Life Cycle and Seasonal Migrations

Abstract

Seasonal and life cycle migrations are mass movements in which individuals move horizontally for long distances to encounter favorable conditions for reproduction and development. Such migrations have been best studied in larger mobile decapod crustaceans, many of which are commercially important. Some decapod shrimps and brachyuran crabs are dependent on productive estuaries for completion of life cycles. In these species, planktonic larvae develop in oceanic waters. Postlarval stages utilize currents and appropriate behaviors to enter estuaries via selective tidal stream transport (STST). After growth, juveniles and subadults leave for the adult oceanic habitats, again using STST. Many subtropical and temperate zone neritic species make seasonal offshore migrations into deeper waters during the winter, with return nearshore in the spring; some high latitude species make these migrations but with seasons reversed. Numerous freshwater shrimps are amphidromic, that is, they live and reproduce in streams and rivers, but their planktonic larvae drift or are released directly into the sea for development and dispersal. Postlarvae find the mouths of streams, and then make spectacular mass migrations as juveniles back upstream to the adult habitat. Adults of terrestrial crabs live inland, but brooding females move into the littoral zone during new or full moon periods to hatch out larvae into high amplitude tides that carry the larvae out to sea for development and dispersal.