New Deep-Sea Worm Pectinereis strickrotti Discovered at Costa Rica Methane Seeps

The deep ocean is an entire realm of alien mystery.

Descend beyond the reach of sunlight and vast ecosystems emerge: creatures shimmer and glitter as they feed in perpetual darkness.

One example comes from a deep-water habitat where methane leaks upwards from beneath the seabed into the water around it. There, researchers have found a striking new worm.

Named Pectinereis strickrotti, it is the 48th new species identified living around methane seeps off Costa Rica's coast. Such environments were once considered far too hostile to support life.

A team led by marine biologist Greg Rouse of the Scripps Institution of Oceanography says the find underlines how little is known about Earth's oceans and the remarkable variety of life they contain.

"We've spent years trying to name and describe the biodiversity of the deep sea," Rouse says. "At this point we have found more new species than we have time to name and describe. It just shows how much undiscovered biodiversity is out there. We need to keep exploring the deep sea and to protect it."

Exploring Costa Rica's deep-sea methane seeps

The ocean's depths are profoundly inhospitable to people. Crushing pressure, freezing conditions and the absence of light make them exceptionally difficult to reach.

Consequently, much of the deeper ocean remains unknown. Humans have explored only 5 per cent of the world's oceans and catalogued an estimated 10 per cent of the life inhabiting their depths.

Where people cannot safely go, however, technology can. Rouse and colleague Bruce Strickrott, the chief pilot of the deep-sea submersible Alvin, after whom the worm was named, first spotted the species during a crewed dive off Costa Rica in 2009. They were roughly 1,000 metres below the surface.

"We saw two worms near each other about a sub's length away swimming just off the bottom," Strickrott recalls. "We couldn't see them well and tried to creep in for a closer look, but it's hard to creep in a submarine and we spooked them."

That was the end of the encounter until 2018, when the researchers returned to the same location. To their surprise, they found the creature again - this time six individuals, moving confidently through the water. The submersible was able to get close enough on this occasion.

The researchers gathered photographs, video footage and specimens before returning to an atmospheric environment to catalogue and examine their collection.

Pectinereis strickrotti and its unusual features

Their specimens proved to belong to a previously unknown marine worm in the Nereididae family, commonly called ragworms. Like its relatives, P. strickrotti is a segmented polychaete with an elongated body lined with leg-like structures known as parapodia. It also has concealed jaws, which extend when it feeds.

Yet P. strickrotti differs from other recognised ragworms in several notable respects. Most ragworms are found in much shallower seas. Living in bathypelagic darkness, P. strickrotti is blind. In addition, while most known ragworms take in oxygen through their parapodia without gills, the parapodia of P. strickrotti are covered in gills.

Male and female life stages of the new ragworm

Nereididae generally pass through two life stages. During the immature atoke stage, they spend most of their lives in burrows on the seabed. In the adult epitoke stage, they emerge, swim freely to reproduce and then die.

Because most of the collected specimens were free-swimming males, while the sole partial female specimen came from sediment, the scientists suspect that only males may become epitokes in this species.

The males also displayed unusual, non-reproductive epitokous traits that seem to be unique within the Nereididae. Further research will be needed to establish how the worm lives and why it requires these features.

Its diet is also unknown, although an investigation of the chemosynthetic food web that develops around methane seeps may provide the answer.

More significantly, this unusual worm offers a glimpse of the immense abyss of human ignorance - and of the strange marvels on Earth that are still waiting to be discovered.

A paper describing the species has been published in PLOS One.