Hidden Beneath the Ice Antarctica’s Secret Seafloor City: Scientists Find Organized Fish Nests Beneath the Weddell Sea

Beneath the ice of Antarctica’s Weddell Sea, scientists have discovered a hidden world — thousands of perfectly arranged fish nests forming vast, geometric colonies on the seafloor. The finding reveals an extraordinary survival strategy and underscores the urgency of protecting these fragile marine ecosystems.

In a remote part of Antarctica’s Western Weddell Sea, an area once hidden beneath a 200-metre-thick ice shelf, scientists have uncovered a new and unusual phenomena: extensive maintained fish nesting grounds arranged in patterns.

When the A68 iceberg, measuring 5,800 square kilometres, calved from Larsen C Ice Shelf in 2017, it opened new access for research. A remotely operated vehicle (ROV) exploring the seafloor revealed more than 1,000 circular nests, each cleared of the layer of plankton detritus that blanketed surrounding areas. These nests were not randomly scattered, instead they were organised into distinct patterns, forming a vast, geometric fish neighbourhood on the seafloor.

The dimples in the sand are the cleaned fish nests. Some are singular, bottom right, some are in curves, centre top, and others in clusters, top left. All have been cleared of the carpet of phytoplankton detritus that covers surrounding areas – seen starkly in the centre top picture.

The Expedition

The Weddell Sea Expedition 2019 had two aims: to conduct an interdisciplinary scientific programme in the waters surrounding the Larsen C Ice Shelf, and to locate, survey, and image the wreck of Sir Ernest Shackleton's ship, the Endurance, which sank in 1915. The scientific motivation was driven by the critical importance of Antarctica's floating ice shelves. These ice shelves play a crucial global role, acting to support and restrain ice flow from the vast interior of the Antarctic Ice Sheet. When ice shelves thin or collapse, this restraint is lost, leading to accelerated ice flow from the continent, which in turn contributes to global sea-level rise. The expedition was particularly timely, following the major calving of iceberg A68. That calving event provided a unique opportunity to study a region undergoing rapid environmental change, specifically the first chance to study previously inaccessible areas of the seabed that were once underneath A68.

Equipped with autonomous underwater vehicles (AUVs) and a remotely operated underwater vehicle (ROV) aboard the South African polar research vessel SA Agulhas II, the team faced the same extreme sea-ice conditions that had crushed the Endurance vessel more than a century earlier. The immense pressure of the thick multi-year pack ice thwarted the 2019 search effort for the Endurance. Although this expedition did not locate the wreckage, the invaluable experience gained in navigating the treacherous ice and deploying submersible technology assisted with the planning and execution of the subsequent Endurance22 expedition on the same ship, which successfully located the remarkably well-preserved wreck in March 2022 at 3,008 meters below sea level.

A Story of Survival

The architects of these nests are a species of rockcod known as the yellowfin notie (Lindbergichthys nudifrons). Each nest would have been guarded by a parent fish, protecting their eggs from predators. This remarkable organisation is thought to be a survival strategy. The dense clusters demonstrate the ‘selfish herd’ theory in action: individuals in the centre of the group gain protection, shielded by their neighbours. The solitary nests on the outskirts are thought to be occupied by larger, stronger individuals better able to defend their nests. The entire community is a dynamic interplay between cooperation and self-preservation.

Why the Discovery Matters

This find is more than a scientific observation; it has critical implications for conservation. It provides evidence that this area contains a Vulnerable Marine Ecosystem, a unique and fragile habitat that is crucial for biodiversity. Importantly, it builds on previous work in the Weddell Sea, such as Purser et al. (2022), who revealed one of the largest known fish breeding colonies on Earth. These discoveries are vital to support the formal designation of the Weddell Sea as a Marine Protected Area. Protecting this area means safeguarding not just the iconic penguins and seals, but also these hidden nurseries that form part of the Antarctic food web. These underwater environments are a powerful reminder that even in the planet's most extremes, life finds a way to build complex, resilient communities.

Original Article: A Finding of Maintained Cryonotothenioid Nesting Sites in the Western Weddell Sea; Frontiers in Marine Science; DOI:10.3389/fmars.2025.1648168

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