A comparative study of the egg morphology of four species of Eubothrium (Cestoda: Pseudophyllidea) with comments on their early development

Abstract

Freshly released eggs from four species of the cestode Eubothrium (Eubothrium crassum, Eubothrium fragile, Eubothrium rugosum, and Eubothrium salvelini) were subjected to morphological and morphometric analysis. The eggs of the two freshwater species, E. rugosum and E. salvelini, were ovoid with a lobed embryophore whereas the eggs of the two marine species, E. crassum and E. fragile, were more circular with a smooth embryophore. However, the morphological differences between species were not readily evident to permit their clear distinction from one another. To discriminate species, a forward stepwise linear discriminant analysis, using six of the seven measured metric characters made on the eggs, was used, which gave 100% correct classification of two species, E. rugosum and E. salvelini, and a high proportion of correct classification for E. crassum (98%) and E. fragile (83%). Of the latter two species, one specimen of E. crassum and five specimens of E. fragile were misclassified between the respective groups. The principal characters used in the classification of the species were the width of the egg, the length of the mediolateral hooks, and the width of the oncosphere. To provide more information on the life cycle of each species, the eggs were used in a series of infection trials to identify appropriate intermediate hosts. Experimental infections with freshwater copepods were successful when exposed to the eggs of E. salvelini, partially successful when exposed to the eggs of marine E. crassum with 10% of the copepods becoming infected, but no infections were obtained when the eggs of E. fragile were used.