Larval Behaviour of Macrobrachium novaehollandiae (De Man) (Decapoda, Palaemonidae)

Larvae of the estuarine prawn M. novaehollandiae tested in the laboratory using point sources and gradients of light are strongly photopositive (tropotactic) during the first 2 of their 12 larval stages. Despite this response few larvae swim near the water surface for long periods of time while illuminated from above, and more larvae swim during darkness than in light. Stage X larvae and subsequent stages are photonegative but stages 111-IX may be either photopositive or photonegative. Most larvae of all stages rest on the bottom for long periods, a mechanism which can prevent large-scale seawards movement down an estuary. Other biotic factors such as salinity, pressure and temperature have no marked influence on orientation and movement of the larvae.