Studies on the life history ofEnteroxenosBonnevie, a gastropod endoparasitic in aspidochirote holothurians

Abstract

The gastropod, Enteroxenos oestergreni, an entoparasite of the holothurian Stichopus tremulus, was studied at two different depths in the Oslofjord (1965-75). 34.5-36.5 % of the host animals were infected. 82.5-85.5 % of the parasites occurred on the host's oesophagus, 8.0-9.5 % on the stomach, the remaining ones on other viscera. 110-429 egg capsules. with a total of21 000-135 000 eggs are produced, all at one time. The population reproduces throughout the year, but each female produces only one batch of eggs in its life. Before or shortly after oviposition the parasites detach from the host's viscera and become free in the body cavity. They are probably expelled in connexion with the annual evisceration of the host. Larval development takes about one year, spent mostly within the capsule, as the ftee life seems to be very short. At metamorphosis shell and operculum are cast off. Completion of metamorphosis in the male larva occurs on an epithelial petiole (male receptacle) in the female's central cavity. To reach this the larva has to pass ftom the lumen of the host's gut through a ciliated tubule, which thereafter soon closes, making entrance of further larvae impossible. The larva's epidermis is lost and replaced by extroverted intestinal cells, and the larva is absorbed into the tissue of the receptacle. Later the young male looses all its tissues except for the rudimentary testis, which expands considerably, and a small area which develops into a vas deferens. The testis is incorporated in the receptacle by apposition of its surrounding basement membrane to that of the receptacle. It is assumed that extroversion of the intestine also occurs in the metamorphosing female larva and that the external epithelial lining of the parasite represents the extroverted intestine. The males in the allied Thyonicola are also implanted in the females. By allowing a continuous and greatly enlarged sperm supply, this condition has made a tremendous increase in the fecundity of the parasites possible.