How Do Swordfish Heat Their Eyes and Brain? The Brain Heater

Why does a swordfish need warm eyes?

The swordfish leads a double life. At night it hunts near the surface, where the water is often 18–24 °C (64–75 °F). At dawn it dives into the twilight zone, usually 200–600 m (650–1,970 ft) deep, where the water can be as cold as 5–10 °C (41–50 °F). This means its body faces large, rapid swings in temperature every day.

For most fish, cold water means a cold brain and cold eyes. Nerves conduct signals more slowly when they are cold, and the retina, the light-sensitive layer at the back of the eye, responds more slowly too. For a predator that hunts quick, darting squid in dim light, a sluggish eye would be a serious handicap. When the American biologist Francis Carey discovered the heater in 1982, he suggested that it protects the brain and eyes from rapid cooling during these daily dives.

Where is the heater and what is it made of?

The heater lies beneath the brain, next to the eyes, and is built from part of one of the extraocular muscles, the small muscles that move the eyeball. A study of its anatomy identified it as part of the superior rectus, the muscle that normally rotates the eye upward. Only part of the muscle has been converted, and the rest still works as ordinary muscle.

Under the microscope, the heater cells look very different from normal muscle. They have lost almost all of the protein filaments that make muscle contract, and instead they are packed with mitochondria, the tiny power plants of the cell, which have been reported to fill up to about 60 percent of each cell. The tissue is rich in cytochrome c, a protein used in burning fuel, and its cells resemble the brown fat cells that help keep human babies and hibernating mammals warm.

The heater is well insulated. Thick layers of fat surround the brain and the eyes, and in the area next to the heater, the bone beneath the brain is reduced to a thin layer of connective tissue, which lets heat pass into the brain.

How does the heater make heat?

In a normal muscle, a nerve signal releases calcium inside each cell, which makes the muscle contract, and the calcium is then pumped back into storage. Pumping it back costs energy. Heater cells keep the calcium-release and calcium-pumping machinery but have lost the parts that contract. Scientists think the calcium is released and pumped back over and over in a cycle that does no mechanical work, and the energy used is given off as heat.

Producing heat is pointless if the blood immediately carries it away to the gills, where it would be lost to the sea. The swordfish solves this with a countercurrent heat exchanger, a dense bundle of small arteries and veins running side by side. Warm blood leaving the heater passes right next to cold incoming blood and warms it, so most of the heat stays near the brain and eyes.

How do warm eyes help a swordfish hunt?

The benefit of the heater was demonstrated in 2005. A team led by researchers in Australia measured how swordfish retinas respond to flickering light at different temperatures. Their key measure was temporal resolution, the ability of the eye to tell apart events that happen in quick succession, much like the number of frames per second a camera can record.

Warming the retina greatly improved its temporal resolution. Depending on how deep the fish is diving, a warm swordfish eye can detect rapid movements more than ten times better than an eye at the same temperature as the surrounding water. For a swordfish hunting squid hundreds of meters down, this can mean the difference between seeing prey dart away and missing it entirely. The researchers suggested that warm eyes give ocean predators such as swordfish, tunas and some sharks a crucial advantage over their cold-blooded prey.

Swordfish also keep their main red swimming muscles warmer than the surrounding water using heat exchangers, and these warm muscles produce more power, which helps explain the fish’s reputation for speed. That reputation is examined in How fast can a swordfish swim?

Which other fish have warm eyes or brains?

The swordfish is not alone. Its relatives the marlins, sailfish and spearfishes have a heater organ built from the same eye muscle, and the butterfly mackerel, a member of the tuna family from cool southern oceans, has its own heater. Studies of the family tree of these fish suggest that the ability to warm the brain and eyes evolved independently more than once, each time in species that moved into colder water.

Other fish warm their eyes and brain without a special heater, using heat-exchanging blood vessels that trap warmth from their muscles. These include tunas, which also keep their swimming muscles far above water temperature, as explained in Are bluefin tuna warm-blooded?, and several sharks, such as the shortfin mako, a fast hunter that sometimes attacks swordfish.

Is a swordfish warm-blooded?

Not in the way mammals and birds are. Most of a swordfish’s body stays close to the temperature of the water around it, so it is not fully warm-blooded. Instead, it keeps a few key parts warm: the eyes and brain, heated by the heater organ, and the red swimming muscles, warmed by their own heat exchangers. Scientists call this regional endothermy.

This targeted heating uses less energy than warming the whole body would, yet it lets the swordfish do something few fish can: hunt actively both in warm surface waters and in the cold, dark depths, with sharp eyes and a quick brain in both.