A new clam: Acesta cryptadelphe

Credit: © Fisheries and Oceans Canada

Dr. Jean-Marc Gagnon: This is the culmination of a story that began decades ago when, as a Ph.D. student, I first observed this clam in an underwater submersible off the coast of Newfoundland.[that was 1984] Originally, we assumed it to be a European species.

In recent years, more samples have been collected off the Grand Banks and in a marine protected area "The Gully" about 220 km off the coast of Nova Scotia. Researchers form the Bedford Institute of Oceanography used ROPOS - an underwater vehicle with cameras and manipulator arms - to collect samples. Through DNA analysis coupled with comparative studies of other giant file clams in museum collections, Gagnon and his colleagues determined these north Atlantic specimens to be a new species.

The giant file clam, about 9 to 15 cm long, is two to three times larger than a regular file clam (so-named because of the sharp ridges on the shell surface). This creature attaches to steep, rocky outcrops in canyons that are home to other deepwater species such as cold-water corals. The clam's scientific name, Acesta cryptadelphe, means "cryptic sibling," which refers to the similarity in shape and structure to the previously described European giant file clam, Acesta excavata.

For the experts: We analyze the morphological and genetic variability within and between seven species of Acesta and specimens recently collected in the northwest Atlantic using traditional morphological measurements, landmark-based geometric morphometrics, and the cytochrome oxidase subunit I (COI) gene sequences, with particular emphasis on North Atlantic species. Shell morphology and external shell appearance do not allow reliable distinction between the widely recognized northeastern Atlantic A. excavata and other northwest Atlantic species or populations of Acesta, with the exception of A. oophaga. Similarly, shape analysis reveals a wide variability within northeastern Atlantic A. excavata, and significant morphological overlap with A. bullisi from the Gulf of Mexico and A. rathbuni from the southwestern Pacific and South China Sea. Specimens from the northwestern and Mid-Atlantic display shell shapes marginally similar to that of A. excavata. These differences are at least partly related to anterior or posterior shifting of the shell body and to the opposite shifting of the hinge line/dorsal region and upper lunule. These morphological variations, along with the midline-width-ratio, explain much of the variability extracted by principal component analysis. Results from a mitochondrial DNA barcode approach (COI), however, suggest that the northwest Atlantic specimens belong to a new species for which we propose the name Acesta cryptadelphe sp. nov. Differences in larval shell sizes between northeastern and northwestern Atlantic specimens are consistent with this result.