The structure of the Cryptocotyle lingua miracidium (Digenea: Opisthorchiidae)

Abstract

In the course of Digenea evolution the transitions to a passive strategy of infection of the first intermediate host occurred multiple times. In these cases, miracidia – ciliated larvae – relinquished their role as active seekers for a suitable snail host and began relying on the chance of being accidently ingested by it. The structural changes resulting from these transitions are diverse, but the majority of them are reductive. In this study we present our ultrastructural survey on the opisthorchioid Cryptocotyle lingua miracidium. The transition to passivity led to extensive reduction in almost all somatic elements of the larva: only four epithelial plates cover its body; just a single nervous cell, a single protonephridium and a single longitudinally-oriented muscle cell have been found in this miracidium. Several undifferentiated cells comprise the germinal material of the larva. The greatest part of its body is occupied by three glandular elements. The position of these glands is consistent within Opisthorchioidea and strikingly corresponds to that of some of the optically studied miracidia of Plagiorchioidea. This similarity is suspicious, because the C. lingua larva appeared to differ vastly from other ultrastructurally described passive miracidia. Here we create a basis for the future comparison of the miracidia of Opisthorchioidea and Plagiorchioidea, expecting the ultrastructural data on the latter to be published someday.