A mussel species thought to have been extinct for 40,000 years has been observed alive

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A bivalve mollusk, baptized Cymatioa cooki, was until now only known through fossils from the Pleistocene. But this little clam was recently spotted alive on a sandy beach off the coast of California.

In November 2018, while searching for sea slugs at low tide on Naples Point Beach, Jeff Goddard, a research associate at UC Santa Barbara’s Marine Science Institute, came face to face with an intriguing pair of tiny translucent clams. The latter had a shell of barely 10 millimeters and a longer foot, with white and shiny stripes. Although Southern California bivalves have been widely studied and documented, Goddard had never encountered this species before.

After taking several pictures of these animals, without disturbing them unduly, he sent these pictures to Paul Valentich-Scott, curator emeritus of malacology at the Santa Barbara Museum of Natural History, who was as surprised as he was. ” I am very familiar with this family of bivalves (Galeommatidae) along the coast of America. It was something I had never seen before “, he tells. The pictures are not enough to identify the animal with certainty, it was important to capture it in order to examine it more closely. But it was not until several months later, and after many attempts, that Goddard finally found a specimen of this strange mollusk.

Similarities to a fossil record

The two researchers could then begin their identification work. The animal’s particularly atypical shell raised the possibility that it was a new species never before described. To confirm this, Valentich-Scott conducted a careful study of all the scientific literature published from 1758 to the present day. One fossil species, described in 1937, particularly attracted the attention of the two researchers.

Lateral (A) and ventral (B) view of live specimen captured at Naples Point (length: 7.4 mm). (C, D) Cymatioa cooki in its natural environment. © P. Valentich-Scott et al.

Illustrations of the bivalve Bornia cookie, found in an article by paleontologist George Willett, was actually very similar to the shell they had discovered. Note that Willett documented the mollusc fauna at Baldwin Hills in central Los Angeles when a sewer line was installed. The work revealed a Pleistocene deposit 20 to 30 cm thick of invertebrate and vertebrate fossils, just over a meter below the surface. These deposits dated between 36,000 and 28,000 years ago.

In his publication, the scientist had recognized a total of 296 species of mollusks and described two new species of Galeommatidae bivalves, Rochefortia Reyana and Bornia cookie (Today Cymatioa cooki), say the researchers. At the time, Willett named the species after Edna Cook, a shell collector who collected the specimens.

type of sample
The type specimen used by George Willett to describe the species Bornia cookie in 1937. © P. Valentich-Scott et al.

After comparing their mollusk to the original specimen described by Willett, housed at the Natural History Museum of Los Angeles County, the researchers concluded that it was indeed the same species. At the same time, Goddard found another specimen at Naples Point, a single empty shell, which also fully matched Bornia cookie.

A search that will undoubtedly lead to other specimens

Finally, researchers have so far found three living specimens of this species: the two individuals observed in 2018 and the specimen captured in 2019. C.Cookie thus joins the list of “Lazarus taxa”, which includes species long considered extinct, then rediscovered. ” It is not that common to find a known species alive for the first time in the fossil record. Goddard noted. How could this mollusk have escaped the watchful eye of experts for so long? ” There is such a long history of shell collecting and malacology in Southern California […] it is hard to believe that no one has found even the shells of this little creature “, he added.

Their preferred habitat is probably located further south, off the Baja California peninsula, which could explain why no one had noticed the presence of C. cookies earlier. ” Given C. cooki’s small size, transparent shell and cryptic habits, it is no surprise that living specimens of the species have been neglected for over 80 years. “, write the researchers, who are convinced that thanks to their new description, other individuals of the species will undoubtedly soon be discovered, especially further south off the coast of Mexico.

According to the two researchers, it is possible that these mussels came up from the south to Naples Point via ocean currents, in the form of planktonic larvae, especially during the marine heat waves of 2014 to 2016. These eventually allowed many marine species to expand their distribution northward, specifically at Naples Point for several of them.

Source: P. Valentich-Scott and J. Goddard, ZooKeys

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