Clownfish

Imagine living among stingers that do not sting you. This little fish, usually orange, does just that, tucked among the tentacles of a sea anemone. It is a ray-finned fish, and its neighbour on this tree is the lined seahorse, a surprising distant cousin. Every clownfish hatches as a male, and each one can change once into a female, but never back again.

Before moving in, a young fish dances over the anemone, gently touching it until a slimy coating keeps it safe from stings. The pair then help each other. The anemone gives shelter from predators and shares meal scraps, while the fish chases intruders away and cleans tiny pests off its host. In each group on an anemone, the largest fish is the one breeding female, with her breeding male next in size. If she dies, he becomes female and the next male down takes his place as the breeding male.

It is about 8 cm long, about as long as a bank card, with females larger than males. It is usually bright orange with three thick white bars edged in black. It lives in warm, shallow coral reefs, about 3 to 15 metres deep, from the Ryukyu Islands of Japan through South-East Asia to north-western Australia and the Northern Territory.