Eelpouts, seamounts and low-calorie ice cream
New species found at methane seep off coast of Costa Rica
Eel-like in appearance, bodies elongated and laterally-compressed, marine eelpouts represent about 300 species of mostly bottom-dwelling, ray-finned fish (family: Zoarcidae), some of which live at great depths.  Scales are small or absent. Many are marked by thick lips and the habit of rolling up. Conspicuously, their dorsal and anal fins run down the body to the caudal fin—which, of course, is why they look much like eels. Curiously, they have a pigment called biliverdin that turns their bones a green color. The majority of species are found in the north Atlantic, north Pacific, and Arctic oceans. The largest species, the Ocean pout (Zoarces americanus) measures over a meter in length.  Genes from this species have been implanted into Atlantic salmon, along with a gene from the Chinook salmon, resulting in elevated growth hormone levels and GMO fish that reach harvesting weight in a third less time. A gene from the fish has also been introduced into a yeast with the goal of producing antifreeze proteins that improve the consistency and storage properties of ice cream. The goal is ice cream with fewer calories that does not suffer from ice crystal formation like other low-calorie formulations.  Few details are known of most eelpouts’ natural history, although evidence suggests they are benthic scavengers or predators and they are one of seven fish families associated with ecosystems situated around hydrothermal vents and methane seeps on the ocean floor.
Live images of Pyrolycus jaco sp. nov., not collected, living among Lamellibrachia barhami and Escarpia spicata colonies. Photo credit: ROV SuBastian/Schmidt Ocean Institute. CC by NC 4.0 — https://creativecommons.org/licenses/by-nc/4.0/
A new deep-sea species of eelpout was recently described in the genus Pyrolycus, one of three genera known to be endemic, or nearly so, to vents and seeps. The new species, Pyrolycus jaco, was discovered during five submersible expeditions off the Pacific coast of Costa Rica. It differs from western Pacific species in having a shorter head, snout, jaw and pectoral fins. It was discovered at the Jacó Scar, a seamount subduction scarp with a high concentration of methane and temperatures about 5 degrees C. That is about 3 degrees above ambient temperature. The species was associated with colonies of tube worms at depths of 1604-1854 m and appears to feed, at least in part, on invertebrates around the seep area.
This fish is the first new vertebrate species discovered from the Jacó Scar hydrothermal seep site, joining more than 20 new species of invertebrates found there. It is also the first known occurrence of the genus in the eastern Pacific Ocean. The fish is small, usually under 150 mm in length. Paedomorphosis is the evolutionary phenomenon involving the retention of immature characteristics in biologically adult individuals. In this case, along with miniaturization, the species shows paedomorphic traits involving latero-sensory canals, fin rays and scales as well as certain reductions of the skeleton.
This discovery reenforces the enticing possibility of additional new species of eelpouts associated with vents and seeps, for several reasons. First, as we were recently and tragically reminded by the implosion of the submersible Titan, as it descended toward the site of the wreck of the Titanic, it is difficult and dangerous to visit deep sea sites. Not surprisingly, species associated with deep hydrothermal vents remain poorly or entirely un-sampled. Second, these fish are notoriously endemic with benthic larvae that do not disperse widely. And finally, the first species of the genus Pyrolycus was only recently discovered, in 2002, off the coast of New Guinea, so the exploration of its species, distribution and habits have only begun.
References
Frable, B.W., Seid, C.A., Bronson, A.W., and P.R. Møller (2023) A new deep-sea eelpout of the genus Pyrolycus (Teleostei: Zoarcidae) associated with a hydrothermal seep on the Pacific margin of Costa Rica. Zootaxa 5230: 70-89.
Machida, Y. and J. Hashimoto (2002) Pyrolycus manusanus, a new genus and species of deep-sea eelpout from a hydrothermal vent field in the Manus Basin, Papua New Guinea (Zoarcidae, Lycodinae). Ichthyological Research 49: 1-6.