The deep-sea eelpout Pyrolycus jaco was discovered by Scripps Oceanography researchers

Pyrolycus jaco, a recently discovered species of eelpout, is the light purple fish in the lower left of this image. It is seen here at the Jacó Scar hydrothermal seep among mussels and tubeworms. Image credit: ROV SuBastian/Schmidt Ocean Institute.

A new species of fish belonging to the eelpout family was found by a team of researchers led by the Scripps Institution of Oceanography at UC San Diego. It resides in the eastern Pacific Ocean off Costa Rica.

The first fish species to be identified from the hydrothermal seep location known as Jacó Scar, which is situated on Costa Rica’s Pacific coast, is the newly discovered species named Pyrolycus jaco. The little eel-like fish, which is about 6 inches long and has a pale pink to lavender body, has been spotted nestled among tubeworms at depths of 1,750 to 1,800 meters. The study was released in the journal Zootaxa on January 19 and describes the new species.

Ben Frable, the main author and collection manager of the Marine Vertebrate Collection at Scripps Oceanography, said that is a really intriguing and distinctive tiny fish, so we called it after the unique place and city of Jacó in Costa Rica. This finding is a remarkable illustration of how far collaborative research can go, made possible by previous expeditions and discoveries made by Scripps marine scientists over the years.

Eelpouts are ray-finned fish that have unusual eel-shaped bodies. Some species also have mouths that appear to be “pouting,” making them easy to identify. There are about 300 species of eelpouts that have been identified, some of which may be found directly off the coast of California.

With the discovery of Pyrolycus jaco, there are now 13 species of eelpout known to exist in the world, all of which may be found on hydrothermal vents or methane seeps, which are microbe-driven ecosystems that run on chemicals from the Earth’s crust rather than sunlight. Natural gas and other compounds come out of the seafloor as methane seeps at the same cold temperature as the water around them. A geyser-like eruption of chemical-rich water with temperatures exceeding 400°C (752°F) occurs at hydrothermal vents. Only four other eelpout species are known to live on hydrothermal vents in the Pacific Ocean, where the new species joins them.

The Jacó Scar methane seep is a harsh area with hydrothermal vent-like features, including high temperatures up to 5.2°C (41.4°F), which is several degrees warmer than the surrounding areas in the deep sea. Pyrolycus jaco is the first eelpout species to be observed there. Due to this seemingly minor variation, Jacó Scar appears to be a sanctuary for several creatures that usually only exist at hydrothermal vents, in contrast to nearby methane seeps.

The new species may also be clearly identified from closely related species by changes in its body size, such as a reduced head length, and in the amount of bones and sensory pores that are distributed throughout its body.

When researchers from the University of Costa Rica and Scripps Oceanography first found the Jacó Scar site in 2009, Lisa Levin, Greg Rouse, and other scientists saw the fish for the first time. They saw many eelpouts swimming among the tubeworms while using the Human Occupied Vehicle (HOV) Alvin of the Woods Hole Oceanographic Institution. The researchers continued to investigate this distinctive ecosystem over succeeding research cruises, including the use of high-resolution video by the Schmidt Ocean Institute’s remotely operated vehicle (ROV) SuBastian in 2019. To date, they have characterized 24 new invertebrate species from Jacó Scar alone, mostly tiny worms.

The current research was based on four eelpout specimens obtained by HOV Alvin in 2018. Three of the specimens are now on display: two are at Scripps Oceanography’s Marine Vertebrate Collection, one is at the University of Costa Rica’s Ichthyology Collection, and the fourth is in Copenhagen.

According to Frable, the original scientific publication documenting Jacó Scar did not classify the fish to the species level because the major objective was to document the existence of this rare seep-vent ‘hybrid’ habitat. Later, in 2021, while working on a more thorough species inventory, Charlotte Seid of Scripps Oceanography requested Frable to identify the eelpout. In order to get more information on the possible species, Frable got in touch with Peter Rask Møller, a famous eelpout expert and curator at the Natural History Museum of Denmark. He identified it right away as being new and belonging to the Pyrolycus genus, which is Latin for “fire wolf.” Then Frable, Seid, and Møller collaborated with Allison Bronson, a Cal Poly Humboldt expert in CT scanning, to create a 3D digital X-ray that could be used to study the bones without damaging the specimens.

The researchers emphasized in the Zootaxa report that this new species finding demonstrates the richness of unique biodiversity at the Costa Rican methane seeps.

According to Seid, collection manager of the Benthic Invertebrate Collection at Scripps Oceanography, this eelpout finding is just one illustration of how exceptional this specific environment is, and by extension, other deep-sea habitats that they have not even identified yet. The researchers have been visiting various locations for years, and the Jacó Scar research site is simply one of the many things they continue to learn about. This report emphasizes the need for further deep-sea habitat exploration and conservation, as well as the technologies required to fully research them.


FRABLE, BENJAMIN & Seid, Charlotte & BRONSON, ALLISON & Moller, Peter. (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. 79-89. 10.11646/zootaxa.5230.1.5.