Article Phototactic behaviour and the nature of the shadow response in larvae of the estuarine crab <i>Sesarma catenata</i>

Abstract

The phototactic behaviour of the first zoea of the estuarine crab Sesarma catenata (Brachyura, Grapsidae) was studied in the laboratory. Light and dark-adapted larvae showed only positive phototaxis when stimulated horizontally with light intensities ranging from 10^-7 W m^-2 to 1 W m. Peak percentage response was at 10^-1 W m^-2 and 10^-4 to 10^-2 W m^-2 in light and dark-adapted larvae respectively. This pattern differs from that common to other brachyuran larvae in the absence of negative phototaxis at low light intensities in light-adapted larvae. The shadow response in S. catenata first zoea was studied in a natural light field. The nature of the shadow response is discussed according to larval responses in the horizontal plane. An interpretation of the mechanics of the shadow response is proposed which does not involve negative phototaxis. The consequences of the phototactic behaviour of larvae for dispersal in the Swartkops estuary are briefly discussed. According to their phototactic pattern, larvae would migrate to the top section of the water column and aggregate there during the day. This would increase their chances of being exported out of the estuary with the ebb tides.