New giant deep-sea isopod discovered in the Gulf of Mexico

Researchers have identified a new species of Bathonymus, the famous genera of deep-sea isopods whose viral internet fame has made them the most famous aquatic crustaceans since Sebastian’s “The Little Mermaid.”

There are about 20 species of living Bathonymus, a mysterious and primitive group that inhabits the benthic zone of the ocean – the deepest reaches, rarely explored in person. Isopod crustaceans are only distantly related to their more famous decapod relatives, the crabs, shrimp and lobsters.

Publishing their findings in the Natural History Magazinea group of Taiwanese, Japanese and Australian researchers unveil the newest creature on this list: B. yucatanensis, a new species that is about 26 cm long, some 2500% larger than the common louse.

Deep-sea isopods belong to the same group that includes the terrestrial isopods known as woodlice, pillbugs, and roly polys, which feed on decaying matter and are probably familiar to anyone who has lifted a rock or dug in the yard . Indeed, they are quite similar, but due to their extraordinary size – the largest grows to almost 50 centimeters. And like wood louse, although they may look a little scary, they are completely harmless to humans.

Their odd features and unusual size have spawned endless memes and a range of products celebrating their endearing craziness, from plush toys to phone cases.

This finding of B. yucatanensis brings a new addition to the isopod pantheon and brings the total of known Bathonymus species in the Gulf of Mexico to three: B. giganteus was described in 1879 and B. maxeyorum was described in 2016.

It was initially thought to be a variant of B. giganteus, one of the largest of the deep-sea isopods. But a closer look at the specimen, which was caught in a bait trap in 2017 in the Gulf of Mexico off the Yucatán Peninsula about 600 to 800 meters below, revealed a series of unique features.

“B. yucatanensis is morphologically distinct from both B. giganteus and B. maxeyorum,” the authors claim.

Held by the Enoshima Aquarium in Japan, the individual studied was subtly different from its relatives. “Compared to B. giganteus, B. yucatanensis has more slender body proportions and is shorter in overall length … and the pereopods [thoracic limbs] are slimmer,” the researchers note. It also has longer antennae. The two species have the same number of pleotelson spines. These spines protrude from the tail end of the crustacean.

“Bathynomus giganteus was discovered more than a century ago and more than 1,000 specimens have been studied so far without any suggestion of a second species with the same number of pleotelsonic spines,” they add. “Superficial research, using only pleotelson spines, can easily lead to specimens of B. yucatanensis being misidentified as B. giganteus.”

“Compared to B. maxeyorum, the most distinguishing feature is the number of pleotelson spines — 11 spines in B. yucatanensis versus 7 in B. maxeyorum.” The blotchy, creamy yellow color of the shell further set it apart from its grayer relatives.

To be sure, the scientists performed a molecular genetic analysis comparing B. giganteus and B. yucatanensis. “Due to the different sequences of the two genes (COI and 16S rRNA), coupled with differences in morphology, we identified it as a new species,” they write. The phylogenetic tree they constructed showed B. yucatanensis as most closely related to B. giganteus.

“B. giganteus is indeed the closest species to B. yucatanensis,” the authors claim. “This indicates that the two species probably had a common ancestor. In addition, there may also be other undiscovered Bathynomus species in the tropical western Atlantic.”

The article also clarifies that South China Sea specimens identified as B. kensleyi are actually B. jamesi. B. kensleyi is restricted to the Coral Sea, off the coast of Australia.

“It is becoming increasingly clear that species of Bathynomus in general can be extremely similar, as well as that there is a long history of misidentification of species in the genus,” the authors warn.

They note that these newly established species distinctions have conservation implications. “Some species of Bathynomus with commercial potential have become targets of deep-sea trawling,” they say. Although giant isopods are only sporadically exploited, “for managing the Bathynomus fishery it is important to know exactly which species are being caught.”