Parasite-mediated shell alterations in Recent and Holocene sub-Antarctic bivalves: the parasite as modeler of host reaction

Abstract. New data on shell reactions elicited by larval digeneans in bivalves from Recent sub-Antarctic populations and late Holocene Patagonian deposits are reported. Shell alterations, which are traces of digenean trematode infections, were found affecting intertidal bivalve populations from Malvinas (Falkland) Islands, Burdwood Bank, Beagle Channel, and from Holocene deposits at Tierra del Fuego (Argentina). The bivalve species involved belonged to the families Nuculanidae, Cyamiidae, and Neoleptonidae. Such reactions consisted of quite unusual dome or igloo-shaped calcifications on the inner shell surface; the similarities and uniqueness of this reaction in different bivalve species reported here suggest that the invasive agent is the same. Based on previous findings of morphologically identical shell alterations in Gaimardia trapesina (Bivalvia, Gaimardiidae) from Magellanic and sub-Antarctic waters, it is suggested that the parasites responsible for the traces reported here belong to a digenean platyhelminth species of the Gymnophallidae genus Bartolius . The host bivalves reported here belong to three different superfamilies, and share a similar crystalline shell microstructure: aragonite with homogeneous structure. After a review of the available information dealing with bivalve shell-mantle reactions against digeneans, it is hypothesized that parasites are responsible for the modeling of the host response they elicit. However, although the specific characteristics of the reaction depend on the parasite, they would probably be constrained by some characteristics of the host shell structure.