Lined Butterflyfish

At 30 cm this is the largest butterflyfish in the world, and on the reef it looks it: a fish the size of a dinner plate cruising over coral heads where the rest of the family are the size of a hand.

It is white, crossed by dozens of fine vertical black lines, with a broad black wedge along the rear of the back that runs down onto the tail base. The dorsal, anal and tail fins are yellow, and a black bar drops straight through the eye. Nothing else in this sea puts that pattern on that much fish, and the black wedge is the mark that survives a bad photograph in poor visibility.

The bar through the eye is working camouflage. A predator aims at the eye because the eye tells it which way the prey will turn, so butterflyfishes hide the real eye inside a dark stripe and, in many species, hang a false spot or a false rear end at the other end of the body. The deep, disc-shaped body then pivots almost on the spot, and the guess costs the attacker its strike.

Lined butterflyfish live in pairs, and the pair is not a breeding-season arrangement. Partners in this family stay together for years, feed within sight of each other and take turns keeping watch. When researchers broke up long-standing pairs and forced new ones, the new partners spent their time in conflict and both fed worse, and the penalty faded only after the new bond had had months to settle.

It eats coral polyps, but not only coral polyps. Anemones, small invertebrates and algae all go down, and that flexibility is the difference between this fish and the strict coral feeders beside it on the same slope: when a reef bleaches, the specialists starve and this one keeps eating. Look for it between 2 and 50 m, usually two at a time. Recorded ages reach ten years.