The enigmatic bivalve genus Paramya (Myoidea: Myidae): symbiotic association of an East Asian species with spoon worms (Echiura) and its transfer to the family Basterotiidae (Galeommatoidea)

Abstract

Paramya is an enigmatic genus of Myidae (Bivalvia: Heterodonta: Myoidea) that includes three uncommon species: Paramya subovata (Conrad, 1845), Paramya recluzi (A. Adams, 1864) and Paramya africana Cosel, 1995. Paramya subovata is known as a commensal living in the burrow of the spoon worm Thalassema hartmani Fisher, 1947 (Annelida: Echiura: Thalassematidae), in North American coastal waters. However, the biology of the other two species remains unknown. In this study, we found P. recluzi living in the burrows of the two thalassematid echiuran species, Ikedosoma gogoshimense (Ikeda, 1904) and Arhynchite hayaoi Tanaka & Nishikawa, 2013, in intertidal flats in the Seto Inland Sea, Japan. Paramya recluzi was embedded in the burrow wall with its short siphons protruding into the host burrow lumen for respiration and filter feeding. To determine the phylogenetic position of P. recluzi, we performed a molecular phylogenetic analysis using the 18S, 28S, COI and H3 genes. Molecular analysis showed that P. recluzi belongs not to the family Myidae, but to the genus Basterotia (Galeommatoidea: Basterotiidae). Morphological examination of P. recluzi revealed that this species has many similarities with Basterotia (e.g. a single cardinal tooth on each valve and short siphons surrounded by tentacles). Thus, we propose that this species should be transferred from the genus Paramya to Basterotia. In addition, we also suggest that the other two species of Paramya (P. subovata and P. africana) should be transferred to the family Basterotiidae based on their shell morphology, anatomy and ecological characteristics.