TriggerfishesBalistidae

The family is named after a lock. The first dorsal spine stands up like a mast, and a second, much smaller spine slides in behind its base and jams it there, so the big spine cannot be folded back down by anything pulling from outside. Release the small one and the whole thing drops flat. A triggerfish uses this to make itself impossible to remove: it backs into a hole, raises the spine against the roof and locks, and a pelvic spine braces it from below. You can pull the tail off one before you get it out.

It swims like nothing else on the reef. The dorsal and anal fins ripple in opposite phase along the back and belly, driving the fish forward, backward or nowhere at all with the tail folded away and held in reserve for a sprint. That buys precision at the cost of speed, which suits an animal whose food does not run away: short jaws, chisel teeth set in heavy bone, and a diet of the armored half of the reef, urchins and crabs and snails, taken shell and all.

The question divers actually arrive with is about the titan triggerfish, and it has a specific answer. In the breeding season a female digs a crater in the sand and stays with it, and the water she defends is not the circle you can see on the bottom: it stands over the nest and widens upward, so a diver who reacts by swimming up is climbing further into it. Move away horizontally, low and level, fins toward the fish. She warns first, turning side on and standing head down over the nest, and the bite that follows comes from a mouth built to break sea urchins.

Everything about the family is armor and independence: leathery skin with plate scales set in it, no true pelvic fins, and small high-set eyes that rotate separately from each other. Eleven species are listed here, and two of them have given up shells entirely to hang in the current picking plankton.