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Dynamics of shell disease in the edible crab Cancer pagurus: a comparative study between two sites on the Gower Peninsula, South Wales, UK.

Shell disease syndrome is a degradative condition of the crustacean exoskeleton which results in the formation of black-spot lesions. Field surveys on the edible crab Cancer pagurus (L.) population at 2 sites in the Gower Peninsula of South Wales, UK, namely Langland Bay and Rhossili causeway, revealed similarities in the prevalence of shell disease but differences in the severity. Male crabs from Langland Bay displayed significantly greater severities of the disease on the dorsal carapace and ventral surfaces than those in Rhossili. The greatest severities of the disease in Langland were attributable to sand abrasion injuries during back-burrowing behaviour and prolonged exposure to high numbers of chitinolytic bacteria. Lower severities of shell disease in males from Rhossili were a reflection of the inability to back-burrow due to the nature of the onshore sediment, and hence the crabs were only exposed to low levels of chitinolytic bacteria in the water column. On areas of the causeway that permitted back-burrowing, the chitinase enzyme activities in sediments were significantly lower than those in Langland Bay. It is concluded that the potential for crustacean shell degradation is higher at Langland than Rhossili, but whether the differences in chitinolytic abilities of the sediments at the 2 sites are a result of differences in pollution levels or simply of different substratum types, remains to be determined.