Boxfish, also called trunkfish or cowfish, are a small group of shallow-water, marine fish in the family Ostraciontidae (order Tetraodontiformes). The family includes the genera Lactoria, Ostracion, and Tetrosomus and is closely related to the poisonous puffer fish of the family Tetraodontidae. To avoid confusion with these poisonous relatives, some people avoid eating boxfish despite their being good for food. Boxfish are generally oval in shape when seen from the side; while being viewed from one end, different species of boxfish resemble triangles, squares, or pentagons.
Boxfish take their shared common name from the hard shell or carapace, composed of strongly joined plates corresponding to the scales of other fish, which surrounds their bodies. Only the eyes, the low-set mouth, the fins, and the tail are not covered by this rigid shell. Boxfish reach lengths of up to 2 ft (61 cm). They inhabit warm waters from the Pacific Ocean to the Caribbean. Boxfish usually prefer shallow waters, and are often found around coral reefs. In some areas the boxfish are dried and used as decorations.
Boxfish possess an intriguing method of defense against predators, such as sharks. Like other species of fish, when disturbed boxfish can secrete molecules known as icthyocrinotoxins or ichthyotoxins—literally, fish poisons—from their skin. The toxins of two Pacific boxfish species, Ostracion immaculatus from Japan and O. lentiginosus from Hawaii, have been characterized by biochemists. These toxins turn out to be esters of choline chloride. Choline is a vitamin in the B complex that makes up part of many of the fatty acids found in the membranes of human cells.
Similar to the ichthyotoxic compounds of fish from other families, boxfish toxins are surfactants. In general they act as biological detergents, promoting the dissolution of fatty acids in water. This occurs because surfactants, like all detergents, include parts that can interact with water and parts that can interact with fats. Thus, fats and water can mix, something that normally does not occur. By this mechanism, these ichthyotoxins cause hemolysis in the laboratory by dissolving cell membranes. When these membranes dissolve, cells break down and die. This irritates and deters predators, saving the boxfish from being eaten.