Mysterious holes in the deep seafloor have popped up in the Bering Sea, and scientists think they have found their makers: small, shrimp-like crustaceans.
Between July and September 2022, a team of scientists embarked on the AleutBio expedition in the North Pacific. The aim was to study the biodiversity of the Aleutian Trench, a plunging, 7-kilometre-deep gash in the seafloor near Alaska. But the research vessel also sampled three locations in the nearby Bering Sea.
Behind the ship, the researchers towed downward-facing cameras thousands of metres below the waves. While watching the footage on board, the team spied odd, half-metre-long lines of small holes punched into the seafloor mud, says at the Senckenberg Research Institute and Natural History Museum Frankfurt in Germany.
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“And then we kept seeing them over and over and over again. We saw them hundreds of times in these three different stations,” she says.
Intrigued by their abundance, the researchers examined the footage to learn as much as they could about the mysterious holes. “Absolutely nothing” was known about them, says Sigwart, but superficially similar, equally mysterious lines of holes – termed “pogo stick trails” – have been observed in the mid-Atlantic Ridge for decades.
Examining footage of the holes from many different angles revealed that they were connected underneath by a horizontal tunnel. In all, the researchers recorded nearly 200 of these tunnels, each with multiple holes. Ninety-six animals were spotted on or within the burrows, including varied invertebrates and giant grenadier fishes (Albatrossia pectoralis).
Sigwart and her colleagues wondered which of these species, if any, could be the engineer behind the flute-shaped burrows. The fish were too large to have made them, and the marine worms were the wrong shape. The urchins were more inclined to bulldoze the structures entirely. Then they saw a pinkish amphipod – a variety of narrow-bodied crustacean – in one of the exit holes. , the chief scientist on the expedition “jumped out of her chair”, says Sigwart, recalling footage she had once seen of a different amphipod digging with the same posture and movement.
The researchers collected one of the spindly-legged crustaceans, which appear to be in the genus Maera. Being about the right size and equipped with legs specialised for digging, the amphipod is the team’s hypothesised architect. It is still unknown what makes the mid-Atlantic burrows, but Sigwart thinks it is a different deep-sea species.
The amphipod is probably making these burrows as a shelter or nest, she says. “They’re also using this to get to a nutrient-rich layer that’s a little bit below the surface. So, it’s also a way of getting them safe access to food.”
Many other animal species seem to congregate around and within the burrows, too. “The abyssal plain is famously very flat. It’s just a lot of mud,” says Sigwart. “Any additional structure that adds a three-dimensional aspect also creates habitat.”
All that digging may also be cycling chemicals in the ecosystem, says at the National Oceanographic Centre in Southampton, UK, who wasn’t involved in the work. In the deep sea, says Durden, there aren’t many physical processes that can mix sediment, so it is mostly burrowing animals – acting like the gophers or moles of the abyss – that shuffle the mud.
The amphipods act as deep-sea ecosystem engineers, with an outsized influence on abyssal ecology. Yet almost nothing is known about their biology.
“We’re fairly sure that [the amphipod] is a new species,” says Sigwart, noting that it still needs to be formally described and named.
Ecology and Evolution
