SECONDARY SEXUAL CHARACTERS IN THE LOACH 
MISGURNUS ANGUILLICAUDATUS CANTOR 
By Kixuo OKAMOTO 
Of the Anatomical Institute of Keio University, Tokyo, Japan 
~ 
ONE PLATE 
Secondary sexual differences are common among mammals, 
birds, amphibians, and insects, as we know; for instance, in color, 
in size, and adornment. Among fishes there are scattered in- 
stances of such differences, but they are comparatively rare. 
Since the appearance of Cunningham’s (1) work on Cynolebias, 
Regan (3) has published a paper on the sexual differences in 
Cynolebias but not on those of the Ostariophysi. While study- 
ing the intestinal respiration of loaches, at the suggestion of 
Prof. N. Yatsu, I observed the secondary sexual characters in 
them. According to the descriptions of Cunningham and others, 
the large size of the body is certainly one of the commonest of 
the secondary sexual differences in female fishes, while the strong 
pectoral fins are one of the commonest in the male. In addition 
to these I observed a remarkable difference; namely, a swelling 
on the posterior sides of the dorsal fin of the male fish. This 
swelling is more or less spindling or somewhat cylindrical in 
shape, tapering at each end and especially so caudad (Plate 1, 
fig. 1a). Its length is about 1 by 10 to 12 centimeters. This 
specific characteristic of the male distinguishes it markedly from 
the female. . 
In some Ostariophysi (for instance, a kind of Cyprinidez), ac- 
cording to Cunningham’s description, the males are distinguished 
by an enlargement, or swelling, of the anterior ray of the pec- 
toral fins, especially in the spawning season; this swelling dis- 
appears soon after breeding has been accomplished. I have not 
observed this character in the loaches, nor does the swelling on 
the back disappear even after the breeding season. In what 
stage does the swelling begin to appear in the male loach? It 
appears as in other cases in mature fishes, according to my ob- 
servation, in specimens more than 7 centimeters in length. 
The second characteristic of the male consists in the enlarge- 
ment of the pectoral fins and the elongated second ray of the 
723 
