Reef Stonefish

A lump of rock with a mouth. Up to 40 cm (16 in) of warty, algae-crusted, mottled brown fish that sits half-buried on rubble and does not move when approached, because moving is not its plan. The eyes are set high, the mouth is vertical, and the outline is genuinely impossible to resolve until the fish is already known to be there.

Thirteen spines stand along its back, each with a pair of venom glands at the base and a groove running up the shaft. They lie flat and rise when the fish is disturbed. Weight from above - a foot, a hand, a knee - compresses the glands and drives venom up the spine under pressure, which is why every serious case is a stepped-on fish rather than an aggressive one. The pain is described as the worst known from any marine animal, and severe stings can kill.

The main component is verrucotoxin, and knowing what it is explains the first aid. Unlike the small stable poisons of puffers and cone snails it is a large protein, and proteins come apart when heated: immersing the limb in water as hot as the patient can stand without scalding, around 45 C (113 F), for 30 to 90 minutes breaks the toxin down and cuts the pain dramatically. Then hospital, because antivenom exists and the wound needs cleaning. Worth knowing before the dive rather than after.

It hunts by not moving at all. Small fish and shrimp come within range, and the strike takes about fifteen milliseconds - among the fastest movements measured in any fish. The mouth opens, the volume of the head triples, and the prey is pulled in by the inrush of water before it has registered anything.