VERTICAL MIGRATION RHYTHMS OF NEWLY HATCHED LARVAE OF THE ESTUARINE CRAB, RHITHROPANOPEUS HARRISII

¹ Duke University Marine Laboratory, Beaufort, NC 28516
² Department of Zoology, Duke University, Durham, NC 27706

Zoea larvae of the estuarine crab Rhithropanopeus harrisii were maintained in constant conditions in the laboratory, and their vertical migrations were followed for two or more days. Larvae which hatched in the laboratory, but which underwent embryonic development in an estuary having semidiurnal tides, often expressed circatidal rhythms in vertical migration. However, first-stage zoea larvae collected by plankton net in the same estuary had circatidal vertical migration rhythms of much greater amplitude and with a constant phase with respect to the natural tidal cycle. Laboratory-hatched larvae of crabs from an estuary with aperiodic tides had more variable vertical migrations, and field-caught larvae from the same habitat never expressed clear migration rhythms. When reared to the third zoeal stage in the laboratory under a diel light:dark cycle, larvae from both estuaries usually migrated arhythmically under constant conditions. Vertical migration rhythms of larvae of this species appear to be strongly predisposed to entrainment by natural tidal cues. Such migrations probably contribute to estuarine retention of the developing larvae.