A small armored box, 20 cm (8 in), that comes in two versions. Males are brilliant blue on the sides with a yellow-brown back; females and juveniles are dull olive spotted with dark brown and carry only a blue tail. They are the same species, and photographs of the two circulate as if they were not.
It lives here and in the 🌊 Gulf of Aden and the 🌊 Persian Gulf, and nowhere else - one of the regional endemics that make this fauna distinct from the wider Indo-Pacific.
The body is a shell of fused plates with holes for the eyes, mouth, fins and tail. Nothing bends, so the fish sculls with fins that look far too small and hovers rather than swims, and it never travels far from a hole.
The defense is chemical and worth respecting in a closed space: stressed boxfish release ostracitoxin from the skin, which is lethal to other fish in a confined volume and is a well-known way to lose an aquarium. In open water it does nothing to anyone, and the fish's habit of hanging motionless a meter away makes it one of the easiest subjects on the reef.
The species. Ostracion cyanurus Rüppell 1828 - cyano- (blue) + oura (tail). ETYFish
- rigid box-shaped body, males brilliant blue with a yellow-brown back
- females and juveniles dull olive-brown with dark spots and a blue tail
- tiny fins sculling constantly, body held perfectly still
- length up to 20 cm
