Search for famed shipwreck reveals geometric nest patterns created by fish in Antarctica

Sea ice covers much of Antarctica's Weddell Sea. Dr. Michelle Taylor/Weddell Sea Expedition 2019 via CNN Newsource

During an expedition to search for a famed shipwreck, researchers discovered something unexpected: striking geometric patterns on the floor of Antarctica’s western Weddell Sea created by fish.

The seafloor had once been obscured beneath a thick ice shelf, but that changed in July 2017 when the massive A68 iceberg calved from the Larsen C Ice Shelf. The iceberg measured 2,239 square miles (5,800 square kilometres) — about the size of Delaware.

Seizing an opportunity to study the previously hidden seafloor, researchers organized the Weddell Sea Expedition 2019 with two goals in mind: exploring the biology of the western Weddell Sea and searching for the wreckage of the HMS Endurance, which became trapped and ultimately ripped apart by ice in 1915.

Researchers set sail in January 2019 for a 49-day expedition aboard the South African polar research vessel SA Agulhas II. Ironically, the expedition experienced extreme sea ice conditions similar to what Endurance faced more than a century earlier, preventing the team from conducting a search for the wreck.

“The sea ice in particular was a challenge as at the time there was a bottleneck and a build of sea ice around that area — we were heading directly towards this, playing chicken with icebergs as we went,” said Dr. Michelle Taylor, senior lecturer in the School of Life Sciences at the UK’s University of Essex. Taylor is the coauthor of a new study published Wednesday in the journal Frontiers about what the expedition uncovered.

Despite not finding the wreckage, the expedition captured invaluable underwater footage of marine life that inhabits the frigid depths of the Weddell Sea. The research vessel’s remotely operated underwater vehicle, nicknamed Lassie, spied a series of nests arranged in distinct shapes sprawling across the seafloor.

“Over a thousand maintained nests within the areas studied goes to show that exploration of our world is still underway, with constant new findings,” lead study author Russ Connelly, postgraduate researcher in the School of Life Sciences at the University of Essex, said in an email.

The discovery showcases the diversity of life that exists in a place experiencing rapid climate change — which is why scientists are petitioning for the Weddell Sea’s fragile ecosystem to be protected by law.

(The HMS Endurance shipwreck was ultimately discovered during a 2022 expedition.)

A geometric neighbourhood

As Lassie passed over the Weddell seafloor, large dimples appeared in the sand. The round spots seemed neat compared with their surroundings, clear of the layers of decomposing plankton spotted elsewhere on the seafloor.

“This was the first time we have seen this area of seafloor, so there is always a realm of mystery to be expected,” Connelly said.

When scientists looked at the video footage that Lassie took, they noticed fish and fish larvae within the dimples. A closer look revealed the fish to be a species of rockcod called Lindbergichthys nudifrons, otherwise known as the yellowfin notie. The fish are found from the Antarctic Peninsula to South Georgia in the Atlantic section of the Southern Ocean, Connelly said.

“These species are extremophiles; they thrive in high-pressure and cold-water environments,” Connelly said. “They build small, circular nests in the fine sediment, and then the males guard the eggs for around 4 months. Their main threat comes from predators on the seafloor, like brittle stars and predatory worms, which try to eat the eggs.”

The researchers also noticed that the 1,036 active nests, located across five sites on the seafloor, showcased six different patterns.

There were clusters of nests found close together or overlapping, and crescents of nests arranged in a curved line, like a crescent moon. Other nests were evenly spaced in a straight line, arranged in an oval shape with even spacing around the perimetre or following a distinct U-shape.

There were also individual, isolated nests, which were typically larger than any that were arranged in a specific shape, according to the study.

The finding marks the first time that complex patterns have been observed in yellowfin notie nests.

Clusters were the most common arrangement, representing more than 42% of the nests, the researchers said.

The team believes the patterns represent a strategy for survival against predators. For those living in cluster patterns, the fish at the center receive the most protection from predators. The researchers think this arrangement represents the “selfish herd” theory, “where individuals reduce their domain of danger by putting other individuals between themselves and an approaching predator,” the study noted.

Previous research has shown that yellowfin notie males will defend territories up to 9.8 inches (25 centimetres) away from their nests, which would mean their neighbours are also protected within the cluster arrangements.

“In the absence of refuge, they can group together to form greater levels of protection,” Connelly said.

The fish are likely protecting against ribbon worms, or Parborlasia corrugata, a type of nemertean, observed living in the vicinity.The ribbon worms detect chemical signals from their environment to find food.

“(The clusters) could create a confusing sensory environment for the nemertean, making it more difficult to detect and target a single nest, a crucial adaptive strategy during the long incubation period (for the eggs),” the authors wrote in the study.

Meanwhile, the isolated nests on the outskirts of the fish communities are thought to belong to larger, stronger members of the species capable of defending their own nests.

Protecting a vulnerable place

The ice-covered Weddell Sea is a challenging place to study, but it’s home to a diverse community of fish, brittle stars, sponges, cephalopods, marine birds and mammals.

Another expedition to the Weddell Sea in 2021 observed a vast colony of icefish, or Neopagetopsis ionah, with see-through skulls and transparent blood.

Dr. Autun Purser, a senior researcher in deep-sea ecology and technology at the Alfred Wegener Institute in Germany, led the 2021 icefish research. While he did not participate in the new research, Purser said he believes both studies benefited from underwater camera systems that can spot patterns within extensive fish nests.

“With mobile camera systems, we can now investigate larger structures and areas of seafloor in studies than was possible in the past,” Purser said.

Return trips to the Weddell Sea have confirmed that such nesting behaviors are more abundant within the icy ecosystem, he added.

He also said he suspects that species within the environment, such as the icefish and the yellowfin notie, benefit from rocks dropped by icebergs as they drift and melt. In the new study, the authors noted that 14.9% of the active nests had pebbles within or around them.

“(The rocks) are ideal to lay eggs on as they allow good oxygenation of the eggs, helping to prevent rotting on the seafloor, whilst also providing a barrier to animals living within the muds to eating the eggs,” Purser noted. “The soft sediments make a ridge wall around these eggs, furthering defense. … It is just an idea, but I like it.”

The study authors believe the new research provides crucial evidence of breeding habitats that strengthens the idea of the Weddell Sea Marine Protected Area, proposed by the Commission for the Conservation of Antarctic Marine Living Resources.

“If successfully designated as (a marine protected area), it would mean these vulnerable marine ecosystems are protected,” Connelly said. “It is incredibly important to explore and study these unique environments before we lose something we never knew we had.”