Taking a closer look leads to rediscovery of a prevalent deep-sea animal
November 21, 2016
Larvaceans are very common marine animals but most people have never heard of them. These tadpole-like relatives of “sea squirts” are usually less than a centimeter in length, but some “giant” larvaceans in the deep sea grow up to nine centimeters long.
In 1900 a scientist identified the first giant larvacean and created a detailed drawing. He named it Bathochordaeus charon, after the mythical ferryman who transports souls of the dead across the river Styx.
MBARI scientists were next to document B. charon, a century later, after a routine sample collection turned extraordinary.
“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,” Rob Sherlock, MBARI senior research technician, said of the first giant larvacean discovery.
Ever since then, scientists collected specimens, took photos, and made drawings, yet struggled to either find or identify another specimen that had features that fit with the original giant larvacean. This cast doubt as to the accuracy of the original description, and that confusion has persisted, leading many scientists to question the certainty of B. charon as a species.
“In many ways we know a lot more about the moon than we do about life in the ocean,” said Sherlock. Sherlock and MBARI Senior Scientist Bruce Robison are interested in larvaceans because of the important role these animals play in transporting food into the deep sea. During their ongoing Midwater Time Series project they’ve documented countless larvaceans.
A giant larvacean was collected during a typical dive using MBARI’s remotely operated vehicle (ROV) Ventana. Near the end of the dive, in the ROV control room, Sherlock watched the monitors as the ROV ascended through the water. When a particularly large larvacean came into view Sherlock asked the ROV pilots to stop and collect the animal. Back in the lab at MBARI, Sherlock took a closer look at the larvacean under a microscope. At about nine centimeters, it was exceptionally large. At first Sherlock was puzzled because it didn’t look right. Then he realized the animal was B. charon, and was thrilled. He went straight to Robison’s office, “We found B. charon. It exists!” Robison put down the paper he was reading, took a look…and agreed.
Part of the reason for the confusion is that these fragile animals are not easily collected in nets. “At MBARI we are fortunate to be able to have very skilled pilots using modern ROVs and advanced sampling equipment,” said Sherlock. Their work collecting the first giant larvacean that could definitively be called Bathochordaeus charon since 1900 is documented in a recent paper.
After Sherlock’s sighting, MBARI researcher Kristine Walz reviewed 25 years of archived ROV video, looking for similar larvaceans. She found 12 more observations of these animals. Sometimes it just takes a closer look to see what’s been there all along.
Article by Teresa L. Carey
Original journal article:
R. E. Sherlock, K. R. Walz, and B. H. Robison (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, DOI:10.1186/s41200-016-0075-9