Hatching rhythms in three species of Diclidophora (Monogenea) with observations on host behaviour

Eggs of three species of Diclidophora were incubated in alternating 12 h periods of light and darkness at 13 degrees C. Eggs of D. merlangi collected at Arbroath hatched during the illumination period with most larvae being recovered in the first 4-6 h; some evidence of a seasonal difference in hatching of these eggs was found. Eggs of D. merlangi collected at Plymouth hatched with a peak of larval recovery in the 2 h period before the light came on. Eggs of D. luscae hatched over 'dusk' while those of D. denticulata hatched after the light was switched off. Neither mechanical disturbance nor the proximity of host tissue caused hatching in D. merlangi or D. luscae. Observations on the behaviour of the host fishes suggest that the hatching rhythms are adapted to specific host behaviour patterns.