An unusual habitat for bivalves: rediscovery of the enigmatic commensal clam Sagamiscintilla thalassemicola (Habe, 1962) (Bivalvia: Galeommatoidea) from spoon worm’s spoon

Abstract

Sagamiscintilla thalassemicola (Bivalvia: Galeommatoidea: Galeommatidae sensu lato) is a white-robed commensal clam that lives on the body surface of spoon worms (Annelida: Echiura). This species was described based on specimens collected from the spoon worm Anelassorhynchus mucosus (Thalassematidae) in Amakusa, Kyushu, Japan, in 1962, but there has been no reliable record since the first description. Thus, some researchers suspected that this species may have become extinct. In this study, we rediscovered this species from the spoon-shaped proboscis of the echiuran Anelassorhynchus sp. in Shikoku and Kyushu Islands, Japan. Although the original description did not mention the details of its host use pattern, we found that this species lives almost exclusively within the proboscis gutter of the host, where it is almost hidden from the outside. Such a cryptic habitat is probably the reason why this species has remained undiscovered for over 50 years. Living hidden in the host probosces probably provides an anti-predation benefit for S. thalassemicola. The host echiurans create ciliary water currents on the proboscis gutter to transfer food particles into the mouth, and the bivalves can utilize this water currents for respiration and filter feeding. Sagamiscinitlla thalassemicola is distinctive among ectocommensal galeommatoideans in having a mantle that fully covers the shell valves. It may be a morphological adaptation to protect the shell valves from sand and other hard particles transferred through the proboscis gutter of the host.