Researchers aboard German research icebreaker Polarstern in February 2021 made an unexpected discovery: the world’s largest icefish nesting ground, located 500 meters (1,650 feet) under the Weddell Sea off the coast of Antarctica.
While imaging ocean floor topography using the vessel’s towed video camera sled system, graduate student Lilian Böhringer upon sighting the vast, incredible icefish nesting “metropole” contacted the bridge where Autun Purser, of the Alfred Wegener Institute, was stationed.
This Antarctic region, known as the Filchner Trough, was of particular interest to the team because of its enigmatic inflow of slightly warmer, topographically guided bottom waters, up to two degrees Celsius warmer than surrounding bottom waters. Such underwater troughs often act as conduits for upflows to the Weddell Shelf, and this one flowed directly over a vast breeding colony unlike anything that had been seen before.
It was here that they found the ocean pockmarked with thousands of circular, crater-like nests, most of which contained a single, ghostly icefish (or Neopagetopsis ionah)—specially adapted to cold water, with transparent blood (or “white blood” lacking hemoglobin) containing antifreeze compounds.
“I would say [the massive colony] is almost a new seafloor ecosystem type,” he added. “It’s really surprising that it has never been seen before.”
The nests were dispersed at a density of 0.26 per square meter, the researchers said, each measuring just over a half meter (1.6 feet) in diameter—the same length as the fish itself. The occupying icefish guarded 1,735 eggs on average. These bowl-shaped excavated nests offer the eggs protection from water currents, while the coarse, rocky detritus found at the bottom of each nest provides aeration and cleanliness for eggs laid over top.
Degraded icefish carcasses were also spotted throughout, in and around the nesting colony, which would provide input for local food webs, amphipod scavengers, and overall biogeochemical processing. The water column above the nesting ground exhibited higher concentrations of such biological matter than elsewhere in the Weddell Sea.
Meanwhile, inside abandoned nests, with their exposed gravel and protection, were found colonization of fauna such as tube dwelling polychaetes, bryozoan colonies, and sponges, thusly enhancing local biomass and diversity. Elevated concentrations of phytodetritus were discovered in many of these abandoned nests, presenting food for anemones, sponges, other suspension feeders, and the like.
Furthermore, adolescent icefish which would spawn from these eggs would frequent shallower depths in the water column, making easy meals for apex predators such as Weddell seals on the shelf above. The researchers observed these Antarctic mammals diving in greater numbers in the vicinity of the Filchner Trough than in other areas.
The incredible discovery of such a colossal nesting ground raises many questions about how this ecosystem behaves over time. To address the matter, Purser positioned two camera systems at depths of 3 meters (10 feet) above the seafloor to monitor the benthos and lower water column for two more years.
