An Analysis of the Genera of Surgeon Fishes (Family Acanthuridae ) 1 
John E. Randall 
The genera of few families of tropical mar- 
ine fishes have been as badly confused as 
those of the Acanthuridae. Ahl (1923: 36) 
and others have pointed out the need for a 
review of the generic classification of the 
family. As may be seen in the key below, the 
genera are easily distinguished; most of the 
inconsistency with respect to use of names 
has been purely nomenclatorial. 
The genera fall into two major groups on 
the basis of the armature of the caudal pedun- 
cle. Four of them, Acanthurus , Ctenochaetus , 
Zebrasoma , and Far acanthurus, are character- 
ized by a single folding spine on each side 
of the peduncle. Naso and Prionurus have one 
to six fixed spines or plates in this region. 
Such a clear-cut distinction might tempt one 
to treat the two groups as subfamilies. There 
is, however, a sharing of other important 
characters by various genera in both groups 
which makes such a consideration untenable. 
In this paper the phylogenetic interrelation- 
ships of the surgeon fish genera are discussed, 
and an attempt is made to put the generic 
classification in order. 
The four genera with the single folding 
spine on the caudal peduncle are being re- 
vised. The revision of Ctenochaetus is com- 
pleted (Randall, 1955). Those of Zebrasoma , 
Paracanthurus, and Acanthurus will follow. 
1 A portion of a thesis submitted to the Department 
of Zoology, University of Hawaii, in partial fulfillment 
of the requirements for the degree of Doctor of Phi- 
losophy. Contribution No. 67 of the Hawaii Marine 
Laboratory in cooperation with the Department of 
Zoology and Entomology, University of Hawaii. Man- 
uscript received November 8, 1954. 
KEY TO THE GENERA OF ACANTHURIDAE 
la. 1 to 6 immovable keel- or thorn-like 
spines or laminae on each side of caudal 
peduncle; least depth of caudal peduncle 
contained 3.5 to 6 times in length of 
head; dorsal and anal spines stout. .... 2 
lb. A single folding spine on each side of 
caudal peduncle; least depth of caudal 
peduncle contained 2.1 to 3.5 times in 
length of head; dorsal and anal spines 
slender (except Paracanthurus) 3 
2a. 1 to 2 pairs of caudal spines or laminae; 
pelvic fin rays I, 3; anal spines II; dorsal 
spines IV to VII; teeth small, conical 
with tips slightly compressed, smooth 
or with very small denticulations. (Indo- 
West-Pacific) Naso 
2b. 3 to 6 pairs of caudal spines or laminae; 
pelvic fin rays I, 5; anal spines III; dorsal 
spines VIII or IX; teeth moderately 
large, flattened, close-set, with large den- 
ticulations. (Japan, Australia, tropical 
eastern Pacific, Galapagos Islands). . . . 
Prionurus 
3a. Pelvic fin rays I, 3; scales on head modi- 
fied to tuberculated plates. (Indo- West- 
Pacific) Paracanthurus 
3b. Pelvic fin rays I, 5; scales on head not 
modified to tuberculated plates 4 
4a. Dorsal spines VI to IX (usually VIII or 
IX); length of longest dorsal ray con- 
tained 3.5 to 6 times in standard length; 
359 
