BoxfishesOstraciidae

Frighten a boxfish inside a bucket and everything in the bucket dies, including the boxfish. Under stress the skin releases pahutoxin, which is not a protein and not the tetrodotoxin the puffers carry but a soap-like molecule that pulls apart cell membranes and wrecks gill tissue. In open water it disperses and does nothing. In a confined volume it does not, which is why a boxfish is a catastrophe in a home aquarium and why a diver should never put one in a bag. Very few fish anywhere defend themselves by putting a chemical into the water rather than into a wound.

It needs something like that, because the body is a box. The plates of the head and trunk are fused into a rigid case with holes cut for the eyes, mouth, fins and tail, and a fish inside a case cannot flex, cannot bend into a turn, and has no escape burst in it at all. Every other reef fish answers a lunge by accelerating. This one cannot, so it pays for the armor with a chemical.

The shape has a second life on land. In 2005 Mercedes-Benz built a concept car around the outline of a yellow boxfish, on the understanding that the box was a low-drag, self-stabilizing form. Work on the living fish a decade later found close to the opposite: the keels on the carapace generate forces that push the body off course, and the boxfish holds a straight line by correcting continuously with those small fins. The shape is not stable. It is maneuverable, which is what an animal that spends its life at walking pace among coral heads actually needs.

Five species are listed here and three of them are cowfishes, carrying horns above the eyes and at the back corners. Sizes run from about fifteen centimeters to well under half a meter, colors change completely between juvenile and adult in some of them, and none of them are in any hurry. A boxfish will let you watch it from arm's length for as long as you have air, which is the compensation for everything the armor cost it.