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-inseminat e when maturity is reached and self-insemination has not been recogniz ed during many hours spent observing living parasites. E. soleae lives on the lower surface of its flatfish host, the common sole (Solea sol ea), in relatively small numbers, most commonly 1, 2 or 3 adults per f ish, but in spite of the low density and extensive habitat, freshly co llected 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 p artners as a result of death or transfer to another host and that para sites have an efficient way of locating a mating partner. Mathematical modelling indicates that random locomotion with searching movements o f the body would lead to mating contacts between 2 individuals on a sm all sole within the reproductive life-time of the parasites, but is un likely to maximize reproductive output because it cannot ensure that m ating will occur soon after sexual maturity is achieved. It is suggest ed that pheromonal attraction may ensure early meetings and that the t hin stagnant layer of sea water between the lower surface of the relat ively inactive sole and the sea bottom is an ideal environment for the operation of such a system and may account for the preference of adul t E. soleae for the host's lower surface.