Fish that spends all day holding its breath

Leaves sulphide-rich mud only at night

July 2010: Scentisst have discovered that a unique fish species has adapted to a hostile environment poisonous to most other organisms, managing to survive in an oxygen-free environment.

SURVIVAL: The beared goby can spend all day in oxygen-free but nutrient-rich mud, where predators are unable to prey on them.

The Benguela-goby, or bearded goby, is found only on the anoxic continental shelf outside Namibia and South-Africa, but since the collapse of the sardine fisheries, this goby has become the new predominant prey species for larger fish, birds and mammals in the region.

Scientists from Norway, South Africa and Namibia have discovered how this little survivor copes in an environment that kills other fish. About 9,000 square kilometres of the continental shelf off Namibia lack oxygen, as a result of a thick layer of rotting mud. Methane gas and hydrogen sulphide build up in the sediment, causing frequent gas eruptions that strip the entire water-column of oxygen.

Previously the belief was that fish could not survive in an environment of sulphide rich mud or between stinging jellyfish. But the bearded goby has become tolerant to these extreme environments.

Survives in extreme environment that would be toxic to its predators
During the day the goby stays on the anoxic or oxygen-free seabed, where it appears to ‘hold its breath' while still eating the mud and associated small organisms. Later, under cover of darkness, the goby swims up to the more oxygen-rich surface waters in order to restore its oxygen debt and to digest its food, says senior scientist Anne Christine Utne-Palm from the University of Bergen.

While holding its breath it feasts on the sulphidic mud, preferring it to alternatives such as sand because the sulphide-rich sediment kills many of the goby's predators. The fish also thrives among jelly.

When gobies swim up to shallower waters at night, they associate with jellyfish. This is a night-time shelter from predators, as well as a food source. The scientists' study showed that goby predators avoid jellyfish, but that gobies are largely indifferent to them, staying close and even swimming inbetween their stinging tentacles, says Professor Anne Gro Vea Salvanes.

The findings are a result of a long-term research collaboration between senior scientist Anne Christine Utne-Palm and Professor Anne Gro Vea Salvanes, University of Bergen, Professor Mark Gibbons, University of the Western Cape, South-Africa and chief biologist Bronwen Currie, National Marine Information and Research Centre (NatMIRC), Namibia..