Mysterious Deep-Sea Animal Rediscovered after 116 Years

Bathochordaeus charon — an extremely rare species of giant larvacean — has been rediscovered, more than a century after the only previous known specimens of the species were found.

Bathochordaeus charon and its ‘house’ in Monterey Bay: giant larvaceans use their oikoplastic cells to secrete complex filters or ‘houses’ that allow them to concentrate and feed on particles; a ‘house’ consists of a large, diaphanous outer structure as well as a smaller, more convoluted and bi-lobed inner structure that functions as a filter; together these serve to concentrate appropriately-sized food particles from the surrounding water; in situ, larvaceans in the genus Bathochordaeus are often visible from several meters away because their houses may span a meter in longest dimension. Image credit: MBARI.

Larvaceans are solitary, free-swimming tunicates, animals with a primitive spinal cord but no real backbone.

These fragile animals are usually less than 0.4 inches (1 cm) in length, but some giant larvaceans in the deep sea grow up to 3.5 inches (9 cm) long.

In 1900, German marine biologist Carl Chun identified the first giant larvacean, Bathochordaeus charon.

Scientists from the Monterey Bay Aquarium Research Institute (MBARI) were next to document this species, 116 years later, after a routine sample collection turned extraordinary.

“Larvaceans in the genus Bathochordaeus are large, often abundant filter feeders found throughout much of the world ocean,” the researchers said.

“The first described species, Bathochordaeus charon, was reported over 100 years ago by Chun.”

“However in the time since, few specimens have matched Chun’s original description, resulting in ambiguity on the validity of B. charon as a species.”

According to the team, larvaceans are not easily collected in nets.

“At MBARI we are fortunate to be able to have very skilled pilots using modern remotely operated vehicles (ROVs) and advanced sampling equipment,” said Dr. Rob Sherlock, MBARI senior research technician.

“What’s amazing is that they were able to collect an animal in the 1890s using the technology of the time and still able to make a great drawing,” he said of the first giant larvacean discovery.

Several specimens of Bathochordaeus charon were collected by MBARI’s ROVs Ventana, Tiburon and Doc Ricketts.

“Using ROVs we have carefully observed and collected Bathochordaeus charon as well as B. stygius,” Dr. Sherlock and his colleagues said.

“A combination of morphological features, house structures and molecular evidence clearly distinguish the two species and provide the first records of B. charon from Monterey Bay as well as off the coast of Oregon, expanding its range into the eastern North Pacific Ocean.”

Details of the research were recently published in the journal Marine Biodiversity Records.

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R.E. Sherlock et al. 2016. The first definitive record of the giant larvacean, Bathochordaeus charon, since its original description in 1900 and a range extension to the northeast Pacific Ocean. Marine Biodiversity Records 9: 79; doi: 10.1186/s41200-016-0075-9

This article is based on a press-release from the Monterey Bay Aquarium Research Institute.