Fig. 343.—Teuthis cœruleus (Bloch & Schneider), Blue Tang. Mexico. Family Teuthididæ.
The principal genus is Teuthis, characterized by the presence on each side of the tail of a sharp, knife-like, movable spine with the point turned forwards and dropping into a sheath. This spine gives these fishes their name of surgeon-fish, doctor-fish, lancet-fish, tang, barbero, etc., and it forms a very effective weapon against fish or man who would seize one of these creatures by the tail. The species have the center of distribution in the East Indies and have not reached Europe. Three species are found in the West Indies. The blue tang (Teuthis cœruleus) is chiefly bright blue. The common tang, Teuthis chirurgus, is brown with bluish streaks, while a third species, Teuthis bahianus, has a forked caudal fin. Very close to this species is Teuthis crestonis, of the west coast of Mexico, and both are closely related to Teuthis matoides, found from India to Hawaii.
Fig. 344.—Brown Tang, Teuthis bahianus (Ranzani). Tortugas, Fla.
Teuthis triostegus, of Japan and Polynesia and the East Indies, is covered with cross-bands alternately black and pale. In Hawaii this is replaced by the very similar Teuthis sandwichensis. Many species are found about Hawaii and the other Polynesian Islands. Teuthis achilles has a large blotch of brilliant scarlet on the tail, and Teuthis olivaceus a bright-colored mark on the shoulder. Teuthis lineatus, yellow with blue stripes, a showily colored fish of the coral reefs, is often poisonous, its flesh producing ciguatera.
Zebrasoma differs from Teuthis in having but 4 or 5 dorsal spines instead of 10 or 11. In this genus the soft dorsal fin is very high. Zebrasoma flavescens, sometimes brown, sometimes bright yellow, is common in Polynesia; Zebrasoma veliferum, cross-barred with black, is also common.
Ctenochætus (strigosus), unlike the others, is herbivorous and has its teeth loosely implanted in the gums. This species, black with dull orange streaks, was once tabu to the king of Hawaii, who ate it raw, and common people who appropriated it were put to death.
In Xesurus the caudal lancelet is replaced by three or four bony tubercles which have no sharp edge. Xesurus scalprum is common in Japan, and there are three species or more on the west coast of Mexico, Xesurus punctatus and Xesurus laticlavius being most abundant.
In Prionurus (microlepidotus) of the tropical Pacific the armature is still more degraded, about six small plates being developed.