Eeltail catfishesPlotosidae

This fish tastes with its skin. A catfish carries taste buds outside its mouth, thickest on the barbels around the jaws but scattered over the flanks and the belly and out onto the fins and the tail as well, so an animal working a sand flat in the dark is reading the bottom chemically with its entire surface and knows what it has just passed over as well as what lies ahead of it. Four pairs of barbels do the fine work. The eyes contribute very little at night, and it does not matter.

A catfish on a coral reef is an odd thing to begin with, because catfish are a freshwater group. Almost all of the thousands of species live in rivers, lakes and floodplains, and only a couple of families ever made a proper living in the sea. Doing it took extra hardware: this family carries a structure no other fish has, a branching, blood-rich organ behind the anus that dumps excess salt, and that is what lets an animal with a river ancestry hold its internal balance in water as salty as this.

It is also one of the genuinely dangerous animals of a shallow reef. The leading ray of the dorsal fin and of each pectoral fin is a serrated spine with venom glands at its base, the spines lock upright, and the sting is out of all proportion to the size of the fish. Deaths have been recorded. This is not something to handle, to net, or to push aside.

Juveniles are the ball, a dense rolling sphere of striped fish crossing open sand, and that is where the species page here starts. Adults take it apart. They turn nocturnal, spend the day packed in a heap under a ledge, and work the sand after dark alone or in small groups, which is why a flat that seemed to be full of them can look empty by daylight. One species is listed for this sea.