Ontogenetic changes of phototaxis, vertical distribution, body density and external morphology during larval development in the horsehair crab Erimacrus isenbeckii reared in the laboratory

¹Akkeshi Laboratory, Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Akkeshi, Hokkaido 088-1108, ²Department of Marine Biosciences, The Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, ³General Planning and Coordination Department, Headquarters, Fisheries Research Agency, Yokohama, Kanagawa 220-6115, Japan

 The larval sinking phenomenon is a cause of larval mass mortality during seed production of the horsehair crab Erimacrus isenbeckii. As a basis for developing larval rearing techniques to prevent this phenomenon, we examined the larval phototaxis, vertical distribution, body density and external morphology such as carapace spines, which may provide drag force while larvae are sinking in seawater. Larvae showed positive phototaxis in the first zoeal stage, and showed strengthened negative phototaxis after the second zoeal stage. Higher incidences of negative phototactic behavior were recorded at longer wavelengths of more than 620 nm in all larval stages. Newly hatched larvae floated in the vertical test chamber, but they tended to sink in the chamber one day after hatching. Larval body density was greater than the specific gravity of the seawater. The length of carapace spines tended to decrease with larval growth. The horsehair crab larvae thus showed a behavioral habit that makes them sink in seawater under the light condition. Further studies should examine methods for rearing horsehair crab larvae under dark conditions or under light of longer wavelengths from the bottom of the vertical chamber.