In the remote depths of the Weddell Sea in Antarctica, scientists have found something astonishing: the world’s largest known fish breeding colony. Spanning around 240 square kilometers, this “fish city” is home to more than 60 million nests of the icefish Neopagetopsis ionah. Each nest, carefully tended by a parent, contains about 1,700 eggs.
This discovery not only reveals an extraordinary natural wonder but also highlights one of the strangest groups of vertebrates on Earth — fish that thrive without hemoglobin, the very molecule that most animals depend on to carry oxygen in their blood.
Icefish and their unusual biology
Unlike other vertebrates, Antarctic icefish (family Channichthyidae) have no hemoglobin in their red blood cells. Their blood is almost transparent, carrying oxygen dissolved directly in plasma. While this would be deadly for most organisms, icefish survive because of the cold Southern Ocean, where oxygen is more abundant in water.
To cope, icefish have enlarged hearts and wide blood vessels that help pump oxygen-rich plasma efficiently throughout their bodies. These adaptations make them unique among vertebrates and place them at the extreme edge of evolutionary possibility.
The nest builders
The breeding colony revealed that Neopagetopsis ionah are more than just physiological oddities — they are also careful architects of the seafloor. Each nest is about 75 cm wide, lined with pebbles, and guarded by a parent fish. The density is staggering: in some places, researchers counted a nest every three square meters.
This concentrated effort transforms the seafloor into a massive nursery, ensuring that the next generation of these rare, hemoglobin-free fish has a chance to survive.
An ecosystem built around the colony
The fish city also fuels a wider web of Antarctic life. Weddell seals dive into the breeding grounds to feed on icefish, and scavengers like starfish and brittle stars feast on the carcasses of dead fish. Abandoned nests provide shelter for sponges and worms, further enriching biodiversity.
In this way, the colony is not just a nursery but also a critical ecological engine for the Southern Ocean.
Why here?
The location of the colony seems tied to ocean currents. Researchers found that the nests are concentrated in areas where slightly warmer modified Warm Deep Water rises from the depths. At about 0°C, this water is just warm enough compared to the surrounding –2°C seas to support egg development, providing a natural incubator beneath the ice.
Conservation urgency
The discovery of the Antarctic fish city carries major conservation implications. It strengthens the case for creating a Marine Protected Area (MPA) in the Weddell Sea, under negotiation by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Protecting this breeding ground means preserving not just the icefish but also the wider ecosystem that depends on them.
Why it matters
The Weddell Sea colony is more than just a natural spectacle — it is a reminder of life’s resilience and diversity. Icefish, with their blood stripped of hemoglobin, demonstrate how evolution can push boundaries. Their fish city, hidden under ice for millennia, shows how much of the ocean still holds secrets waiting to be discovered.
Suggested Reading
Purser, A., Hehemann, L., Boehringer, L., Tippenhauer, S., Wege, M., Bornemann, H., … & Wenzhoefer, F. (2022). A vast icefish breeding colony discovered in the Antarctic. Current Biology, 32(4), 842-850.
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