Why are clownfish unharmed when they swim among the stinging tentacles of sea anemones? Lawson Wood has at least part of an answer
The symbiosis (a word which literally means living together) between the clownfish and thr host anemones has fascinated scientists and marine biologists for generations. It is now proving equally captivating for snorkellers, divers and underwater photographers. The host anemones constitute the territory of the clownfish (sometimes known as anemone fish). And because the anemones are sedentary animals, you can - once you have discovered your anemone and your fish - be sure of finding them again on future dives. Both can be approached closely, but often the fish's fearless, aggressive nature makes it difficult to photograph. Of the 1000 or more species of anemone found in all corners of the world, only 10 species act as hosts for clownfish. All of these anemones are found in the Indo-Pacific region, and they are host to a total of only 28 species of true clownfish. This often results in a number of different varieties being found in the same location. The most prolific areas are around Papua New Guinea, where ten varieties of clownfish have been recorded from one location. There are six recorded from Sipadan, off the east coast of Borneo. As you travel further from this central 'hub', the species diversity reduces and quite often results in only one type of fish being found on a number of different host anemones, such as Amphiprion bicintus in the Red Sea. There is only one species off the Comores, five off the Great Barrier Reef, and none recorded off Hawaii, although there are host anemones. Several locations, such as the Seychelles, Maldives, Oman, Madagascar, Mauritius and Chagos, have their own indigenous species. Many, of course, overlap, and quite often different species can be observed very close to each other. All are found in shallow water (less than 50m). How, exactly, do clownfish manage to live in such proximity to anemones without falling victim to the tentacles' stinging cells? It is, it appears, a very personal thing. For it has been discovered that it is the individual fish's relationship to its particular host which allows it to remain unharmed by the stinging cells of the anemone. In aquariums, where a clownfish has been introduced to a new host, it has been observed that the fish exhibits behaviour consistent with being stung. However, afterwards, the fish has returned to the host time after time, swimming in an elaborate 'dance' among the tentacles. Slowly it allows the tentacles to touch its fins, then gradually the rest of its body, until it appears to be immunised against the sting. The clownfish's immunity derives from the coating of mucus which covers its body. During its dance, it would appear not only that the clownfish's mucus is spread onto the anemone's tentacles, but also that the mucus from the tentacles is spread over the fish. The exact mechanisms involved are not understood; but the result is that the fish is afforded complete protection. And the anemone - what does it gain by having the clownfish on board? Although anemones can exist quite happily without their clownfish, it appears that, as well as acting as cleaners, these little fish also help, through their fiercely territorial behaviour, to protect their host from would-be predators.