Sunday, April 29, 2001		Nature

SOME marine animals have warm muscles, they are usually kept warmer than the surrounding water. It’s a useful evolutionary trick which keeps them one step ahead of their prey.

One such fish is tuna, when travelling at speed, it pulls its fins into grooves along the body, giving it a smooth, hydrodynamic outline. Blue-fin tuna are the fastest among these fish. With these adaptations, the above species can reach the speed of up to 45 mph over short distances, mostly while trying to escape from one of their main enemies, the swordfish.

The powerful swimming muscles of the great white shark, and its relatives porbeagle and the mako, are kept 45-50 degrees warmer than the surrounding water. And for each 10-degree rise in temperature, these predators obtain a threefold increase in muscle power. These animals achieve this warmth with the help of a special blood-supply system to the swimming muscles, which looks rather like an old-fashioned, central-heating radiator, and acts like a heat exchanger. Basically, warm blood is prevented from being carried to certain parts, such as the gills, where heat would be lost to the surrounding seawater.

Besides this the fish’s swimming efficiency is further enhanced by its torpedo-shaped body, and also by the texture of its skin. The skin of sharks is not as smooth as that of many other fishes, instead it has minute teeth-like structures all over that are known as dermal denticles, which not only protect the skin but also reduces the drag and lessens the fish’s resistance in water.

Another highly active undersea predator, the swordfish, swims in short, fast bursts in pursuit of fast-swimming prey. It does not have warm muscles, instead, it has a heater in its head. The brain, the retina of the eye and brown tissues on the underside of the brain case can be up to 57°F (14°C) warmer than the surrounding water. Like the great white shark, the swordfish has a heat exchange mechanism that prevents any serious loss of heat from the blood and directs that warm blood, not towards the muscles, but towards the brain.

During day swordfish go down up to a depth of 2,000 ft and stay in semi-darkness, returning to the surface at night. It is one of the main reasons why not much is known about the biology of this animal. It does not swim continuously like many fish do in search of food, instead it acts like a cheetah and is known as a ‘stalker and sprinter’. If it is to remain alert and ready to respond to opportunities that might present themselves, it must have a sensory system that can respond immediately. And this is precisely that swordfish has. In order to spot and chase passing prey, particularly in cold depths, the fish warms up its eyes and brain, an ability it shares with the white marlin and the sailfish, the fastest fish in water.

Swordfish and its relatives swim so fast that sometimes they cannot stop abruptly. Confronted by an unexpected obstacle, they can be in trouble. Broken swords of swordfish have been found embedded in whales and wooden ships. Predators in the open sea, though, can normally afford to overshoot the mark, but those attacking prey close to the bottom are in danger of crashing into the mud. The large-mouthed bass swims slowly towards its target, using vision to guide itself. At the last moment, it darts at the prey, oblivious to any change of course. If the prey moves, the bass misses, but it is careful not to overshoot the original spot. It makes rapid braking movements just before the actual point of prey capture so as not to crash.