Rediscovery of a Caribbean living fossil: the bivalve Pholadomya candida

Pholadomya candida Sowerby, 1823 is an anomalodesmatan bivalve belonging to the ancient family Pholadomyidae, which is a group of burrowing bivalves living on earth since at least the Early Carboniferous (330 million years B.P) and reached a high degree of diversification in Jurasic to Cretaceous times. The genera Panacca and Pholadomya are probably the only living representatives of this ancient lineage. A few Recent species of Panacca occur mainly in deep, cold waters in the southern Pacific, whereas Pholadomya candida is the only living species of the nominal genus and occurs in shallow settings at certain places in the Caribbean; additionally, it is relatively well represented in the fossil record of Northern South America since the Miocene (ca. 15 million years B.P.).

P. candida had been collected alive only twice, at least in the sense of being available for scientific purposes; both specimens were found before 1842 and presumably in the same area (Virgin Islands). Since living specimens had not been recorded for nearly 140 years, some authors considered that the species was extinct, but new discoveries of fresh-looking shells from Venezuela and Colombia provided evidence that it still lives.

On January 26, 2008, while diving in an inlet of the National Natural Park Tayrona, not far from the city of Santa Marta, at the Colombian Caribbean coast, Dr. Juan M. Díaz, the Colombian CoML country coordinator, detected a pair of small holes on the sandy bottom that called his attention: the size and shape of the openings were characteristic of the long, large, and bifid siphonic tubes of anomalodesmatan bivalves that live burrowed in the sediment. Carefully using his diver knife for digging out the hidden animal, a treasure came out: a specimen of the living fossil Pholadomya candida (see photo).

The importance of this specimen is extraordinary: it is the third specimen ever collected alive and a definitive proof that the species is not extinct. Moreover, of the other two specimens collected before, one was presumably destroyed along with many thousands of specimens of the collection and documents when the Bristol Museum was blitzed in 1940, and the other, at the Zoological Museum of the University of Copenhagen, was dissected and anatomically described by Bryan Morton in 1980; the recently found specimen, now at the Marine Natural History Museum of INVEMAR, at Santa Marta, of which small pieces of tissue were removed and deposited in the Tissue Bank of the Institute Alexander von Humboldt, is therefore the only one that is now available for genetic studies.