Once the remote-operated camera glimpsed the bottom of the Weddell Sea, more than a thousand feet below the roof’s ice cap, Lilian Boehringer, a student researcher at the Alfred Wegener Institute in Germany, watched the ice fish’s nests. Sand craters scattered to the sea floor, each the size of a collar and collar and less than a foot long. Each crater has solid ice fish, dark pectoral fins spread like bat wings over a cluster of eggs.
The aptly named ice fish thrives in waters above freezing with just a few gigantic hearts and blood that flows as clear as vodka. Their blood is transparent because it lacks red blood cells and hemoglobin to carry oxygen throughout the body. Hemoglobin fish’s loss of hemoglobin genes was not so much an evolutionary adaptation as a happy accident, which allowed them to suck oxygen-rich Antarctic waters through their skin.
The sighting occurred in February 2021 in the camera room aboard the research vessel, Polarstern, which had come to the Weddell Sea to study other things, not ice fish. It was three in the morning near Antarctica, which means the sun was setting but most of the ship was asleep. To Ms. Boehringer’s astonishment, the camera continued to transmit images as it moved with the ship, revealing a continuous horizon of icefish nests every 20 seconds.
“It didn’t stop,” Boehringer said. “They were everywhere.”
Half an hour later, Autun Purser, a deep-sea biologist at the same institute, joined Ms. Boehringer. On the camera feed left only nests.
“We were like, is this ever going to end?” Dr. Purser said. “How has anyone never seen this before?”
The nests persisted throughout the four-hour dive, with a total of 16,160 camera-recorded. After two more dives by the camera, the scientists estimated that the Neopagetopsis ionah ice colony extends across 92 square miles of the calm Antarctic Sea, with a total of 60 million active nests. The researchers describe the site – the largest fish-breeding colony ever discovered – in a paper published Thursday in the journal Current Biology.
“Holy cow,” said K. Christina Cheng, an evolutionary biologist at the University of Illinois-Urbana-Champaign, who was not involved in the research. “This is really unprecedented,” she said. “It’s insanely dense. It’s a big find.”
Mario La Mesa, a biologist at the Institute of Polar Sciences in Bologna, Italy, who was not involved in the research, said the paper provides “evidence for a complex and as yet undescribed benthic ecosystem in the Weddell Sea.” “I wouldn’t be surprised to find other huge fish-breeding colonies elsewhere,” said Dr. La Mesa, who last year described the nest-guarding behavior of Antarctic ice fish species from sites near the newly discovered colony.
Each of the newly discovered nests contained, on average, 1,735 large yolk eggs – low fecundity of the fish. An unprotected clutch would be an easy snack for predators such as starfish, multiheaded worms and sea spiders, Dr. Cheng said. So the males stand guard to ensure their offspring are not devoured, at least not before they have had a chance to hatch, and may clean nests with their outstretched lower jaws, according to Manuel Novello, a researcher at the Bernardino Rivadavia Museum of Natural Sciences. In Argentina, who did not participate in the research.
About three-quarters of the colony’s nests were guarded by a single fish. Others had eggs but no fish, and a fish carcass covered in bacteria or nothing at all. Near the edges of the colony, several unused or abandoned nests housed many carcasses of ice fish, many with sea stars and octopuses feeding on their eyes and soft parts.
“If you die in the fish’s nest area, you rot there,” Dr. Purser said. “But if you die at the edges, it looks like everyone grabs you and starts eating you there.”
The researchers noted that the colony occupied an unusually warm spot of deep water, with temperatures reaching about 35 degrees Fahrenheit — practically impervious compared to other Antarctic waters.
Although the discovery of nests contributes to scientists’ understanding of the life cycle of ice fish, it also raises more questions. How often are nests built and are they reused? Does the fish die after the eggs hatch? Or perhaps the most obvious: “Why is there?” Dr. Cheng asked.
The authors have no confirmed answers, only speculations. Perhaps the deep, warm currents direct the fish to land. Perhaps there is a bounty of zooplankton to be ingested by larvae. Or maybe something else.
But there must be something special about the location of the active colony. About 31 miles to the west, researchers found a patch of seafloor full of nests: all empty. These nests have been abandoned and overrun by sponges and corals — creatures that live long and take years to grow, Dr. Purser said.
It also hosts the waters above the vast settlement of hungry ice fish, in search of Weddell seals. When the researchers collected satellite tracking data from the seals during the flight and analyzed it using historical data, they found, unsurprisingly, that the seals primarily dive into the nests of ice fish. “They’re having a great dinner,” Dr. Purser said.
Before the end of the trip, the researchers deployed a camera that will photograph the site twice daily for two years, and hopefully reveal more about the ice fish’s life cycle. Dr. Novello said he is looking forward to seeing what the camera captures. “This may be the first field observation of courtship behavior and/or nest preparation,” he wrote in an email.
New insights into how ice fish reproduce and contribute to polar food webs could help manage and maintain populations. The authors argue that the new paper provides sufficient evidence to protect the Weddell Sea under the Convention on the Conservation of Antarctic Marine Living Resources.
“The sea floor is not just barren and dull,” Dr. Purser said. “Such huge discoveries persist to this day in the twenty-first century.”