146 
BULLETIN OF THE BUREAU OF FISHERIES 
fins plain translucent. The female with a black blotch on each side of abdomen just above and in 
front of vent when gravid, increasing in size with the development of embryos and becoming very 
prominent before parturition takes place. 
Many specimens were collected from various restricted areas, ranging in length from 19 to 42 
millimeters to 1 54 inches), and these form the basis for the above description. The species is 
recognized by its small size (although not unlike Lucania parva in size) , the plain greenish or grayish 
coloration, and the depressed head. The males are readily distinguished by the peculiar prong- 
shaped development of the anal fin, which occurs in no other minnow in the vicinity of the Chesa- 
peake. Geiser recently (1923, pp. 175 to 188) has shown that Gambusia of the eastern part of the 
United States differ from those of the Mississippi, 
drainage in the detailed structure of the modified 
anal fin (intromittent organ; also called gonopod by 
Geiser). The eastern form has a larger number (11 
to 14) of antrorse teeth on the distal ossicles of the 
third ray; and the larger ossicles, situated toward the 
base of the fin from the distal ones, are posteriorly 
denticulate, whereas in the Mississippi fish these 
segments are entire. The posterior branch of the 
fourth ray usually has seven ossicles, with teeth on 
the posterior margin in the eastern fish, while the Mississippi Valley fish has only about five ossicles 
with teeth. These differences, together with other minute differences, appear to be constant and 
therefore of specific value. Consequently the eastern Gambusia must be regarded as distinct from 
the central western one; and, as pointed out by Geiser, the Atlantic slope one, in accordance with 
rules of nomenclature, becomes G. holbrooki Girard and the Mississippi Valley fish should stand as 
G. affinis (Baird and Girard). 
This fish feeds on a large variety of foods, which, however, are taken principally at the sur- 
face, the habit of surface swimming and feeding being more strongly developed in Gambusia than 
in any of the other so-called “top minnows” inhabiting the waters of the United States. In 15 
stomachs examined from specimens in the present collection the principal food consisted of insects 
and insect larvre, although a few small crustaceans and a few egg masses, too, were present. 
The young of this species are born alive and well developed, being 8 to 10 millimeters long. 
The first young appear 
in May, and spawning 
continues until about 
September. One female 
may deliver several 
broods of young during 
a single season. The 
number of young pro- 
duced at one time may 
vary from a few to a 
hundred or more. The 
largest brood on record 
(Hildebrand, 1921, p. 12) 
consisted of 211 young. 
As a rule a small female 
has fewer young than a large one, although there are many exceptions to this rule. The average 
number of young in a brood probably does not exceed 25. A female kept in the aquarium (Hilde- 
brand, 1917, p. 6) once produced six broods of young during one summer. The young grow rapidly, 
and usually when about 20 millimeters long the sexual differentiation in the anal fin takes place, 
and the fish are sexually mature very soon thereafter. Young born in May and reared in the 
aquarium have been known to produce young by September of the same year. 
The largest specimen taken during the present investigation was only 1 % inches in length. 
This would indicate that the fish run rather small in the vicinity of the Chesapeake, for the maxi- 
mum size attained by the species is about 2 inches. The males, however, are much smaller, 
rarely reaching a length of 1J4 inches. 
// x 
/ cm 
» 
Fio. 79 . — Gambusia holbrooki, gravid female 
Fig. 78 . — Gambusia holbrooki, adult male 
