Scientists have discovered a new bivalve species that prefers living on the body surface of earthworm-like sea cucumbers.

Most of the bivalve species can be found in sand, mud or attached to rock surfaces. However, the newly discovered species of bivalve, described in a paper published in the journal ZooKeys, was found attached to the body of a sea cucumbers living in mudflats at the mouth of the Souzu River in southwestern Shikoku Island, Japan.

The new bivalve, dubbed as Borniopsis mortoni (Galeommatoidea), uses its foot and byssal threads to attach itself to the body surface of its potential host, an earthworm-like sea cucumber called Patinapta ooplax (Synaptidae). Reaching only up to 4.1 mm in length, B. mortoni is considered to be one of the smallest species of the genus.

B. mortoni is characterized by its elongated ovate shells covered by a tan to dark brown periostracum. Researchers believe that the small size of the B. mortoni is adapted to fit in the very narrow burrows of sea cucumbers, which is most likely used to shelter them from predators.

According to a press release, several numbers of B. mortoni can be observed in a single host, but sometimes, more than 10 individuals can occur together. To determine if the bivalves discovered in Souzu River belongs to a new species, the researchers collected samples and compared its morphology and hosts association with the other Borniopsis species. Molecular phylogeny has partially confirmed the new species. With the discovery, there are now three Borniopsis species that present commensal associations with synaptids.

The discovery was made by Ryutaro Goto, a postdoctoral fellow at the Museum of Zoology and Department of Ecology and Evolutionary Biology, University of Michigan, and Hiroshi Ishikawa, an amateur malacologist in Japan. B. mortoni was named in honor of British malacologist Brian Morton, a professor at University of Hong Kong who has described many interesting Pseudopythina species from mudflats in Hong Kong, now assigned to the genus Borniopsis.

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