Sunday, August 5, 2001		Nature

UNLIKE many predators who use stealth, speed and camouflage to catch prey there are some who pretend to be innocuous, nonchalant or doing something that will not harm the prey, but once the prey comes into striking range, the predator springs a surprise attack at the last minute. Sea-anemones that attach themselves firmly to the substrata by sucker-like base and can not move around freely are highly active predators.

Contrary to popular belief, these soft-bodied coelenterates (aquatic invertebrates having simple tube-shaped or cup-shaped body with a double skin) are neither blind nor deaf. They are quite aware of the world around them. They can hear and smell their prey approaching and are always ready to strike. While they appear totally at rest, they tune into a high frequency — about 55Hz or 55 cycles per second.

The hair-like cilia which detect vibrations in the water are relatively short and might pick up fish movements or something similar. If, however, a favourite species of shrimp approaches, chemoreceptors detect the smell of the prey ( in fact, the chitin-like sugars of the shrimp’s shell) and the vibration detectors lengthen. This enables them to pick up vibrations at 5Hz, the frequency of the swimming movements of the shrimp. In this way, the sea-anemone is alert at the moment it is brushed by the shrimp, and it can act immediately and immobilise the shrimp before it has time to escape.

Sea-anemones open slowly and their crown of tentacles curl around the food item, conduct it to the central mouth and it is swallowed. In less than a minute, the sea-anemone has unfurled and is ready to catch again. Although they may resemble elegant undersea flowers, they are, in fact, voracious undersea predators.

Sea anemones that festoon the coral reefs of the world carry millions of tiny stinging-cells that are capable of immobilising ordinary fish, which are then drawn into the anemone’s mouth and devoured. But there is a group of about 30 species of small reef fish, all of which are immune to the poison in the sea anemone’s tentacles.

Known as anemone fish, damsel fish, or clown fish, these brightly marked reef-dwellers spend most of their lives lolling in the lethal tentacles of their poisonous partners. Their secret defence is a thick layer of special mucus that completely covers their bodies. It possesses a chemical quality that stops the anemone’s poison-cells from firing when they are touched by the fish. The chemical involved is still unknown and the precise way it operates remains to be explained, but it somehow manages to deceive the anemone into thinking the fish is not food.

Some investigators believe that the fish’s trick is to transfer part of the anemone’s mucus covering to it own body mucus. The anemone’s many tentacles are constantly rubbing against one another, and must have some kind of sting-inhibitor covering them or the animal would keep stinging itself. If the fish thoroughly impregnates its own mucus shield with this inhibitor, it will give the impression that its body is part of the anemone and it will be completely safe. This is indeed what seems to happen. A special characteristic of these particular fish must be the possession of a body-surface mucus that will ‘marry’ with the tentacle mucus. Other fish lack this property, the mucus marriage never takes place, and they are stung to death.

The immunity of anemone fish may have a genetic basis but it requires considerable acclimatisation to function properly.