The Antarctic cod puts itself into a state similar to hibernation for the winter, researchers have found, which is highly unusual for a fish.
Hamish Campbell has a close encounter on the sea floor
Scientists with the British Antarctic Survey (Bas) found Notothenia coriiceps lowers its metabolic rate during winter, saving energy.
As with hibernating mammals, the fish rouse themselves now and again from their dormant state for short periods.
Researchers suspect the "hibernation" is triggered by changes in sunlight.
The sea temperature varies by only about 2C between summer and winter, which is probably too small a difference to induce such a significant change in behaviour.
"It appears the fish utilise the short Antarctic summers to gain sufficient energy from feeding to tide them over in winter," said Keiron Fraser from Bas.
"The hibernation-like state... is presumably a mechanism for reducing their energy requirements to the bare minimum.
"The interesting question we still have to answer is why these fish greatly reduce feeding in winter when food is still available."
The research team caught their Antarctic cod - not a true member of the cod family - and fitted them with acoustic transmitters, which meant they could be tracked using hydrophones, and re-caught later.
A smaller number of fish were fitted with heart-rate monitors and kept in sea cages.
The main conclusion was that they showed a significant set of physiological changes with the onset of winter.
Their overall metabolic rate and growth both declined by a factor of five. They lived within a much smaller area, and swam shorter distances each day, travelling about 20 times less far.
The finding, reported in the journal Public Library of Science (PLoS) One, came as something as a surprise.
"Fish are generally incapable of suppressing their metabolic rate independently of temperature," observed Hamish Campbell, who led the study from the University of Birmingham.
"Winter dormancy in fish is typically directly proportional to decreasing water temperatures.
"The interesting thing about these Antarctic cod is that their metabolic rates are reduced in winter, even though the seawater temperature doesn't decrease much."