Deep-sea biologist Autun Purser and his team were not expecting to make a significant scientific discovery when they dropped their specially designed camera rig into the icy waters of Antarctica's Weddell Sea. They were in the area studying ocean currents and carbon cycles and weren't really scanning the depths for icefish nests. But, boy, did they find them.

It was February 2021, and the crew aboard research vessel the RV Polarstern were carrying out routine work on mooring lines fixed with sensors. This provided Autun an opportunity to drop his ocean floor observation and bathymetry system into the murky deep. The hefty camera device is designed to be towed behind the vessel in order to record photos and videos and capture measurements of deep-sea habitats. At the time, the seafloor topography in the area they were working in looked a little "boring" – just the edge of a trough nowhere near an intersection with the continental shelf, miles from any area where ecosystems come together. It didn't look like the sort of spot that could yield deep-sea secrets.

Nonetheless, the team launched the camera and were instantly rewarded with the sight of a cluster of circular nests, many of them guarded by an adult icefish. These nests were nothing new and had been documented before but as the camera drifted on, visuals of the stone-lined circles kept coming. "Such huge densities in one place were never envisioned," Purser told us via email. For four hours, the team watched nothing but fish nests. The shallow indentations, spaced about 25 centimetres (10 inches) apart dotted the seafloor in every direction and stretched out over an area the size of the United Kingdom. An estimated 60 million nests were recorded, each with an average of 1,735 eggs cradled inside.

"I’d never seen anything like it in 15 years of being an ocean scientist," Autun Purser, of the Wegener Helmholtz Center in Marine and Polar Research in Germany and lead author of a study on the discovery, told CNN.

An adult icefish guards its nest. Image: Alfred Wegener Institute/PS124 OFOBS team

Icefish belong to a peculiar clade of deep-sea dwellers that have developed unusual physiological traits in response to the frigid waters they call home. Most curious is their colourless blood, which is void of the hemoglobin that gives our 'lifejuice' its crimson hue. They are the only known vertebrates to lack this oxygen-binding protein as adults. To compensate for the strange adaptation, icefish have extra large hearts and wider capillaries to better move oxygen through their bodies. Given the chilly conditions in which they live, their translucent blood also contains anti-freeze proteins to avoid ice crystals forming in their veins. 

While there is still much to learn about the ecology of these notothenioid fish and their en-masse breeding behaviour, the recent discovery suggests that ocean temperatures may play a role in their nesting habits. The clusters of nests "happened to correspond spatially with a tongue of warm water that’s pushed up from the deeper area in the Weddell Sea," Purser explained to Science Friday. "We found that this tongue of warm water matched exactly where the fish nests were. So you were in the zero degrees Antarctic water, and then at two degrees, as soon as you went into this tongue of water, the fish nests started."

The researchers were aboard the icebreaker RV Polarstern at the time of the discovery. Image: Alfred Wegener Institute/PS124 OFOBS team

Previous research shows that icefish typically swim to the surface after they hatch to feed on zooplankton that survive below the ice on a diet of photosynthetic algae. Purser and his team hypothesise that the icefish are meeting at the breeding site, reproducing and then millions of freshly hatched fish rise to the surface. 

As the fish grow bigger they will become attractive to predators like Weddell seals eager to take advantage of the considerable bounty. Intel from tagged seals shows that the animals are active in the area where the icefish are breeding and have been diving these waters for at least the last decade.

Researchers like Purser are hoping to learn more about the complex web of life that exists below Antarctica's ice floes. The recent discovery "means the food webs for this bit of Antarctica need to be rewritten," Purser explains. Cameras have been put in place to monitor the breeding site over the next years and researchers have laid plan to return in April 2022 to survey the surrounding waters. "We would like to know what happens when the eggs hatch. Do they all hatch? Do some juveniles eat the rest? Do all the adults die? And what happens next year? Do fish return to the same nests?"