Cannibalism in juvenile blue-swimmer crabs Portunus pelagicus (Linnaeus, 1766): effects of body size, moult stage and refuge availability

Abstract

Factors influencing the rate of cannibalism in juvenile blue-swimmer crabs Portunus pelagicus were investigated under controlled conditions using time-lapse video recordings. This study was undertaken to improve blue-swimmer crab culture and experimentally addressed (1) prey vulnerability (2) cannibal–victim interactions, and (3) activity patterns of juveniles in varying degrees of refuge. Crabs used in the study were aged 15 weeks and sorted into two size classes; small (≤60mm carapace width (CW)) and large (≥65mm CW) of a similar sex ratio. Vulnerability and thus survival was influenced by body size variation, moult stage and refuge availability. Crabs with carapace width ≤ 60mm were more vulnerable than larger individuals, as indicated by significant differences in survival rates. As predicted, juveniles in transition stages associated with ecdysis were especially vulnerable. Premoult (redliner) crabs appeared to be in a high state of agitation as evidenced by the frequency of agonistic encounters and this may be a contributing factor to the high mortality observed at this critical premoult stage. Increases in refuge density increased survival of juveniles proportionally, indicating that the quantity of shelter is important for reducing cannibalism in this species. Cannibal–victim interactions were frequently asymmetrical in terms of size and moult stage. Cannibals were significantly heavier than victims, and were predominantly at intermoult stage. Sexual biases among cannibals and victims were not found in this study. Activity patterns of juveniles were influenced by the experimental conditions. Crabs provided with high refuge showed reduced aggressive activity and increased time spent resting, but unchanged locomotion or feeding activity. Regular grading as well as the presence of suitable shelter for newly moulted crabs is recommended for improving culture of P. pelagicus. Research into inducing synchronous moulting may also yield promising results.