TIDAL RHYTHMS OF ACTIVITY AND PHOTOTAXIS OF AN ESTUARINE CRAB LARVA

Abstract

1. Tidal rhythms in swimming speed and phototaxis were measured in stage III zoeae of the crab Rhithropanopeus harrisii. Larvae were captured in the Newport River estuary, North Carolina, and maintained under constant conditions in the laboratory. 2. Swimming speed increases during rising tide with a maximum several hours after low tide, and decreases to a minimum about 2 hr after high tide. It is argued that this pattern is the basis of the endogenous tidal vertical migration, in which the larvae ascend on rising tides and descend on falling tides. 3. The tidal rhythm in phototaxis consists of an increase in negative responsiveness which is strongest around the middle of the rising tide, and a decline to a minimum during falling tides. Positive phototaxis shows the opposite pattern. There is no convincing evidence that phototaxis participates in vertical migration. It is hypothesized that the phototactic pattern represents a tidal rhythm in a shadow response used for predator avoidance.