Tails from the deep

Cartoonish-looking rattail discovered in Indian Ocean

Coelorinchus zinjianus, lateral view.  Scale bar = 30 mm.  From Prokofiev & Iwamoto (2023)

Specimens of a cartoonish-looking new fish are deposited in the Shirshov Institute of Oceanology of the Russian Academy of Sciences in Moscow.  They were collected during Soviet expeditions north and south of Madagascar in the 1980s.  This new fish joins more than 120 species in the genus Coelorinchus, including about twenty species inhabiting the western Indian Ocean.

Its name, zinjianus, is derived from Zinj, the ancient Arabian name for East Africa and nearby islands.  Its long snout, not entirely supported by bone along its anterolateral margin, is one of several features shared by this and three related species in the flabellispinis group.

These rat-tailed fishes are members of the Macrouridae, a family of deep-sea fish marked by slender bodies, whiplike tails, and inordinately large heads bearing a single barbel on the chin and forward projecting snouts.  Most macrourids occupy the demersal zone of the water column near the seabed at depths from 200 to 2000 meters, but some are found as deep as 7000 meters.  Weak swimmers, they are, as a group, distributed from the Arctic to the Antarctic and are among the most common fish of the deep.  Some are solitary while others swim in large schools.

Within the family, mature fish vary from less than 4 inches to nearly 7 feet in length.  The new species is nearer the lower end of this range, being from about 5 to nearly 9 inches in length. Efforts have been made to introduce some larger species of the family to the marine fisheries market, but so far they are considered unpalatable.  This is just as well for the fish whose populations do not seem to be very resilient and could be depleted were they to be overfished.

These fish are peculiar in a number of ways beyond their looks.  They are bioluminescent with an abdominal light organ based on a symbiotic relationship with certain bacteria.  In fact, they constitute about half of all known bioluminescent fish species.  It has been reported that unusual muscles attached to their swim bladders enable them to produce sound, perhaps important to mate location and courtship behavior. 

Most are believed to be generalists feeding on other fish, crustaceans, squid, echinoderms, and carrion on the ocean bottom.  Many are attracted to structures on the sea floor such as hydrothermal vents, seeps and shipwrecks.  Given the depths inhabited by most species, it is not surprising that their natural history remains poorly known.  They produce massive numbers of eggs, over 100,000, that have lipid droplets making them buoyant.  They float up to the thermocline, the interface between warmer surface waters and colder deep waters; juveniles thus begin life in shallower water, gradually migrating to greater depths as they mature.  Some species are known to be semelparous, that is, to die after spawning, while others are documented to live more than 50 years.  Coelorinchus zinjianus is just the latest chapter in a fascinating book we have only begun to read.

Acknowledgment

I thank Dr. Tomio Iwamoto for a copy of his paper and permission to use the photograph of Coelorinchus zinjianus that is Figure 1 in that publication.

Reference

Prokofiev, Artem M. and Tomio Iwamoto (2023) A new Coelorinchus from the western Indian Ocean with comments on the C. tokiensis group of species (Teleostei: Gadiformes: Macrouridae).  Zootaxa 5301: 137-150.