An icefish colony discovered in Antarctica is world's largest fish breeding ground

A breeding colony of 60 million fish has been discovered in Antarctica's ice-covered Weddell Sea - a unique and previously unknown ecosystem that covers an area the size of Malta.

The fascinating find shows how little is known about the ocean depths.

The vast colony, believed to be the world's largest, is home to the remarkable icefish (Neopagetopsis ionah), which has a see-through skull and transparent blood. Icefish are the only vertebrates to have no red blood cells.

To survive at such low temperatures, it has evolved an anti-freeze protein in its transparent blood that stops ice crystals from growing.

The breeding colony was discovered in February 2021 by the German polar research vessel Polarstern, which was surveying the seabed about half a kilometre below the ship. It used a car-sized camera system attached to the stern of the ship that transmits pictures up to the deck as it's being towed.

The expedition was focused on ocean currents and the discovery of the nests, made distinct from the muddy seabed by a circle of stones, was a surprise.

"We just saw fish nest after fish nest for the whole four hours, and during that time we covered maybe six kilometres of the sea floor," said Autun Purser, a postdoctoral reseacher at the Alfred Wegener Institute in Bremerhaven, Germany. He's the lead author of a study on the icefish colony that published in the journal Current Biology on Thursday.

"I'd never seen anything like it in 15 years of being an ocean scientist," Purser said. "After that dive, we emailed the experts on shore who know about fish like this. They said, yep, this is pretty unique."

DRAMATIC EXTENT

Four further camera dives revealed the dramatic extent of the breeding colony - and its striking uniform nature.

"This is indeed a surprising discovery," said John Postlethwait, a professor of biology at the University of Oregon, who studies the fish. He wasn't involved in the research.

"It is also significant. The extent of the biomass is to me at least unexpected. And the extent to which the fish change the bottom structure of sediment creates [a] habitat for a community that ripples up the foodweb to support a huge variety of species," he added.

The colony covers more than 240 square kilometres, the researchers said. With, on average, one nest for every three square metres, they estimated that the colony includes about 60 million active nests.

Each of the evenly spaced nests was about 15 centimetres deep and 75 centimetres in diameter, contained on average 1,735 eggs. Most were guarded by one adult fish. Some nests only contained eggs, and some were unused.

"The spacing of the nests is kinda like the spacing of birds on a telephone line," Postlethwait added via email.

"Some animals like to be social, but there's a limit. Congregating may give them advantages for finding mates but provides a rich point source for predation."

The fish appear to be attracted by an area of warmer water, which is around 2 C warmer than the surrounding sea bed, which is a chilly 0 C, said Purser. Sea water freezes at a lower temperatre than fresh water.

The researchers have deployed two camera systems to monitor the icefish nests until a research vessel returns. The hope is that photographs will catch more details about the fish nest ecosystem.

One question researchers want to answer is how long the adult fish guard the eggs - experts suspect it could be months - and whether it was the male or female standing watch.

"It appears that that the reproductive behaviour of most, if not all, icefishes, revolves around male courtship of females through building a 'good' nest, said icefish expert H. William Detrich, professor emeritus of biochemistry and marine biology at Northeastern University, via email. He wasn't involved in the research.

The findings reveal a globally unique ecosystem, according to the researchers, and they say it should be designated a protected area.

"The implications for conservation of this species are clear -- a marine protected area should be established in the Weddell Sea to prevent exploitation of this icefish species, " Detrich added.

SEAL FOOD?

While the Weddell Sea is covered with sea ice all year round, the ice is relatively thin - three feet thick - meaning that photosynthesis can still take place and life can thrive. Purser said the Weddell Sea floor is far from barren, with sea sponges, corals, octopuses and star fish lurking along the seabed.

Around 2,000 seals also live in the area and likely dive in the breeding area and feed on the icefish, he said, although they didn't have definitive evidence.

Purser said while there were species of freshwater fish that make similar kinds of nest, scientists had "never seen any colonies like this in the deep sea."

"I guess we've only filmed maybe 1 per cent of the Weddell Sea floor, and who knows what else is hidden around the place. I am convinced there's many gaps in our knowledge of the deep sea."