It looks an impossible job when a soft-bellied gastropod like the triton tries to demolish a spiky crown of thorns starfish. But the starfish is always the loser.
By Kurt Amsler
The giant crown of thorns starfish (Acanthaster planci), which has been notoriously responsible for the destruction of large areas of coral reef, has basically only one natural enemy: a football-sized mollusc called the triton (Charonia tritonis).
The starfish, which spends its life browsing on coral polyps, can grow up to 1m across, and is capable of demolishing 4m2 of reef surface a month. It uses the thousands of suckers on the undersides of its 16 arms to extract from their cells the minute, plankton-eating polyps whose calcareous secretions build a coral reef.
In recent years crown of thorns starfish have proliferated alarmingly, devastating reefs which may have taken hundreds of years to build up. Various environmental factors have favoured this population explosion, but one of the main causes is the popularity among collectors of the triton's shell. The brown and white-mottled shell, which may measure 40cm across, fetches high prices. But the real price is paid in dead corals, as fewer of the giant gastropods roam the ocean floor, searching for starfish to hoover the insides out of.
Two species of triton are found in the tropical and sub-tropical waters of the Indo-Pacific region, the Mediterranean and the Atlantic. They hunt the seabed by night, relying on their sense of smell to identify prey. Once they have registered the scent of a starfish, they give chase - if you can call the pursuit of a big starfish by a big snail a chase. And no matter how desperately the crown of thorns tries to escape, it is almost always caught and devoured.
The pictures shown here were taken one night after I had spent 30min following an enormous triton along the floor of the Indian Ocean at a decidedly brisk clip. I was excited, because I knew crown of thorns starfish were active in the area, and I dared to hope I might see an encounter between the two species. Suddenly, by the soft light of my lamp, I saw the triton come to a halt, change direction, and set off at a speed that was anything but snail's pace. My light beam wandered across the surrounding seabed and, sure enough, soon fell on a big, thorny starfish.
The starfish seemed to realise that it was in imminent danger, and tried to make off. Both creatures were moving quickly now, but the starfish did not have a chance. The triton slid along like an antediluvian monster until, only inches away from its prey, it extended an enormous proboscis to sieze one limb of the starfish and pull it under its mantle.
Twice the size of the triton, the crown of thorns tried to throw off its predator by twisting and heaving; but the giant mollusc had fastened on like a suction cup, slipping its deadly probe between the starfish's tubular limbs.
A paralysing poison injected by the triton rendered the starfish distictly less energetic, and when the hefty snail jerked violently to one side it overturned its victim to reveal that it had already devoured a good portion of the peduncles and soft parts of the arms. Reviving briefly, the starfish attempted escape with its remaining functional arms, but the triton gripped it with deadly determination, and continued to feed.
Running out of air, I had to return to the surface before the drama concluded, but the outcome was already plain. Such duels may last for an hour or more. But as the triton's paralysing injection takes effect, the starfish invariably succumbs; and I'm sure the hunter which I had watched spent the remainder of the night devouring the soft interior of a starfish which had eaten its last coral polyp.
Appeared in DIVER - December 1997