Clownfish find their hosts by remembering their smell, which drifts over them as embryos.
Sea anemones have long been known to release very powerful olfactory compounds, which might best be described as perfumes. It seems that clownfish become "addicted" to the perfume as an embryo in the egg, lying next to the sea anemone covered in perfume-like compounds it releases.
This hypothesis had to be proved underwater by locating a lot of clownfish egg-clutches nestled by host anemones. Over several weeks of successful dive-investigations, we observed about 40 pairs of clownfish with egg-clutches and, without exception, they had all placed their clutch right next to their host anemone’s column, not even a millimetre away.
By good memory, and a keen sense of smell, juvenile clownfish can quickly track down a suitable sea anemone by smelling their way to the perfume, when they settle on the reef two weeks later. Earlier American and Japanese research on the fate of reef fish larvae established that clownfish larvae emerge from the egg case after seven to nine days, and immediately swim to the surface. They swim around in the ocean, sometimes drifting with the currents, vulnerable to the environment, and feeding on plankton.
After two weeks, the juvenile clownfishes are about a centimetre long and ready to settle on the reef. This always happens in the middle of the night, when there is less chance of being discovered by a hungry predator. Once the fish reach a coral reef, the search is on for a suitable host sea anemone to take them under its protective tentacles. Not all of them will be fortunate enough to find one, and the unlucky ones will end up on the menu of many larger fish or invertebrates on the reef. But for the lucky surviving ones, the smell they instinctively remember is their passport to safety.
This chemical imprinting may be more widespread among reef fishes than is thought, and smell the trigger which leads most fish to coral reefs when they come to settle. It is thought that olfactory cues, perhaps released from corals, similar to the perfume released from sea anemones, might be the trigger for many juvenile coral reef fish to settle. ■