A newly identified giant isopod species offers clues to how these deep-sea crustaceans spread around the world, including a possible return to the Americas via Antarctica.
A giant crustacean brought from Puerto Rico to Germany as a teaching specimen has turned out to belong to a species unknown to science. Tübingen paleontologist Professor Dolf Seilacher (1925–2014) acquired the animal at a research station in Puerto Rico in the 1980s. The dried specimen, rediscovered in storage at the University of Tübingen’s Paleontological Collection, represents a species still living today.
Paleontology student and isopod expert Benedikt Kästle and the collection’s curator, PD Dr. Ingmar Werneburg of the university’s Senckenberg Center for Human Evolution and Palaeoenvironment, examined the animal using advanced imaging techniques. They identified it as a new species of giant isopod and compared its anatomy with that of other giant isopods around the world.
Deep-sea giants that resemble woodlice
These marine crustaceans resemble the much smaller woodlice found in caves and damp basements. Their bodies are armored with an exoskeleton made of calcium-rich, scale-like segments. Members of their genus, Bathynomus, inhabit all the world’s oceans, living as omnivores at depths reaching 2,500 meters, or about 8,200 feet. They can grow up to 50 centimeters, or nearly 20 inches, long and weigh several kilograms, placing them among the largest invertebrates.
“Our Tübingen specimen from Puerto Rico, measuring 32.2 centimeters in length, falls into the category of ‘colossogiants’,” says Kästle.
The researchers named the species Bathynomus tuebingoborikensis, combining a reference to their university town with Boríken, the Indigenous Taíno name for Puerto Rico. Werneburg points to the discovery as evidence of the value of natural history collections as global archives of nature. Their study and species description, published in Scientific Reports, are expected to help scientists understand the broader evolution of deep-sea isopods.
When Panama was a seaway
To investigate that history, Kästle and Werneburg carried out a phylogenetic analysis, reconstructing evolutionary relationships among species. Their results suggest that Bathynomus originated in what is now the Gulf of Mexico during the Miocene, more than 15 million years ago. The route out of that region was different from the geography familiar today.
“At that time, the Isthmus of Panama was open, leaving a strait between North and South America, and the marine animals were able to spread westward, initially as far as Japan,” says Werneburg.
As they spread through the Indian and Pacific Oceans, the isopods diversified into many new species. “This gave rise to the so-called ‘giant’ isopods that are up to 15 centimeters long, and the ‘supergiants’ with a body length of more than 15 centimeters,” adds Kästle. Their expansion also took them to Australia and East Africa, and, before the collision of the Arabian and African continental plates, to Europe, where they became extinct at the end of the Miocene.
Colossogiants emerge as Earth cools
A distinct group emerged in the Indo-Pacific as Earth cooled at the boundary between the Miocene and Pliocene epochs, 5.333 million years ago. “These veritable colossi grew to more than 25.2 centimeters in length and were physiologically perfectly adapted to the new climatic conditions,” says Werneburg.
The researchers propose that these larger isopods spread from Australia via Antarctica to South and Central America. There, they encountered smaller, more distantly related isopods whose ancestors had remained behind during the genus’s original westward expansion. The Puerto Rican specimen belongs to this larger group, and its anatomical relationships connect it to another species in the region.
“The new species is very closely related to another giant species found off the Yucatán Peninsula in Mexico,” Kästle says.
Reference: “A new Caribbean species of the giant isopod genus Bathynomus (Crustacea, Isopoda, Cirolanidae) with notes on its evolutionary history” by Ingmar Werneburg, and Benedikt Kästle, 8 October 2026, Scientific Reports.
DOI: 10.1038/s41598-026-69097-y
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