Speleonectes

Xibalbanus tulumensis (previously Speleonectes tulumensis – Speleonectes = 'cave swimmer'; tulumensis = 'occurring at Tulum' – Xibalbanus = 'from Xibalba') is a venomous, hermaphroditic crustacean found in anchialine caves on the Yucatán Peninsula in the Caribbean Sea. This blind remipede liquefies the body contents of other crustaceans with a venom similar to that of rattlesnakes, and which includes digestive enzymes and a paralysing toxin.

In her 1987 description, Smithsonian biologist Jill Yager wrote 'Body elongate, slender, without eyes or pigment. Cephalic shield small, tapering slightly at anterior end. Trunk segment numbers increasing with age, maximum number examined was 36 segments'. The numerous legs are used as oars for swimming. The species is 25–30 millimetres (1.0–1.2 in) long, and found in cenotes close to the sea, with salty seawater lying beneath the fresh water, and grading smoothly from one type to the other (halocline).