Circatidal activity rhythms in ovigerous blue crabs, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>: implications for ebb-tide transport during the spawning migration

Female blue crabs (Callinectes sapidus Rathbun) with mature embryos have a spawning migration in which they: (1) undergo ebb-tide transport for movement seaward from estuaries, (2) release their larvae, and (3) reverse direction by undergoing flood-tide transport for up-estuary movement. The study determined whether ebb-tide transport during the spawning migration is based upon an endogenous rhythm in vertical migration. Under constant conditions in a rectangular container, which limited horizontal and vertical movements, females with young and mature embryos had circatidal rhythms (periods=12.11-12.95 h) in migratory restlessness (swimming activity) and egg maintenance behavior (abdominal pumping). However, the rhythms were out of phase, as migratory restlessness occurred during the expected time of ebb tide in the field, and egg maintenance behavior, during the time of flood tide. Under constant conditions in vertical columns (1.32 m high), crabs with mature embryos had a circatidal rhythm (periods=12.2-13.7 h) in which they had frequent bouts of swimming to the surface of the column during the expected time of ebb tide in the field and remained on the bottom during the time of flood tide. This rhythm was not present in crabs with young embryos and disappeared after larval release. Thus, an endogenous rhythm in vertical migration does underlie the ebb-tide transport behavior of ovigerous blue crabs with mature embryos during their spawning migration.