EGG HATCHING IN THE MONOGENEAN GILL PARASITE DISCOCOTYLE SAGITTATA FROM THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Discocotyle sagittata displays a clearly defined nocturnal egg-hatchin g rhythm. When eggs of D. sagittata were incubated in alternating 12 h periods of light and darkness at 13 degrees C, the majority of larvae hatched within the first 2 h of darkness. Larvae were rarely recovere d after 4 h of darkness, and none hatched in the light. Reversal of th e light/dark cycle resulted in immediate reversal of the hatching rhyt hm, so that larvae emerged at the beginning of the new dark period. Th is suggests that hatching is a direct response to exogenous cues and n ot endogenously driven. In continuous darkness hatching was arrhythmic ; hatching occurred in continuous light although total numbers of larv ae emerging were suppressed. Field experiments confirmed that eggs exp osed to the natural day/night cycle throughout development also demons trated the precise hatching rhythm seen under controlled conditions. M echanical disturbance played no part as a hatching stimulus. However, larvae hatched in the presence of both host mucus and gill tissue. D. sagittata does not show an egg-laying rhythm. The hatching responses h ave adaptive advantage related to the host's activity pattern. Trout a re generally visual predators, inactive in darkness, providing a stati onary target for emerging larvae responding either to darkness or to h ost hatching factors.