An endogenous circadian hatching rhythm in the monogenean skin parasite Entobdella soleae, and its relationship to the activity rhythm of the host (Solea solea).

1. When the eggs of the skin parasitic monogenean Entobdella soleae are subjected in the laboratory to alternating periods of light and darkness (LD 12:12, LD 6:18 or LD 18:6) at constant temperature (selected from the range 12-16 °C), hatching is rhythmical, most of the oncomiracidia emerging during the first 4 h of the period of illumination on each successive day. 2. Rhythmicity in response to alternating 12 h periods of light and darkness persists in a wide range of lighting conditions including natural daylight and in dim blue light, the latter being similar in intensity (65 nW/cm) and in spectral quality to light at the depths where the host Solea solea lives. 3. The hatching rhythm persists with a periodicity of about 24 h in constant darkness and in constant light, provided that the eggs have been exposed previously to LD 12:12 until hatching begins. This indicates that there is a strong endogenous component to the rhythm. 4. Eggs laid and maintained in total darkness develop and hatch, but the oncomiracidia may emerge at any time during each 24 h period and there is little evidence of rhythmicity. 5. When alternate 12 h periods of light and [darkness are reversed after the commencement of hatching, the hatching pattern also reverses after 24 h, so that emergence of larvae occurs at the beginning of the new illumination period. 6. Mechanical disturbance plays little or no part as a hatching stimulus. 7. Larvae, which had emerged from their eggs at some time between 20 and 80 min previously, responded to host skin, suggesting that an initial uninfective period is of short duration or absent. 8. Hatching soon after dawn may have high survival value for E. soleae in view of the host's activity pattern; soles are active at night and partly bury themselves in the sediment (where the eggs of the parasite lie) at dawn, and the fish is inactive and a stationary target during the day.