First anaerobic multicellular animal

The first confirmed anaerobic multicellular animal is the fish parasite Henneguya salminicola, a tiny multicellular species of myxozoan, belonging to the taxonomic order Cnidaria, which also contains jellyfishes, coral and sea anemones. Unlike all other animals, its cells do not contain any mitochondria, the organelles that convert oxygen into energy. Consequently, via some form of biochemical mechanism not yet discovered, it apparently obtains its energy via anaerobic (non-oxygen-driven) means instead, presumably deriving it from its host species, which comprise various species of salmon in Alaska, USA. The discovery of the parasite's unique physiology was described in the journal PNAS on 24 February 2020.

As reported in BMC Biology on 6 April 2010, three species of loriciferan (microscopic but multicellular marine invertebrates) were discovered living buried within deep-sea, oxygen-lacking sediments at the bottom of the Mediterranean Sea, and were assumed, therefore, to respire anaerobically. However, they still possess mitochondria, albeit highly modified versions known as "hydrogenosomes", and some biologists say that genomic studies are needed to confirm that these loriciferans really have lost the ability to respire oxygen. So at least for now, H. salminicola is the only genomically confirmed anaerobic multicellular animal.