# Pyrolycus jaco

Pyrolycus jaco is a species of eelpout (the Zoarcidae family). These organisms are typically distinguished by their eel-like morphology and, in certain species, facial features that evoke an appearance of "pouting". This species was discovered at Jacó Scar, hydrothermal seep site in the Pacific Ocean.

## Discovery

Researchers from Scripps Institution of Oceanography discovered the species, and the naming was influenced by the unique location of Jacó Scar, leading to the name Pyrolycus jaco. It was first detailed in a paper published in the journal Zootaxa on January 19, 2023.

## Geographic Range

Pyrolycus jaco is a newly discovered species found in the Eastern Pacific Ocean off the coast of Costa Rica. This species is exclusively found in the Jacó Scar hydrothermal seep. This was the first finding of this fish with only four specimens collected, and it is currently not found anywhere else in the world. This species is the fourth member of the Zoaridae family associated with the deep sea hydrothermal and methane sites in the eastern Pacific to be discovered following Thermacres andersoni and T. cerberus, Rosenblatt & Cohen 1986, and Pachycara rimae, Anderson 1989 (Frable 2023).

## Physical Description

Pyrolycus jaco is long and slender with an eel-like appearance. Its body is a very pale pink and purple color that is semi-translucent. The underside of the fish as well as the snout and jaw are darker than the rest of the body. The fins are very light in color, similar to the body, and are also semi-translucent. There is a very faint pattern on most of the fish that is a light brown color. This fish contains no lateral line or scales and has a gelatinous flesh. The skin resembles that of human flesh in color and texture. The pectoral fin has between fourteen and fifteen rays within them and sixteen to seventeen gill rakers. Fins of this fish appear to be smooth and seamlessly connected to the body having a thick membrane between each fin ray. Pyrolycus jaco has two types of teeth which are a mixture of palatine and vomerine teeth. A long gill slit is present and reaches to above the pectoral fin. This fin includes seven to eight dorsal fin rays and is found close to the midline of the body. Located anterior to the pectoral fin is the beginning of the pelvic fin. Just above the middle of that pectoral fin is where the dorsal fin begins. The anal fin then starts close to the middle of the body. No information is given on the caudal fin of this species because it was unable to be preserved and was not documented before the destruction occurred. There are five suborbital bones present arranged in an L-shaped pattern. This species includes many differences in the number of pores from close relatives setting it apart from others within the same family. Pores of this fish are very prominent on the skull just below the eyes and above the gill slits. There are two nasal pores, three postorbital pores, two occipital pores which are the smallest of the pores, and a lack of the interorbital pore. The head of Pyrolycus jaco has a long neurocranium and small rounded eyes. Not extending past the middle of the eye is the maxilla. The snout of this species does not go past the upper jaw and is relatively round. The Pyrolycus jaco also has two nubs of pyloric caeca present within it. The Zoarcidae family is commonly referred to as eelpouts, a group containing almost 60 genera and nearly 300 species, though not true eels like those in the order Anguilliformes (Frickle 2023).

## Habitat

Pyrolycus jaco is only found in a hydrothermal seep named Jacó Scar. It is extremely rare for families of fishes to adapt to live in places like hydrothermal vents and methane seeps. The Pyrolycus jaco belongs to the family Zoarcidae which is one out of seven families of fishes that can live in these extreme environments. Jacó Scar is a unique habitat in that it is a composite reducing habitat that acts like a vent. Composite reducing habitats use composite materials to reduce the mass of habitats while maximizing the strength of the structure. This habitat ranges from 1,604 to 1,854 meters in depth, measures at about five degrees Celsius and contains plants and animals typically found near vents. The Pyrolycus jaco is found living within tubeworm colonies and has not been found outside of this population.

## Development

There is not enough information on the development of Pyrolycus jaco specifically. The Zoarcidae family develops slowly no matter the temperature they are found in but tend to develop extremely slow in colder environments. They develop significantly quicker and tend to live shorter lives in warm climates. This may be caused by less energy being available for molecular repair due to high growth rates to avoid predatory and oxidative stressors. Decrease in temperature typically causes slower development in most fishes, this is not unique to the Zoarcidae. What is unique is that they still develop slower than average in all temperatures. Due to the precise location and lifestyle of Pyrolycus jaco, individuals of this species, possibly including this genus, are likely to be minimized in size compared to other zoarcids, accompanied with reduction or absence of scales and fins, a simplified skeleton, and coloration differences.

## Reproduction

Not enough is known about the reproduction of Pyrolycus jaco. Another type of Eelpout in family Zoarcidae is the Lycenchelys sarsi which lays about twenty to forty large (up to 5.4mm in diameter) eggs buried under the sediment found in their habitats. This number and behavior is typical of all species within Zoarcidae. Two years into an Eelpout's life, they begin to reach their prime sexual maturity. The offspring that comes from the maturity is typically released between November and December. This fish has a gestation period of seven to eight months. This is very long compared to most live bearing fish which on average have a gestation period of only three to five weeks. On average the female Eelpouts can produce between twenty to about seventy-two offspring within their lifetime. Though not much is known about the parental care habits of P. jaco, zoarcids as a group occasionally exhibit parental care characteristics. Their large eggs that hatch benthic larvae that are not widely dispersed are likely a prominent factor in the species' extreme endemism to hydrothermal vents and methane seeps.

## Life Span/ Longevity

Not enough is known about the longevity of Pyrolycus jaco so the longevity will refer to species within the same family Zoarcidae. Some members of the Zoarcidae family can live to be almost three decades old while others may live to be only 11 years old. By living in or near hydrothermal vents, leading to colder ambient temperatures, this causes an evolutionary change that slows their developmental rates. This slower developmental rate causes this family of fishes to develop slower and therefore reach sexual maturity at later ages. This is true of most fish in colder environments, they develop at a slower rate, but live longer than fish in warmer temperatures.

## Behavior

The behaviors of Pyrolycus jaco are unknown as they are not widely understood and studied. The only known behavior of Pyrolycus jaco is that they are commonly found living among tubeworms in the Jacó Scar hydrothermal seep. There is not information on how or why this species was found living within tubeworm communities, but it can be assumed that they live among one another because they occupy similar niches. In Zoarcidae the knowledge of their behavior is also limited. One species within the family relies on algae that is drifting to provide shelter and food resources.

## Food Habits

The Pyrolycus jaco mostly feed on snails and other limpets based on the gut contents. Also revealed from this method was that they were ingesting invertebrates found near hydrothermal seeps that do not move around. Other species within the same family as Pyrolycus jaco also have a similar diet. This suggest that Zoarcids do not move around very much because the organisms consumed were very localized and often sessile.

## Predation

Not enough is known about Pyrolycus jaco to understand predation within this species. The closely related Eelpouts prey on small invertebrates and have been known to be preyed on by arctic skates. Skates are known to be one of the top predators in marine communities, with their diet mostly consisting of small fish. They have been caught ingesting Zoarcidae on two different occasions. This is rare because Eelpouts are relatively large but they are still occasionally eaten. In general there is a low amount of predation among Zoarcidae.

## Ecosystem Roles

Not enough is known about Pyrolycus jaco roles within an ecosystem. Eelpouts in the Baltic and North seas are being used as key indicator organisms to determine the environmental health of the surrounding area. Researchers use the Eelpout’s larvae sex ratio and reproductive success of eelpouts to determine the presence of pollution in the marine environment.

## Economic Importance

There is not enough known about Pyrolycus jaco. All fish are important to make an ecosystem flow. The family Zoarcidae can be infected with viruses that can dramatically impact their population. Rhabdovirus infected and killed a large population of Eelpout known as Zoarces viviparous in 2014. With these fish dying, cascading effects can occur and can have dramatic effects to a population that would want to be avoided such as extinction and decreased diversity.

## Conservation Status

For the Pyrolycus jaco they are not well known enough to have an official conservation status. Four specimens of this species have been collected, but since this species is only known from a limited habitat the habitat should be protected along with the species itself being listed as endangered. Other members within the family are not a good comparison because they are more well-known and common so they could have a different conservation status.