The prevalence of the pea crab, Pinnotheres sinensis, and its impact on the condition of the cultured mussel, Mytilus galloprovincialis, in Jiaonan waters (Shandong Province, China)

Incidence of Pinnotheres sinensis, a parasitic pea crab of the cultured mussel, Mytilus galloprovincialis, was investigated at Jiaonan, Shandong Province, China during December 2000 to December 2002. The impact of the pea crab on the host was evaluated by comparing the index of condition (percentage meat yield) of infested and uninfested mussels. The 25-month inspection revealed that the prevalence of infestation varied from 1.9% to 62.1%, with the lowest and highest prevalence occurring in July 2001 and November 2002, respectively. The number of pea crabs found in each mussel was 1 to 6, and most of the infested mussels were infested by single female (53.8%), or single male (21.4%), or a couple of a female and a male crab (18.1%). The monthly average intensity varied from 1.00 to 1.44 (total average: 1.18). Evaluated by the index of condition, female crabs showed significant impact on the condition of mussels (P < 0.05), but male pea crabs did not (P > 0.05). In 23 of 25 months, the mussels infested by female crabs had the index of condition significantly lower than that of uninfested individuals (P < 0.05), and the female crabs caused more deleterious effect to the mussels in summer than that in other seasons.