Hatching rhythms in the the capsalid monogeneans Benedenia lutjani from the skin and B. rohdei from the gills of Lutjanus carponotatus at Heron Island, Queensland, Australia.

Spontaneous hatching of eggs of Benedenia lutjani and B. rohdei occurred after incubation for 4-6 days and 6-10 days, respectively, at a constant temperature in the range 22-28 degrees C when exposed to natural illumination or to alternating 12-h periods of light and darkness (LD12:12; light on, 06.00 h; light off, 18.00 h). Under these conditions, hatching of the eggs of both species was rhythmical, all larvae emerging only during periods of illumination. Hatching was not confined to particular times with the illuminated period. Evidence for an endogenous component to the rhythm was revealed by transfer of eggs from LD12:12 to continuous darkness (DD) near the end of the incubation period. Hatching, also occurred only during periods of illumination when eggs of each species were incubated under a DL12:12 regime (i.e. period of illumination 18.00 h to 06.0 h; period of darkness 06.00 h to 18.00 h). When the eggs of B. lutjani and B. rohdei were laid and incubated in DD or continuous illumination (LL), some degree of rhythmicity persisted, raising the possibility that the eggs inherit circadian rhythmicity from their parent. The hatching patterns of these 2 species of monogeneans are discussed in relation to host finding, host behaviour and limited observations on the behaviour of the oncomiracidia after hatching.