Previously it has been shown that eggs of the skin-parasitic monogenean Entobdella soleae, maintained free of host contamination at constant temperature (12 °C) and exposed to alternating 12 h periods of light and darkness, hatch during the first few hours of each period of illumination as a result of endogenous processes in the larva in conjunction with the illumination cycle. Washings, prepared by immersing the parasite's host (the common sole, Solea solea) for 1 h in seawater (just sufficient to cover the body), enhance this ‘morning’ hatching. Furthermore, the application of host washings during the latter half of the period of illumination or during the dark period also stimulates hatching. Experiments have shown that the hatching factor is present in sole skin mucus, is not destroyed by boiling for 5 min or by freezing and is produced by small soles measuring 4–5 cm in length as well as by larger soles (15–21 cm long).When eggs are incubated at about 12°C in the absence of fish washings, hatching begins about 30 days after laying, but there is evidence that some of the larvae inside their shells are fully developed and capable of hatching in response to host hatching factor 1–3 days earlier.When fish washings are added to eggs which range in age from 25 to more than 30 days (at 12 °C) many eggs are stimulated to hatch (in addition to ‘morning’ hatching as a result of endogenous rhythmical processes and the illumination cycle) and others fail to hatch. These remaining unhatched eggs, in the absence of further treatment with host hatching factor, will complete their development and subsequently will hatch during the ‘morning’ hours in response to the illumination cycle. Further contact with host hatching factor is likely to stimulate the hatching of some of these remaining eggs.The host hatching factor is not specific to S. solea; washings from plaice, dab, halibut, whiting and ray induce hatching in E. soleae. In experiments in which oncomiracidia were offered scales from dab and sole, most of the attached larvae were found on sole skin, irrespective of whether the hatching stimulant had been provided by dab or by sole.The relationship between the behaviour of the common sole and hatching phenomena in the skin parasite is discussed; hatching rhythms and the use of host hatching factors adapt the parasite to take advantage of most opportunities to infect the host.
Critical lengths, mortality rates and relative yield per recruit of the common sole Solea solea from the Egyptian Mediterranean coast off Alexandria
