CANNIBAL-PREY DYNAMICS IN YOUNG JUVENILES AND POSTLARVAE OF THE BLUE-CRAB

Although cannibalism can act as a density-dependent regulator of popul ation size in terrestrial systems, little is known of its effects in t he marine environment. Herein we investigate the influence of cannibal ism upon the early life history stages of the blue crab, Callinectes s apidus Rathbun, emphasizing cannibalism between juveniles and postlarv ae (i.e. megalopae) of the same year class. In laboratory mesocosms we examined various factors modulating cannibal-prey dynamics, specifica lly: (1) the effects of habitat and presence of conspecifics on postla rval metamorphosis rate; (2) the effect of metamorphosis rate on the m ortality of postlarvae from both intra- and inter-cohort cannibalism; (3) the effects of habitat and predator density on the functional resp onse of young juvenile blue crab predators to varying densities of pos tlarval prey, and (4) the effects of prey size and habitat on predatio n mortality. Inter-cohort cannibalism caused significant mortality in every crab size and habitat type combination, and was lower in grass t han sand for all prey smaller than fifth instar. Cannibalism between p ostlarvae was associated with metamorphosis and was density-dependent in sand, but not present in grass. Metamorphosis rates of postlarvae w ere inversely density-dependent in sand, but density-independent and h igher in grass, indicating that habitat and intra-cohort agonism likel y affects postlarval metamorphosis rates. Inter-cohort cannibalism was negatively correlated with metamorphosis rates of postlarvae. The fun ctional response of young juvenile cannibalistic blue crabs differed s ignificantly between sand and grass habitats, and between medium and h igh 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 dis played a weak type III, density dependent response, yielding significa ntly lower mortality at low prey densities. Thus, habitat complexity c hanges the form of the functional response in cannibal-prey interactio ns and grass provides a relative habitat refuge from cannibalism. Doub ling the number of predators in grass decreased the consumption rates per predator significantly and eliminated the density-dependence, indi cating that intraspecific density can qualitatively change the form of the functional response. In the crab size experiment, only prey small er than fifth instars received a habitat refuge from cannibalism in gr ass, whereas fifth instars received a relative size refuge in sand. Ou r results demonstrate that intra-year class cannibalism can cause mort ality upon settling megalopae and first juvenile instars that is depen dent on prey density. We expect inter-cohort cannibalism to cause loca l extinction of cohorts settling in sand, especially at low settlement densities, and high mortality at moderate settlement densities in gra ss. Satiation of predators at high settlement densities in grass sugge sts that episodic settlement can overwhelm predators locally. Furtherm ore, 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 neces sary lead to an increase in predation pressure. Finally, a relative si ze-refuge from inter-cohort cannibalism for fifth instar crabs support s an ontogenetic habitat shift around this crab size, which may be inf luenced by density-dependent agonistic behavior within cohorts. We sug gest that intra-year class cannibalism is a major process regulating b oth survival and dispersal in megalopae and juvenile blue crabs. (C) 1 997 Elsevier Science B.V.