Pouncing on prey quickly is crucial for swordfish living in deep ocean waters (Image: K Fritsches) Tuna keep their whole bodies warm, giving them a visual edge (Image: K Fritsches)

Swordfish heat up their eyes to improve their tracking of fast-moving prey in deep, cold water, suggests a new study.


Researchers already knew that certain large ocean predators – such as swordfish, tuna and some sharks – keep at least their eyes and brains warmer than the ambient ocean temperature. But this is the first convincing biological explanation for why, says the team led by Kerstin Fritsches at the University of Queensland, Australia.

In laboratories onboard research vessels in the Pacific Ocean, Fritsches and her colleagues took freshly caught swordfish and immediately removed the retinas. They varied the temperature of the retinas from 5°C to 25°C and exposed them to light flashes of different frequencies, at a range of intensities. They found that temperature had a big impact on the retinas’ ability to distinguish between individual flashes, using measurements of the retinas’ physiological response.

Swordfish usually maintain the temperature of their eyes and brain at between 19°C and 28°C. Depending on the diving depth of the swordfish – and so the ambient temperature and intensity of light – this improves their ability to track their prey by up to ten times, says the team.

“Swordfish inhabit a virtual desert, with few sources of food scattered over huge distances. When they do come across extremely fast and highly manoeuvrable prey – such as large flying squid – the ability to pick up fast movement will significant increase their chances of catching their prey,” says Fritsches.

Seven times better

It is not clear exactly how keeping the eyes and brain warmer prevents a deterioration of their vision. “But a temperature drop must affect transmission speed in nerves and other molecular and neurochemical processes, slowing the whole nerve response down,” Fritsches says.

It is common to find swordfish in the Pacific Ocean at a depth of 300 metres. At this depth, the ambient temperature could be as cold as 3°C. A swordfish retina kept at 20°C at this depth would be seven times better at resolving moving images. The greatest benefit of having warm eyes would be when the water is cold but brightly lit.

In swordfish, a part of one of the muscles that moves its eyes has adapted to produce heat instead. It warms up the blood, which is then moved towards the eye and the brain. Other fish, such as tuna, keep their whole bodies warm. And similar light experiments by the team with retinas from freshly caught tuna suggest that warming also confers a visual advantage in these species.

Since all bony fish that warm themselves heat their eyes and brain, the adaptation is probably to prevent any debilitating effects from fast temperature changes to the nervous system – and the effect on the speed of vision is probably the most noticeable of these, Fritsches says.

Journal reference: Current Biology (DOI: 10.1016/j.cub.2004.12.064)