Threefin blennyTripterygiidae

Most of the fish flesh eaten on a coral reef comes from fish that no diver has ever noticed.

Triplefins are one of the families supplying it. A grown one is a couple of centimeters of fish sitting on a rock, and it lives the way that implies: it matures in weeks, breeds again and again through the year, and is eaten by more or less everything with a mouth, from a hawkfish to a young grouper to a shrimp. When biologists traced where the flesh in a reef food web actually comes from, it was not the visible fish. It was this hidden size class, present in small quantities at any one moment and replaced constantly, feeding the reef the way grass feeds a pasture. Their larvae add to it: rather than going offshore, they stay near the reef they were spawned on, so the replacement supply is local.

The family takes its name from the only feature that separates it cleanly from the other small fish on the same rock, three separate dorsal fins in a row along the back where almost everything else has one or two. That is a photograph's diagnosis rather than a diver's. In the water what you have is a stub of fish on a vertical face, propped on its front fins, moving in short hops and never leaving the rock.

Five species are listed for this sea, some of them named only in the last few decades, because a fish this size has to be collected on purpose before anyone can describe it. The number is not a measure of what is down there. It is a measure of how many people have gone looking at that scale.