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

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A bivalve mollusc, baptized Cymatia cookie, was previously only known from Pleistocene fossils. But this tiny clam was recently spotted alive on a sandy beach off the California coast.

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

After snapping several photos of these animals without unduly disturbing them, he sent those photos to Paul Valentich-Scott, Curator Emeritus of Malacology at the Santa Barbara Museum of Natural History, who was just as surprised as he was. ” I know this family of mussels (Galeommatidae) from the coast of America very well. It was something I had never seen before “, he says. The photos are not enough to identify the animal with certainty, it was essential to capture it in order to examine it more closely. But it wasn’t until a few months later, and after many attempts, that Goddard finally found a specimen of this strange mollusc.

Similarities to a fossil record

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

Lateral (A) and ventral (B) views of a live specimen caught at Naples Point (length: 7.4 mm). (CD) Cymatia cookie in its natural environment. © P. Valentich-Scott et al.

Illustrations of the seashell Bornia biscuitfound in an article by paleontologist George Willett were actually very similar to the clam they discovered. Note that Willett was documenting mollusc fauna at Baldwin Hills in central Los Angeles when a sewer line was installed. The work uncovered a 20 to 30 cm thick Pleistocene deposit 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 identified a total of 296 mollusk species and described two new species of Galeommatidae mussels. Rochefortia reyana and Bornia biscuit (today Cymatia cookie), say the researchers. At the time, Willett named the species after Edna Cook, a shell collector who collected the specimens.

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

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

A search that will undoubtedly lead to more specimens

Finally, the researchers have so far found three living specimens of this species: the two individuals observed in 2018 and the specimen captured in 2019. C. Cooki thus joins the list of “Lazarus taxa”, which includes species that were long thought to be extinct and were then rediscovered. ” Finding a known species alive for the first time in the fossil record is not that common. ‘ remarked Goddard. How did this mollusc escape the watchful eyes of experts for so long? ” There is such a long history of shell collecting and malacology in Southern California […] It’s hard to believe that the shells of this little creature haven’t even been found yet “, he added.

Their preferred habitat is probably farther south off the Baja California peninsula, which could explain why no one had noticed its presence C. Biscuit so far. ” Given C. cooki’s small size, translucent shell, and cryptic habits, it’s 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 redescription, more individuals of the species will surely be discovered soon, 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 in the form of plankton larvae via ocean currents, particularly during the marine heat waves of 2014 to 2016. The latter allowed many marine species to expand their distribution northward, particularly at Naples Point for some of them.

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

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