A small, near-transparent jellyfish that swims rather than drifts, and the most venomous animal in the water.
A cube-shaped, almost colourless bell with tentacles trailing from each of the four corners, from a couple of centimetres across in the smaller species to about 30 cm in the largest. Unlike the drifting jellyfishes it swims actively, at a speed that outpaces a swimmer, and it has true eyes — clusters of them on each face of the bell, some with lenses and retinas — and avoids obstacles. Several genera occur in the region and separating them underwater is not realistic, so this entry is pitched at the class.
The venom of the larger species acts on the heart and is among the fastest-acting known; smaller species cause Irukandji syndrome, in which a mild sting is followed half an hour later by severe pain, cramping and a hypertensive crisis. Records from Indonesian and Philippine waters are scattered and mostly come from stings rather than from surveys, so the honest position is that box jellyfish are present across the region, are not common at the sites covered here, and are close to invisible in the water. A full-length exposure suit is the practical answer, and a severe sting is a medical emergency rather than something to be dealt with in the water.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.