A massive, ‘creamy yellow’ relative of Woodlouse was found living at a depth of about 600 to 800 meters (2,000 to 2,600 feet), off the Yucatán Peninsula.
Scientists have identified a new species of Bathonymus. The Little Mermaid.
There are about 20 types of life Bathonymus, a mysterious and primitive group that inhabits the benthic zone of the ocean – its deepest reaches, rarely explored in person. Isopod crustaceans are only distantly related to their more familiar decapod siblings, the crabs, shrimps and lobsters.
A group of researchers has just revealed the latest creature to this list – B. yucatanensis, a new species that is about 26 cm (10 in) long. This makes it about 2,500% larger than the usual wood louse. The scientists, from Taiwan, Japan and Australia, published their findings on August 9 in the peer-reviewed Journal of Natural History.
Deep-sea isopods belong to the same group that includes the terrestrial isopods known variously as woodlices, pillbugs, and roly polys. These feed on decaying matter and are probably familiar to anyone who has picked up a rock or dug around in the garden. Indeed, they look quite similar but for their extraordinary size – the largest of them grow to almost 50 centimeters (20 inches). And, just like woodlice, although they may look a little scary, they are completely harmless to humans.
This find of B. yucatanensis adds another addition to the isopod pantheon and brings the total of known species of Bathonymus 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 variation of B. giganteus, one of the largest of the deep-sea isopods. However, closer examination of the specimen, which was caught in the Gulf of Mexico off the Yucatán Peninsula in 2017 at about 600 to 800 meters (2,000 to 2,600 feet) below, revealed a variety 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 lobster.
“Bathynomus giganteus was discovered more than a century ago, and more than 1,000 specimens have been examined without so far suggesting a second species with the same number of pleotelson spines,” they add. “Superficial research, with only pleotelson spines, could easily result in specimens of B. yucatanensis is misidentified as B. giganteus.”
“Compared to B. maxeyorumThe most distinctive feature is the number of pleotelson spines – 11 spines in it B. yucatanensis against 7 in B. maxeyorum.”
The mottled, creamy yellow color of the shell further distinguishes it from its grayer relatives.
To be sure, the researchers carried out a molecular genetic analysis that compares 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 built 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 be other undiscovered ones as well Bathynomus spp. in the tropical western Atlantic.
The paper also clarifies that specimens from the South China Sea identified as B. kensleyi are actually B. jamesi. B. kensleyi is restricted to the Coral Sea, off the coast of Australia.
“It’s becoming more and more clear that kind of thing Bathynomus can be very similar in general appearance, and also 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 implications for conservation. “Some species of Bathynomus with commercial potential have become the targets of deep-sea trawling,” they say. While giant isopods are only sporadically exploited, “for the management of Bathynomus fishing, it is important to know exactly which species are caught.”
Reference: “A new kind of Bathynomus Milne-Edwards, 1879 (Isopoda: Cirolanidae) from the southern Gulf of Mexico with a redescription of Bathynomus jamesi Kou, Chen and Li, 2017 from Pratas Island, Taiwan” by Ming-Chih Huang, Tadashi Kawai and Niel L. Bruce, 9 August 2022, Journal of Natural History.
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