‘Supergiant’ sea creature could lurk in more than half of the deep ocean

A “gigantic” deep sea creature first discovered in the 1800s, but considered relatively rare, could inhabit up to 59% of the world’s deep oceans according to a new study from the University of Western Australia.

Alicella gigantea, the largest-known species of amphipod on Earth, reaches lengths of up to… 34cm.

An animal the size of your forearm may not, at first, appear to deserve the “supergiant” label, but according to deep sea researcher and lead author of the study, Dr Paige Maroni, they’re “ginormous” compared to other amphipods.

“Which is just fantastic when you see them, because amphipods are probably the biggest, baddest predator of the deep sea,” Maroni told Cosmos.

“Most other deep-sea amphipods max out at about 20cm, if that.”

Amphipods are small, shrimp-like crustaceans that inhabit aquatic environments worldwide.

“They’re so prolific, and they eat everything and anything. So they are fantastically biodiverse, and they thrive where other bigger predators, like fishes, disappear. So, anything deeper than 8,000m or so.”

But despite a long history of sightings spanning 75 sites across the Indian, Pacific, and Atlantic oceans, Alicella gigantea has, historically, been considered quite rare.

This is because so few of them rock up to baited traps.

“If you’re trying to collect amphipods from the deep sea, you use something that smells really good to them, like dead fish or dead squid, or anything oily and fatty,” says Maroni.

“When amphipods come, they come in masses of sometimes hundreds, even after a really short amount of time.”

Alicella gigantea are captured alone, or very rarely in groups of up to 4 individuals, and this takes weeks of effort to accomplish.

Considering how little we know about the deep sea – a recent study determined that only about 0.001% of the deep seafloor has been visually observed – it may come as no surprise that the new study predicts that A. gigantea is likely far more common than we realised.

Because so little is known about Alicella gigantea apart from the depths at which the creatures have been captured, the team’s Habitat Suitability Model uses depth data as a proxy for other environmental variables.

“In the deep sea depth is an amazing indicator of pressure and temperature and current speed or direction, because it is quite homogeneous across these really deep, vast places,” says Maroni.

Data from the 195 historic collections of the supergiant amphipod indicates the species lives in the lower abyssal and upper hadal zones at depths of between 3,890-8,931m. The Habitat Suitability Model therefore projects Alicella gigantea could inhabit as much as 59% of the world’s oceans and all 6 major ocean bodies.

“What the ideal situation would be is that we one day figure out more about its breeding habits or its feeding habits or its mating strategies, and then we can refine that model over and over again, and [potential habitat] may shrink,” says Maroni.

“It may not, it may get bigger.”

The model predicted that Alicella gigantea should be found in the northeast Pacific Ocean, where it had never been captured before. So, a group of researchers ventured out to the Murray Fracture Zone during the 2023 Trans-Pacific Expedition to find them.

“They managed to put cameras … on the sea floor and finally visualised these amphipods in their masses. And this had only ever been done a few times before.

“To finally visualise them … in 20s, rather than just 1s or 2s, was like, okay, they may not be rare. They’re just maybe not the easiest things to collect, or maybe our methods aren’t suitable for collecting multiple of them,” says Maroni.

The team sequenced the nuclear and mitochondrial DNA of newly collected specimens to explore the global distribution and evolutionary history of this weird, abyssal creature.

This allowed them to conclude that specimens, collected from across the globe and separated by oceans and continents, are, genetically, the same species.

Maroni says this is not the outcome she expected when she started working on Alicella gigantea: “I was like ‘I’m definitely going to find 7 new species!”

“[Alicella gigantea] just found its niche, and it’s maintained status quo for potentially millions of years.

“The [deep sea] environment is so stable, and it has been so stable for estimated, the last 30-40 million years, but it may go back even further than that.

“The same or similar amounts of oxygen and cold water are protruding from the poles into the deep sea every year. The current speeds are pretty regulated. The global oceanic overturning pathway has been pretty interrupted until, of course, the new wave of climate change is certainly altering it.

“So, it’s very different from, for example, a volcanic system or a sea mount system that has localised volcanic eruptions, which are these natural disturbances which promote things like speciation and diversification.”

The deep sea is the largest ecosystem on our planet, she says, but “for so long, has been out of sight, out of mind.”

“What a lot of people don’t realise is what happens in the deep sea 100% affects what happens on our coastal shores, on our islands, our climate, our seafood, monsoon seasons, farming.

“Working on species by species is one way that we can start to bring the deep sea to people’s front door and show them that it is a fantastically biodiverse and beautiful place.

“We’ve still got millions of species left to discover, and we are in a biodiversity crisis around the world.

“I hope papers like this show people that it is all connected.”

The research is published in Royal Society Open Science.