Size Distribution in Spider Crab Populations: Spatial and Temporal Variation

The size distribution of the spider crabs Hyas coarctatus and Inachus dorsettensis was investigated in several areas close to the Isle of Man. Samples were collected in 1958-1959 and again in 1988-1990. One area was sampled during both periods. On both occasions each species displayed clear differences between areas in the size distribution of the terminal instar. This is attributed to variation in the size at which the terminal pubertal molt occurred; the contributions of genetic and environmental factors to this variation are discussed. In prepubertal specimens males exceeded females, but females predominated among postpubertal crabs. This swing in sex ratio is discussed. In the area sampled in both periods, the abundance of each species declined to about a fifth of that recorded 30 years ago. This is considered a result of heavy scallop dredging throughout that interval. An expected decrease in the size of the terminal instar was not observed, and, in fact, an increase occurred in Inachus dorsettensis. Possible explanations are considered.