A tidal rhythm in phototaxis of larval grass shrimp(Palaemonetes pugio)

Abstract

An endogenous phototaxis rhythm was measured in newly hatched larvae from the estuarine shrimp Palaemonetes pugio. Shrimp were maintained in constant conditions and phototaxis tested using a 500‐nm light stimulus. Larvae exhibited large temporal variations in both positive and negative phototaxis, with peaks in responsiveness at 12—14h intervals. A constant phase relationship was observed between these peaks and tidal cycles in the field, regardless of the diel timing of the tides. Least squares fits of the data to a sine wave demonstrate a strong circatidal cycle in photoresponsiveness. The circa tidal pattern persists for 2–3 cycles in constant conditions, and is still expressed in larvae after 3 tidal cycles of embryonic development in the laboratory. We conclude that an endogenous circatidal rhythm affects the photobehavior of P. pugio larvae. If coupled with a tidal cycle in vertical migration in the field, the negative phototaxis rhythm could help the larvae avoid predators such as ctenophores.