WrasseLabridae

Wrasses are the second largest family of fish in the sea, and the range inside it is absurd: from a five-centimeter possum wrasse hiding in rubble to the humphead wrasse at over two meters and two hundred kilograms, with several hundred species in between. Parrotfish are wrasses too, specialized for biting rock. If you cannot place a colorful reef fish, the odds are good it is one of these.

They all row. A wrasse swims with its pectoral fins, beating them like oars and keeping the tail folded for emergencies, which gives the family its unmistakable bustling flight and lets them hold position precisely while picking at something.

Most of them change sex. A wrasse matures as a female, and the largest fish in a group converts to male, rebuilding its gonads and repainting itself into a terminal-phase pattern so different from the female that many species were originally described twice, once for each phase. Remove that male and the biggest remaining female starts the change within days.

The cleaner wrasse is the one to watch. A small blue and black species sets up shop at a fixed spot on the reef and other fish queue for it, including predators that could swallow it, and hold still with their gills flared and mouths open while it works over their skin and inside their mouth removing parasites. The station is a genuine institution: clients from all over the reef, regular hours, and roughly two thousand visits a day for a busy cleaner. It also has a con artist's problem, because a cleaner would rather eat the client's mucus than its parasites, and clients that get bitten leave or chase the cleaner, so the cleaners behave better in front of an audience of waiting customers. That work made this small fish one of the most-studied animals in behavioral biology.

At night many wrasses bury themselves in sand or spin a mucus envelope to sleep in.