octopus vulgaris is presented from 3 observations made in the Ría de Vigo (NW Spain), which were filmed by scuba divers. These records document common traits in cannibalistic behavior: (a) it was intercohort cannibalism; (b) attacks were made by both males and females; (c) in 2 of the records, the prey were transported to the den, which was covered with stones of different sizes; (d) the predator started to eat the tip of the arms of its prey; (e) predation on conspecifics occurred even if there were other abundant prey available (i.e., mussels); and (f) the prey/predator weight ratio in the 3 cases ranged from 20% to 25% body weight. The relationships between this behavior and sex, defense of territory, energy balance, food shortage, competition and predation, as well as how the attacker kills its victim are discussed.

Inter-Cohort Cannibalism of Early Benthic Phase Blue King Crabs (Paralithodes platypus): Alternate Foraging Strategies in Different Habitats Lead to Different Functional Responses

Blue king crabs (Paralithodes platypus) are commercially and ecologically important in Alaska, USA, but population abundances have fluctuated over the past several decades likely resulting from a combination of environmental and biological factors, including recruitment variability. Cannibalism between cohorts may be a source of mortality limiting recruitment success in the wild, but the degree of inter-cohort cannibalism is unknown for early benthic phase blue king crabs. In laboratory experiments, we evaluated the effects of habitat type (sand and shell) on the predator functional response and foraging behavior of year-1 blue king crabs as predators of year-0 conspecifics and examined the effects of predator presence on crypsis of prey crabs. In sand, consumption rates increased with predator size and prey density until satiation, while predation rates in shell were low regardless of predator size or prey density. These differential predation rates yielded a type III functional response in sand but a type I functional response in shell habitat. Crypsis of prey crabs was generally high and did not change in the presence of predators. Predator foraging activity was reduced in shell and may be an adaptive behavior to balance foraging efficiency and susceptibility to larger predators. Our results demonstrate that early benthic phase blue king crabs are cannibalistic between cohorts in the laboratory and that shell material is extremely effective for reducing encounter rates with conspecific predators. The distribution and abundance of such habitat may be important for recruitment success in some populations. Future studies should compare benthic habitat and species assemblages in areas with variable abundances, such as the Pribilof Islands and Saint Matthew Island in the eastern Bering Sea, to better understand possible mechanisms for recruitment variability.

Conspecific cues affect stage-specific molting frequency, survival, and claw morphology of early juvenile stages of the shore crab Carcinus maenas

Benthic conspecific cues are used by competent larvae of many marine invertebrates to locate and settle on suitable habitat. However, aggregations of conspecifics can generate strong intraspecific competition and inter-cohort cannibalism. We investigated the effects of adult conspecific cues on general fitness parameters of juvenile Carcinus maenas (stages J1–J5), and used geometric morphometrics to investigate patterns of allometric growth indicative of life-history strategies and resource use potential. Cues induced faster metamorphosis and slightly shortened intermolt time in J2 individuals, at the expense of acute mortality in J1 crabs. These effects are cumulative but compensatory processes nullify differences by the end of the experiment. Allometric carapace change toward the adult standard remained unchanged, but conspecific cues induced first a change in size (J1) and then in shape (J5) of claws. In both control and cued juveniles, heterochely was incipient but apparent in J5 crabs. Independently of body side, conspecific cues triggered a very marked increase of the propodus posterior margin, presumably enhancing general strength. Therefore, early benthic stages may grow slightly faster to a size refuge, and develop stronger claws providing competitive advantage for the use of high-value food items when population density-dependent processes are more probable.

Cannibalism and inter-specific predation in early stages of intertidal crab species that compete for refuges

Most benthic species with dispersive life stages suffer high mortality rates in hours to days following settlement. Predation exerted by other species and predation by self (cannibalism) are commonly the chief sources of post-settlement mortality and can profoundly influence population dynamics and adult spatial distribution. In Chile, two predatory crab species (Acanthocyclus gayi and Acanthocyclus hassleri) coexist in the intertidal zone. Both settle preferentially in mussel beds, but adults show remarkable spatial segregation, apparently as a result of asymmetric competition for refuges. Although early recruits of A. gayi are an order of magnitude more abundant than A. hassleri, late juveniles are similarly abundant. Recruits of A. gayi are probably subjected to higher mortality before competition for refuges intensifies. Here, through laboratory experiments, we quantified the strength of intra and inter-cohort cannibalism and inter-specific predation as probable sources of differential post-settlement mortality. Intra-cohort cannibalism (among recruits of same size) accounted for the mortality of up to 30% of recruits in both species, with no evidence of density-dependent effects on mortality. Rates of cannibalism between juveniles and recruits (inter-cohorts) were also similar between the two species. Both species exhibit type III functional responses of juvenile predators with a tendency to consume heavily upon the most abundant recruits (A. gayi in the field), which could potentially provide A. hassleri recruits with a “virtual refuge” from the inter-cohort predation in the field. The combination of these different sources of mortality might contribute to the large reduction in abundance of A. gayi recruits by the time they reach juvenile stages. Our results illustrate the complexity of mechanisms that can underlay patterns of distribution and relative abundances among competitors through different life stages, especially among coexisting species in which attacking peers can provide higher rewards later in development than just the energy obtained from other food.

Features of intercohort cannibalism of Vendace (Coregonus albula (L.)) under laboratory conditions

Cannibalism in Vendace (Coregonus albula (L.)) was studied under laboratory conditions by exposing Vendace eggs (100) or newly hatched larvae (20) to an older individual in aquaria. Predation on the eggs was not detected whereas both biting and ingesting of newly hatched larvae were observed in 23% of older Vendace individuals. Small individuals (<100 mm in total length) were significantly more likely to attack larvae than larger individuals. Differences were also found in attack rates between individuals. The rate ranged from no attacks to attacks towards every larva exposed to the cannibalistic individual. These results confirm that intercohort cannibalism is neither an exceptional nor a universal phenomenon when freely swimming Vendace larvae are exposed to older conspecifics. Field observations and bioenergetic modelling are recommended to assess whether cannibalism plays a role in recruitment variation, especially since Vendace show a biennial cycle in the strength of their year classes.

Feeding strategy and cannibalism of the Argentine hake Merluccius hubbsi

The diet composition and feeding strategy of the Argentine hake Merluccius hubbsi in the San Matías Gulf were analysed in order to use this information for the sustainable management of the fishery. Merluccius hubbsi behaved as an opportunistic predator. Small M. hubbsi consumed planktonic crustaceans, whereas medium and large fish ate numerous prey taxa with low frequency of occurrence and variable specific abundance. Intra- and intercohort cannibalism were detected in all size groups and were particularly significant in large M. hubbsi. Medium-sized M. hubbsi consumed small conspecifics and large-sized M. hubbsi consumed both small and medium M. hubbsi. These results indicate that the removal of large M. hubbsi by fishing may increase the risk of overfishing by two combined effects: a direct effect of recruitment-overfishing and an indirect effect of growth-overfishing through an enhanced cannibalism of medium M. hubbsi on small M. hubbsi. Intra- and intercohort cannibalism and other trophic relationships in the M. hubbsi should therefore be considered explicitly in stock assessment models.

Body size, temperature, and seasonal differences in size structure influence the occurrence of cannibalism in larvae of the migratory dragonfly, Anax junius

The aim of this study was to test the hypotheses that body size and seasonal differences in temperature and size structure influence cannibalism in larval dragonflies. In the first two experiments, larvae that were either similar or different in size were paired to examine the potential for intra- and intercohort cannibalism. In the third experiment, size structure of an assemblage of larvae and water temperature were manipulated to explore the seasonal dynamics of cannibalism. Cannibalism was common between individuals that differed in body size by one or more instars. Cannibalism also occurred between individuals similar in size but the rate varied across developmental stages. Results suggest that cannibalism may be most common when water temperatures are warm and late-instar larvae are present at high densities. These results highlight the importance of intra- and intercohort cannibalism as factors that can influence the population dynamics of generalist predators.

Occurrence and intensity of intercohort cannibalism of post-hatching stages in the broad-nosed pipefish,Syngnathus typhle

In the broad-nosed pipefish, Syngnathus typhle, males brood their offspring in a ventral brood pouch, providing them with nutrients and oxygen. The pregnancy is costly to the males as they feed less and grow more slowly than females during the breeding season (Svensson, 1988). The juvenile fish, once completely developed, leave the pouch to lead independent lives. At this stage, the juvenile fish can be in risk of predation, even by conspecific adults according to field results obtained in other related Syngnathus species (Franzoi et al., 1993; Teixeira and Musick, 1995). This form of cannibalism has been defined as intercohort cannibalism of juvenile stages by Smith and Reay (1991). In this study, incidence and intensity of intercohort cannibalism have been assessed in a sampling of a broadnosed pipefish population inhabiting the Venice Lagoon, at the peak of the breeding season.

Cannibalism of Juveniles by Adult Common Snook (Centropomus undecimalis)

Cannibalism is probably common to many fish species, but has been documented for relatively few. This study provides the first documentation of intercohort cannibalism (juveniles cannibalized by adults) for common snook, Centropomus undecimalis, all instances of which occurred in putative nursery habitats.

Interference competition for space in nursery habitats: density-dependent effects on growth and dispersal in juvenile shore crabs Carcinus maenas

In marine organisms with dispersing larval stages, current ecological theory suggests that recruitment of benthic juveniles may be limited by juvenile habitats and regulated by early post- settlement processes. However, few studies have attempted to identify the density-dependent mech- anisms responsible for these demographic bottlenecks. I conducted a series of experiments to assess if interference competition for space, within nursery habitats, could result in density-dependent juve- nile growth and dispersal from refuge habitats, and regulate local populations of juvenile shore crabs Carcinus maenas in shallow nursery areas in Sweden. In laboratory mesocosms, increased natural densities of similarly sized crabs resulted in decreased growth and increased per capita emigration from mussel habitats, even though food was provided in excess, suggesting that competition for space and mutual interference mediated the density-dependent growth and dispersal. This was supported in a field experiment where a density-dependent relation was found between the abundance of juve- nile crabs in nursery areas and their migration rates to experimental mussel patches. The results sug- gest that interference competition for space and density-dependent emigration from refuge habitats, coupled with habitat-specific mortality, is an important regulating mechanism for juvenile shore crab populations, in particular for older juvenile cohorts that have escaped inter-cohort cannibalism through a size refuge. Density-dependent effects on growth were found only at unusually high nat- ural densities and may, therefore, be less important as a regulating mechanism. However, negative effects on feeding rates and growth at high conspecific densities may represent an important selec- tive force in juvenile migration behavior. Density-dependent dispersal was found at juvenile densi- ties that are regularly found in nursery areas, suggesting that nursery habitats become saturated with juveniles during the recruitment season and represent a limiting resource for local populations, con- sistent with field observations of shore crab populations in the study area.

Patterns and Processes of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae) Recruitment in Mar Chiquita Coastal Lagoon, Argentina

Recruitment patterns of crabs and other benthic invertebrates with planktonic larvae are determined by a combination of pre- and post-settlement factors. Recruitment is considered settlement limited when there is a positive correlation between the postlarvae available in the water column and the number of recruits, and regulated when abundance of individuals is mainly affected by post-settlement factors, such as competence and inter or intraspecific predation. Temperate southwestern Atlantic saltmarshes are inhabited by Cyrtograpsus angulatus and Chasmagnathus granulata, two intertidal grapsid crabs. These crabs are considered key species in the ecosystem in Mar Chiquita coastal lagoon, Buenos Aires, Argentina. In this work, we compare the recruitment pattern of Cyrtograpsus angulatus and Chasmagnathus granulata on the basis of a 3-year sampling programme. Furthermore, we examine experimentally, in laboratory and field, settlement and postsettlement processes that can help explain these patterns: habitat selection, intra- and inter-cohort cannibalism and predator avoidance behaviour. Finally, we integrate our results with the previous knowledge about distribution, growth and reproductive biology of both species, to propose a model of recruitment of each species. Both species had a consistent recruitment pattern through 3 years. Recruitment of both species started at the beginning of summer, and continued to autumn, but recruits were present until the beginning of spring. Densities of recruits and juveniles of C. angulatus were not correlated, whereas, there was a relationship between abundance of recruits and juveniles of C. granulata. In the field, recruits of C. angulatus suffered high mortality caused by intra- and inter-cohort cannibalism. Megalopae of C. angulatus selected a substrate (crevices in Ficopomatus enigmaticus reef) that gave them refuge against cannibals. First crabs of C. granulata selected adult conspecific substrate (mud from adult habitats). Recruits of C. angulatus avoided adults of either species. A qualitative model of recruitment of both species is proposed. Post-settlement processes strongly affects the recruitment pattern and, consequently, both species have regulated recruitment. The degree of such regulation, however, is more intense in C. angulatus than in C. granulata.

Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

The grapsid crabs Chasmagnathus granulata and Cyrtograpsus angulatus are considered as key species within the benthic communities of estuaries and brackish coastal lagoons in the southwestern Atlantic region. In controlled laboratory experiments, we studied the intensity of interspecific predation as well as intra- and intercohort cannibalism in setllers in relation to refuge availability, predator characteristics (species, size, sex, nutritional state), and the presence or absence of an alternative food source ( Artemia nauplii). In both species, the intensity of intracohort cannibalism among recently settled crabs (instars I and II, “settlers”) was low, with ca. 5% mortality during 48 h experimental observation periods. Larger juveniles and adults of both species, by contrast, preyed heavily on the settlers. Predation was significantly reduced when refuges were available for the settlers. Hunger of the predators enhanced in general the predation rate. In the presence of alternative food, the consumption of settlers was significantly reduced. In C. granulata, adult females ate more settlers than the males, probably as a consequence of differences in the morphometric traits of their chelae. Cannibalism and predation by juvenile and adult crabs may play an important role in the regulation of recruitment success for both species and hence, in the structure of estuarine benthic communities.

Diel foraging cycles and interactions among juvenile Atlantic cod (Gadus morhua) at a nearshore site in Newfoundland

In coastal Newfoundland, age 0 Atlantic cod (Gadus morhua) settle in shallow, nearshore regions occupied by older conspecifics. Although intercohort cannibalism is common, little is known of the behavioural responses that limit interactions among juvenile cohorts in the wild. We examined the day-night differences in catch and diet of age 0 and age 1 cod to delineate peak foraging activity and resource use in eelgrass (Zostera marina) habitat. Cod exhibited a size-related shift (age 0 to age 1) from feeding predominantly on zooplankton by day to benthos at night. Intercohort cannibalism occurred when age 1 cod were approximately three times larger than their prey. Concentrations of age 0 cod foraged in the water column during the day, ceased feeding at night, and appeared to disperse to the bottom. Seasonally decreasing daylength and prey size contributed to a decline in daily ration, yet age 0 cod still maintained a strict diurnal foraging cycle. The nocturnal decrease in feeding coincided with an increase in the catch of older conspecifics (age 2 and age 3) and increased foraging activity of age 1 cod, suggesting that avoidance of older conspecifics is an important factor influencing the diel foraging and activity cycles of age 0 cod.

Cannibal–prey dynamics in young juveniles and postlarvae of the blue crab

Although cannibalism can act as a density-dependent regulator of population size in terrestrial systems, little is known of its effects in the marine environment. Herein we investigate the influence of cannibalism upon the early life history stages of the blue crab, Callinectes sapidus Rathbun, emphasizing cannibalism between juveniles and postlarvae (i.e. megalopae) of the same year class. In laboratory mesocosms we examined various factors modulating cannibal–prey dynamics, specifically: (1) the effects of habitat and presence of conspecifics on postlarval metamorphosis rate; (2) the effect of metamorphosis rate on the mortality of postlarvae from both intra- and inter-cohort cannibalism; (3) the effects of habitat and predator density on the functional response of young juvenile blue crab predators to varying densities of postlarval prey, and (4) the effects of prey size and habitat on predation mortality.Inter-cohort cannibalism caused significant mortality in every crab size and habitat type combination, and was lower in grass than sand for all prey smaller than fifth instar. Cannibalism between postlarvae was associated with metamorphosis and was density-dependent in sand, but not present in grass. Metamorphosis rates of postlarvae were inversely density-dependent in sand, but density-independent and higher in grass, indicating that habitat and intra-cohort agonism likely affects postlarval metamorphosis rates. Inter-cohort cannibalism was negatively correlated with metamorphosis rates of postlarvae. The functional response of young juvenile cannibalistic blue crabs differed significantly between sand and grass habitats, and between medium and high predator densities. Juvenile crabs displayed a type II, inversely density-dependent functional response in sand, resulting in very high mortality at low densities of postlarval prey. In grass, the crabs displayed a weak type III, density dependent response, yielding significantly lower mortality at low prey densities. Thus, habitat complexity changes the form of the functional response in cannibal–prey interactions and grass provides a relative habitat refuge from cannibalism. Doubling the number of predators in grass decreased the consumption rates per predator significantly and eliminated the density-dependence, indicating that intraspecific density can qualitatively change the form of the functional response. In the crab size experiment, only prey smaller than fifth instars received a habitat refuge from cannibalism in grass, whereas fifth instars received a relative size refuge in sand.Our results demonstrate that intra-year class cannibalism can cause mortality upon settling megalopae and first juvenile instars that is dependent on prey density. We expect inter-cohort cannibalism to cause local extinction of cohorts settling in sand, especially at low settlement densities, and high mortality at moderate settlement densities in grass. Satiation of predators at high settlement densities in grass suggests that episodic settlement can overwhelm predators locally. Furthermore, density-dependent mutual interference within large cohorts in the grass beds likely reduces their predation efficiency, indicating that aggregation of conspecific predators in grass habitats does not necessary lead to an increase in predation pressure. Finally, a relative size-refuge from inter-cohort cannibalism for fifth instar crabs supports an ontogenetic habitat shift around this crab size, which may be influenced by density-dependent agonistic behavior within cohorts. We suggest that intra-year class cannibalism is a major process regulating both survival and dispersal in megalopae and juvenile blue crabs.