Insemination and population density in Entobdella soleae, a monogenean skin parasite of the common sole, Solea solea.

Immature specimens of the monogenean skin parasite Entobdella soleae, experimentally isolated from other individuals, fail to self-inseminate when maturity is reached and self-insemination has not been recognized during many hours spent observing living parasites. E. soleae lives on the lower surface of its flatfish host, the common sole (Solea solea), in relatively small numbers, most commonly 1, 2 or 3 adults per fish, but in spite of the low density and extensive habitat, freshly collected wild parasites were found to be inseminated. If it is assumed that E. soleae is unable to self-inseminate in the wild, then it seems likely that single isolated parasites on wild soles have lost their partners as a result of death or transfer to another host and that parasites have an efficient way of locating a mating partner. Mathematical modelling indicates that random locomotion with searching movements of the body would lead to mating contacts between 2 individuals on a small sole within the reproductive life-time of the parasites, but is unlikely to maximize reproductive output because it cannot ensure that mating will occur soon after sexual maturity is achieved. It is suggested that pheromonal attraction may ensure early meetings and that the thin stagnant layer of sea water between the lower surface of the relatively inactive sole and the sea bottom is an ideal environment for the operation of such a system and may account for the preference of adult E. soleae for the host's lower surface.