Abstract
Evidence is accumulating about the way strategic decisions over an organism's lifetime may depend on its state and on external circumstances. In the parasitic copepod Pachypygus gibber, sex is environmentally determined when the free-living infective stage (Copepodid 2) enters the host, the sea squirt Ciona intestinalis. Whereas in a rich trophic environment the Copepodid 2 develops into a typical male or into a typical female (both are unable to swim and must remain inside the host), in poor trophic conditions it develops into an alternative free-living stage able to swim and to leave the host, the atypical male. Thus this copepod life cycle is characterised by 2 fundamentally different free-living stages. We conducted an experimental study to compare the priority decisions of the Copepodid 2 and of the atypical male in their habitat selection behaviour. We showed that while Copepodid 2s are mainly attracted by chemicals released by the host into the water, atypical males only respond to female and typical male cues. We discuss the adaptiveness on this behavioural difference in light of the ecological constraints met by the different stages of P. gibber.