A breeding colony of 60 million fish has been discovered in the ice-covered Weddell Sea off Antarctica – a unique, and previously unknown, ecosystem that covers an area the size of Malta.
The fascinating discovery shows how little is known about the depths of the ocean.
The vast colony, believed to be the largest in the world, is home to the remarkable icefish (Neopagetopsis ionah), which has a transparent skull and blood. Icefish are the only vertebrates that do not have red blood cells.
To survive such low temperatures, the species evolved a transparent blood antifreeze protein that prevents ice crystals from growing.
The breeding colony was discovered in February 2021 by the German polar research vessel Polarstern, which surveyed the seafloor about half a kilometer below the vessel. He used a car-sized camera system attached to the stern of the ship that beams images to the deck as it is being towed.
The expedition was focused on ocean currents and the discovery of the fish nests, distinguished from the muddy sea floor by a circle of stones, was a surprise.
“We saw nest after nest of fish in the four hours [da expedição] and during that time we covered about six kilometers of the sea floor,” said Autun Purser, a postdoctoral researcher at the Alfred Wegener Institute in Bremerhaven, Germany. He is the lead author of a study on icefish colonies that was published in the journal Current Biology on Thursday.
“I’ve never seen anything like it in 15 years as an ocean scientist,” Purser said. “After this dive, we sent an email to the shore experts who know about fish like this. They said yes, this is very unique.”
Four more camera dips revealed the enormous extent of the breeding colony – and its uniform and impressive nature.
“This is really an amazing discovery,” said John Postlethwait, a professor of biology at the University of Oregon in the United States who studies this type of fish. He is not involved in the research.
“This is also significant. The extent of the biomass is at least unexpected to me, and the extent to which fish change the structure of the bottom of the sediment creates [um] habitat for a community that surrounds the food web to support a huge variety of species,” he added.
The colony covers more than 240 square kilometers, the researchers said. With, on average, one nest for every three square meters, they estimate that the colony includes about 60 million active nests.
Each of the evenly spaced nests was about 15 centimeters deep and 75 centimeters in diameter, containing an average of 1,735 eggs. Most were guarded by an adult fish. Some of the nests contained only eggs and some were unused.
“Nest spacing is like the spacing of birds on a telephone wire,” Postlethwait added by email.
“Some animals like to be social, but there is a limit. Gathering together may give them an advantage in finding mates, but it provides a rich source for predation.”
The fish appear to be attracted to an area of warmer water, which is about 2°C warmer than the surrounding seafloor, which is 0°C cold, Purser said. Sea water freezes at a lower temperature than fresh water.
The researchers deployed two camera systems to monitor icefish nests until a research vessel returns. The hope is that the images will capture more detail about the fish’s nest ecosystem.
One question the researchers want to answer is how long adult fish guard eggs — experts suspect it could take months — and whether it’s the male or female that keeps watch.
“It appears that the reproductive behavior of most, if not all, icefish revolves around male courtship of females through building a ‘good’ nest,” icefish expert H. William Detrich, professor emeritus of biochemistry and marine biology at Northeastern University in the United States. He is not involved in the research.
The findings reveal a globally unique ecosystem, according to the researchers, and they say the site should be designated a protected area.
“The conservation implications of this species are clear – a marine protected area must be established in the Weddell Sea to prevent exploitation of this icefish species,” added Detrich.
Although the Weddell Sea is covered in sea ice year-round, the frozen surface is relatively thin—about three feet thick—meaning photosynthesis can still occur and life can still thrive. Purser said the Weddell Seafloor is far from barren, with sea sponges, corals, octopuses and starfish that can be seen along the seafloor.
About 2,000 seals also live in the area and are likely to dive at the nest site and feed on the icefish, he said, although they have no definitive evidence.
Purser said that while there are species of freshwater fish that make similar types of nests, scientists have “never seen colonies like this on the seafloor.”
“I think we only filmed 1% of the Weddell Seabed, and no one knows what else is hidden there. I am convinced that there are many gaps in our knowledge of the seafloor.”
This content was originally created in English.
Reference: CNN Brasil