Scientists discover an underwater city of 60 million fish on the Antarctic seafloor

An international team of scientists has confirmed the existence of the largest known fish breeding colony on Earth beneath the Filchner Ice Shelf in the southern Weddell Sea, Antarctica. The discovery reveals a hidden ecosystem made up of roughly 60 million icefish nests spread across an area of about 90 square miles, an expanse comparable in size to the U.S. city of Orlando. The finding marks an unprecedented breakthrough in our understanding of Antarctic marine biodiversity and highlights the existence of complex ecosystems that remain unexplored beneath the continent’s ice.

The colony was first identified during a 2021 scientific expedition led by the Alfred Wegener Institute (AWI) under the direction of researcher Autun Purser. To locate it, scientists used the Ocean Floor Observation and Bathymetry System (OFOBS), a platform capable of surveying the seafloor in one of the most inaccessible regions on the planet. A subsequent mission deployed the autonomous underwater vehicle LASSIE, designed specifically to monitor the colony over extended periods. The robot operated for 48 hours beneath hundreds of meters of ice, confirming both the colony’s vast extent and the remarkable density of its nests.

Life in the ice

The architects of this enormous “underwater city” are blackfin icefish (Neopagetopsis ionah), a species that has evolved extraordinary adaptations to survive in Antarctica’s frigid waters. Unlike most vertebrates, these fish lack hemoglobin, giving their blood an almost transparent appearance. They also produce antifreeze glycoproteins that prevent ice crystals from forming within their bodies, allowing them to thrive in waters colder than the freezing point of freshwater.

Each fish builds a circular nest measuring about 30 inches in diameter, carefully clearing away sediment to expose small stones where it deposits roughly 1,700 eggs. The male typically remains behind to guard the clutch until the eggs hatch. The exceptionally high nest density, averaging 0.26 nests per square meter, creates a vast reproductive network stretching as far as the underwater vehicle’s cameras could see.

Researchers also uncovered the reason millions of fish congregate in a single area of the seafloor. Measurements showed that a slightly warmer deep-water current flows through the Filchner Trough, keeping this region as much as two degrees Celsius warmer than the surrounding waters. While temperatures remain extremely cold, that small difference appears to create ideal conditions for the species’ reproduction.

Yet this massive nursery also becomes a major hunting ground each season. Satellite tracking data show that Weddell seals regularly visit the colony during the breeding period to prey on adult fish. The significance of the discovery has also strengthened calls for greater protection of the area. Ongoing monitoring with autonomous vehicles will allow scientists to track how these populations evolve in the coming decades and support proposals to expand Marine Protected Areas in the Weddell Sea.

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