Largest fish colony

The largest fish colony ever recorded is a breeding ground of Jonah's icefish (Neopagetopsis ionah) discovered on the seafloor under an ice shelf in the southern Weddell Sea near Antarctica by a research team in February 2021. The colony covers at least 240 square kilometres (93 square miles) and is estimated to contain more than 60 million nests in active use. Each nest contains one adult fish and 1,500-2,000 eggs, which extrapolating means the total number of eggs in this single mega-colony is estimated to be approximately 100 billion. This colossal colony is believed to be several decades old.

Each nest is about 15 cm (6 in) deep and resembles a blue circle on the seafloor

Icefishes are small ray-finned fishes related to perches, and are zoologically famous for being the only known vertebrates to lack haemoglobin in their blood as adults. Red blood cells are also absent or, even if present, non-functional, hence icefish blood is colourless. They are able to survive without blood by having a low metabolic rate and because oxygen is more soluble at the very low temperature in Antarctic waters. These factors collectively ensure that their blood does carry enough oxygen around their body to sustain them.