THE INFLUENCE OF LIGHT IN THE DEPTH REGULATION OF CRAB LARVAE

¹ Department of Zoology, Duke University, Durham, North Carolina

1. Experiments were conducted to determine the phototaxis responses of each larval stage of two species of Xanthid crabs and to assess the influence of light on depth regulation.

2. Two primary orientations in response to light are described.

3. Secondary orientation in response to light results in positive phototaxis in all four zoea stages and the megalopa of each species.

4. In response to changes in light intensity, the first zoea stage of P. herbstii demonstrated high photokinesis. Although some increase in swimming rate in response to increased light intensity was noted in larvae of L. floridanus, particularly in the first zoea stage, the differences were not statistically significant. It is suggested that larvae of L. floridanus may swim at their capacity at low light intensities, in contrast to larvae of P. herbstii.

5. Light response dominates that of gravity in the first zoea stage; in the megalopa, gravity response dominates that of light in a majority of individuals.

6. Light aimed down the axis of the observation tank exerts a negligible effect upon the net vertical distribution of the first zoea stage. However, the net distribution of later larval stages is shallower in light than is the case in darkness.

7. The response of the larvae to light thus superimposes diurnal vertical migration upon the basic pattern of differential vertical distribution through ontogeny. These two characteristics of larval development will result in considerable vertical scattering during the dispersal phase.