Egg hatching in 3 species of monocotylid monogenean parasites from the shovelnose ray Rhinobatos typus at Heron Island, Australia

Eggs of Neoheterocotyle rhinobatidis, Troglocephalus rhinobatidis and Meriz ocotyle icopae (Monogenea: Monocotylidae) from the shovelnose ray Rhinobato s typus (Rhinobatidae) have a distinct hatching pattern linked to light per iodicity. Larvae of these 3 monogenean species emerge only during daylight when exposed to natural illumination or when incubated in alternating 12 h periods of light and dark (light on 06.00 h, light off 18.00 h). N. rhinoba tidis larvae emerge with a distinct peak during the first 2 h of light; thi s peak is nor as pronounced in T. rhinobatidis or M. icopae. Eggs of N. rhi nobatidis incubated in a reverse light/dark cycle (light on 18.00 h, light off 06.00 h) hatched only during periods of illumination, again with a peak during the first 2 h of light. Evidence suggests that the hatching pattern s observed in all 3 species represent true circadian rhythms because eggs i ncubated in 24 h light or 24 h dark conditions continued to hatch with a rh ythm. Shadows, disturbance and host tissue did not promote hatching in N. r hinobatidis or T. rhinobatidis but there were indications that host tissue may promote hatching in M. icopae. The hatching patterns observed are discu ssed with respect to their adaptive responses to host behaviour and predati on pressure.