Abstract: The crab resources in the offshore waters of midsouthern East China Sea (26°00′–28°30′ N, 121°00′–126°00′ E) were investigated in May (spr
ing), August (summer), November (autumn) 2006 and February (winter) 2007, with the characteristics of their community structure, including species composition, index of relative importance (IRI), diversity, and biomass analyzed. A total of 49 species were identified, belonging to 30 genera and 12 families. In both spring and autumn, the crab species number was higher in northern  than in southern part of the Sea; in summer, it had a relatively uniform distribution; while in winter, less difference was observed between the southern and northern parts of the Sea, and the crabs were mainly concentrated in the exterior areas. A total of 13 dominant species (IRI>1000) with Charybdis bimaculata, Calappa lophos, Charybdis miles, Ovalipes punctatus, Carcinoplax longimana, Charybdis riversandersoni, and Portunus argentatus as the representatives appeared all the year round. The diversity index of the crabs was lower in summer, but had no significant difference among the other three seasons. The catching mass of the crabs in the investigated sea area was averagely 5197.62 g·h^-1, being 8830.79 g·h^-1 in summer>4877.50 g·h^-1 in spring > 4268.30 g·h^-1 in autumn > 2813.87 g·h^-1 in winter. The crabs in the offshore waters of mid-southern East China Sea could be categorized as 3 ecological communities, i.e., eurythermal and eurysaline, hyperthermal and eurysaline, and hyperthermal and hysaline.

Key words: crabs, community structure, dominant species, diversity, offshore waters of mid-southern East China Sea, Brachionus calyciflorus, sibling species, temperature, algal density, fitness, Jiulian Pond.