Ecology of the velvet swimming crab Liocarcinus puber (L.) (Brachyura: Portunidae)
Currently, Liocarcinus puber is commercially fished in the United Kingdom and exported to Southern Europe. The species appears particularly vulnerable to overfishing and has a history of overexploitation. The present study has examined the reproduction, growth and diet of L. puber, to provide information on which to manage this fishery. Immature Liocarcinus puber were first observed in late summer at a size of 6-10mm long carapace width (LCW). Growth in immature crabs was rapid, and sexual maturity occurred after approx. one year at 45mm (for males) and 40mm (for females) LCW. To differentiate the modal growth of mature crabs, probit analysis and computer models (ELEFAN) have been used. The life expectancy for L. puber is estimated at between 4-6 years, with males attaining a maximum size of approx. 95-100mm LCW and females 85-90mm LCW. From the end of the second year, moulting in both sexes was annual, and occurred in early summer for males and later in the year (generally late summer to autumn) for females. The reproductive cycle was also strongly seasonal with mating occurring at the time of the female moult. Evidence from the seasonal occurrence of different egg stages, and the ovarian development of ovigerous females, suggests that older females (II+ years) produce more than one brood each breeding season (January to July). Stomach analyses established that the diet of Liocarcinus puber contained quantities of both animal and algal material, however, algae were the most abundant prey item. Animal prey items included a broad range of benthic organisms and variation in the diet was noted between depth zones, with the availability of prey items appearing to largely determine the diet. Analysis of laminarinase activity in the hepatopancreatic tissue showed levels comparable to the highest recorded for any crustacean. L. puber can survive for extended periods, and moult on an algal diet, however, it appears that an algal diet contains insufficient protein to fully support long-term growth in this crab.