AGGREGATIVE MATING OF TANNER CRABS, CHIONOECETES BAIRDI

In April 1991, a high-density mating aggregation of Tanner crabs, Chio noecetes bairdi, was discovered at 150 m depth near Kodiak, Alaska, wi th the research submersible DSRV Delta. The aggregation consisted prim arily of oldshell, multiparous female crabs which formed mounds 1-2 m in diameter, 0.5-1.0 m high, and spaced at intervals of 1-2 m. Mounds contained hundreds of crabs each at densities >100.m(-2) Male crabs we re found mating with females at the periphery of the aggregation, with sex ratios (male to female) varying from 1:10 to 1:100. The entire ag gregation covered an area of about 2.2 ha and included approximately 1 00 000 crabs. No mounds were observed in May 1992, but recently spawne d female crabs were found buried in the sediment at high densities (up to 2.2m(-2)) over an area of about 25 ha, suggesting that aggregation had reoccurred at the same location. Newly ovigerous females exhibite d a cycle of nocturnal activity and diurnal burial whereas unmated cra bs remained exposed on the sediment surface. Many body pans and presum ably dead crabs were observed. We conclude that aggregative mating is a major mode of reproduction in the family Majidae.