Tissue reaction of Tagelus plebeius (Bivalvia: Psammobiidae) against larval digeneans in mixohaline habitats connected to the south-western Atlantic

Abstract

This paper describes the organism–organism relationship between a bivalve host and larval digeneans. The studied population of the stout razor clam Tagelus plebeius from the mixohaline Mar Chiquita lagoon (37° 32′S 57° 19′W) showed 100% of prevalence of infection by a larval digenean (metacercaria) of the family Gymnophallidae. The larvae occupied the extrapallial space just below the hinge of the bivalve. The tissue reaction against larval digeneans by T. plebeius consisted of hyperplasia and metaplasia of the outer layer of the mantle epithelium adjacent to parasites, forming an invagination to generate a sac that surrounded gymnophallid larvae. According to the intensity of infection, each sac progressively enlarged to harbour a variable number of larvae. In highly infected clams, a great number of sacs are formed. Eventually, the older sacs containing larvae may become closed, losing their communication with the extrapallial space and sinking into the dorsal part of the visceral mass. Larvae within sacs grow and remain alive until they reach an appropriate definitive bird vertebrate host. Older lesions commonly showed remains of dead larvae which had undergone resorption. As a result, an orange to brownish amorphous material accumulated in the space once occupied by the larvae leaving conspicuous orange marks on the inner surface of the valves. In some cases, calcifications in the form of pearls or blisters were observed.