RELATIONSHIP BETWEEN THE PEA CRAB PINNIXA-TUMIDA AND ITS ENDOBENTHIC HOLOTHURIAN HOST PARACAUDINA-CHILENSIS

The ecology of the pea crab Pinnixa tumida living in the endobenthic h olothurian Paracaudina chilensis, which buries its body in the fine sa ndy bottom in the sublittoral zone, was investigated. Almost all of th e pea crabs collected from their host holothurians were mature females . Mature pea crabs were found to live singly in the alimentary canal o f the larger hosts, which are mainly distributed just below the low wa ter level of the spring tide. However, immature crabs were not found i n either large or small hosts. In the laboratory, mature crabs searche d for the tip of the host's tail, which appeared on the sand's surface , and then entered the host's alimentary canal through the opening of the anus at the tail's tip. These results indicate that the pea crab e nters larger hosts only when it is mature, or just. before maturity. P ea crabs showed 2 types of feeding preference in the laboratory: suspe nded particles and the mucus secreted in the alimentary canal of the h ost. Ovigerous females were found in the hosts between late February a nd mid-May, carrying eggs which developed slowly and synchronously wit h those of almost all of the other ovigerous females observed througho ut this period, indicating that the pea crab has an annual breeding cy cle. The biology of the pea crab was compared with that of crabs livin g in bivalves and on irregular echinoids.