This fish could expand what we know about one odd deep-sea ecosystem

Off the Pacific coast of Costa Rica sits a deep-sea chimera of an ecosystem. Jacó Scar is a methane seep, where the gas escapes from sediment into the seawater, but the seep isn’t cold like the others found before it. Instead, geochemical activity gives the Scar lukewarm water that enables organisms from both traditionally colder seeps and scalding hot hydrothermal vents to call it home.

One resident of the Scar is a newly identified species of small, purplish fish called an eelpout, described for the first time on January 19 in Zootaxa. This fish is the first vertebrate species found at the Scar and could help scientists understand how the unique ecosystem developed. 

Jacó Scar was discovered during exploration of a known field of methane seeps off the Costa Rican coast and named for the nearby town of Jacó. It is “a really diverse place” with many different organisms living in various microhabitats, says Lisa Levin, a marine ecologist at Scripps Institution of Oceanography in La Jolla, Calif.

Levin was on one of the first expeditions to the Scar but wasn’t involved in the new study. She recalls the team finding and collecting one of the fish during this early excursion, but the researchers didn’t recognize it as a new species.

Several more specimens were snagged during later submersible dives. Charlotte Seid, an invertebrate biologist at Scripps who is working on a checklist of organisms found at the Costa Rican seeps, brought the fishy finds to ichthyologist Ben Frable, also of Scripps, for formal identification.

Frable says he knew the fish was an eelpout. They look exactly as one would expect based on their name: like frowning eels, though they aren’t true eels. But he was having trouble determining what type. Eelpouts are a diverse family of fish comprised of nearly 300 species that can be found all over the world at various ocean depths.

Because the physical differences between species can be subtle, they are “kind of a tricky group” to identify, Frable says. “I just was not really getting anywhere.” So the team turned to eelpout expert Peter Rask Møller of the Natural History Museum of Denmark in Copenhagen, sending him X-rays, pictures and eventually one of the fish specimens.

Møller narrowed the enigmatic eelpout to the genus Pyrolycus, meaning “fire wolf.” Turns out, the tool, called a dichotomous key, that Frable had been using to identify the specimens was outdated, made before Pyrolycus was described in 2002. “I did not know that genus existed,” Frable says.

Because the other two known Pyrolycus species live far away in the western Pacific and have different physical features, the team dubbed the mystery fish P. jaco — a new species.

The first eelpouts most likely evolved in cold waters, Frable says, but many have since made their home in the scalding waters of hydrothermal vents. Of the 24 known fish species that live only at hydrothermal vents, “13 of them are eelpouts,” Frable says.

The new finding raises questions about how the known Pyrolycus species came to live so far apart. It may have to do with the fact that methane seeps are more common than previously thought on the ocean floor, and if some are lukewarm like Jacó Scar, the new species could have used them as refuges while moving east.

And by comparing P. jaco to its vent-living relatives, researchers may be able to figure out how it adapted to live in the tepid waters of the Scar — which may provide clues to how other species living there did too.

The eelpout is part of a medley of other species that form Jacó Scar’s composite ecosystem, along with, for example, clams typically found at cold seeps and bacteria found at hydrothermal vents. Jacó Scar is a “mixing bowl” of species found in other parts of the world, Seid says. Figuring out how this eclectic bunch interacts “is part of the fun.”