Living Fossil: Tiny mollusc makes big impression on marine biology world

Cover image:  Live monoplacophoran sighted during a remotely operated vehicle (ROV) dive on Utu Seamount in the waters of the American Samoa, February 2017. Image courtesy of NOAA Office of Exploration and Research.

A February 2017 dive by the NOAA Ship Okeanos Explorer yielded an exciting discovery. Scientists spotted a live monoplacophoran, a rarely observed type of mollusc that is thought to be the closest living relative of the ancestors of modern day bivalves (e.g. clams and mussels) and gastropods (e.g. snails).

For years, monoplacophorans were known only as fossils. Traces of their shells have been found in rocks from the earliest years of the Paleozoic era – 345 million years ago. This changed on May 6, 1952, when a Danish deep-sea trawling research expedition off the coast of Costa Rica unexpectedly hauled up ten living specimens from a depth of 3590 meters, each only about 3 cm long. This sudden discovery of an organism long thought to have been extinct was a shock, and scientists hailed it as “the most dramatic [discovery] in the history of [mollusc science].”

Monoplacophoran fossil. Image courtesy of the Museum of Comparative Zoology and Harvard University.

Since this initial discovery in 1952, a handful of other current-day monoplacophoran specimens have been discovered and at least 5 species have been described. A live specimen remains an extremely rare sight. These small, single-shelled animals inhabit deep ocean environments, which means that their habitats are difficult to access. Live specimens have been collected at depths ranging from 2000 meters to more than 6000 meters. Interestingly, some monoplacophoran fossils are associated with relatively shallow environments, which suggests that they may have more recently evolved to live in the deep ocean.

Anatomy of a typical monoplacophoran. They are “limpet-like” with a single shell. They underside of the animal would not have been preserved through the fossilization process, so obtaining live specimens allows scientists to investigate and analyze these otherwise unknown internal features. Monoplacophoran anatomy image courtesy of Ivy Livingstone, © BIODIDAC.

Studying live monoplacophorans has given scientists new insight on the development of major invertebrate groups. The discovery of live specimens of an animal previously thought to be extinct allows scientist to make observations beyond what can be preserved through the fossil record. The rarity of this organism, along with the story of its rediscovery and evolutionary significance, made its recent sighting a highlight of both the ROV dive and the February 2017 cruise as a whole.

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