Sex and Growth in Male Mollies
H. J. Crier
published in Livebearers this reprint taken from ALA special publication 3
The Genus Pecilia mollies, limias and related species available from the
ALA see their web pages.
One of the most desirable native aquarium fishes is the sailfin molly,
Poecllia latlpinna, Which experienced breeders have developed into a
number of interesting color varieties that are generally available to the
tropical fish hobbyist. Undoubtedly, the most popular and familiar variety
of P. latipinna is the black molly. However, the "wild" green
sallfin, which ranges from Mexico through the Gulf states as far north as
Virglna, often is available along with albino, marble and chocolate
varieties. In its natural habitat P. latipinna is found in either brackish
or fresh coastal waters of shallow depth, The species is extremely hardy
and adaptable to salinity changes, easily tolerating extremes from 2% to
twice the salinity of sea waters Remarkably, adaptation to any salinity
within the above range can be achieved within 24 hours, for the most part,
without unduly stressing the fish. In the aquarium, P. latipinna ranks as
one of the most avid algae eaters, at least the wild type, that this
author has encountered, rivalling that of any catfish whose reputation is
based on an ability to clear green glass.
In its Florida habitat, Where the author has collected extensively the
size of wild P. latipinna is quite variable depending upon collection
site, with the Largest specimens generally coming from brackish water
habitats. The size achieved by males is particularly variable with only
the largest possessing a sallfin as the illustration depicts (the
illustration was to dark to scan. It depicts two male sailfin mollies the
small one is 3mm long and the large one 10mm long with a fully developed
sailfin). The two males pictured were raised in the same aquarium, came
from the same brood and ate the same food. They were from the third brood
born to a female originally collected from brackish water habitat in St.
Petersburg, Florida. The female had been maintained with a single sailfin
male. Both of the fish illustrated were preserved in formalin and
photographed when one year old. Only one fact can explain the size
discrepancy observed between the two fish. The smaller male matured within
six weeks of birth, along with a similar-sized sibling and ceased to grow
further. The larger male, and several others from the same brood, did not
begin to mature until three months of age when a slight elongation of the
anal fin, or future gonopodium, was noted. Elongation of the gonopodium
continued for a period of six months during which time the fish grew to
their adult size, When the large, late-maturing males from this brood were
between 11 and 12 months old, the fleshy pall at the tip of the gonopodium
began to form. It was not until after complete formation of the gonopodlun
that the sailfin began to develop. Of 14 fish reared to maturity from this
brood, two were sma11, early-maturing males of similar size, 5 were
late-maturing males all of which exceeded 70 mm in standard length. The
remaining fish were females of equal size to the late-maturing males.
From these observations, it appears that certain aspects of size and
physical traits achieved in male mollies are linked to the time of sexual
maturation. After males mature, they either cease to grow any further or
grow very little. On the other hand, females continue to grow after
becoming sexually mature. Early-maturing male mollies do not achieve large
body size before endocrine factors, probably associated with androgen
production by the testis, result in cessation of the growth process.
Early-maturing male mollies remain small throughout life, late-maturing
male mollies which undergo an extending period maturation and growth,
attain large body size before becoming sexually mature. Once mature,
further growth ceases although sailfin development only commences after
the gonopodium has fully developed and the male is capable of and does
court females. While the authors captive late-maturing male P. latiplnna
took about nine months to mature completely (from the beginning of
gonopodial development until formation of the Sailfin), observations of
wild populations indicated that this process occurs in about three or four
months. The reasons for this discrepancy are not immediately evident.
In addition to wild P. latipinna, there is also a link between final
adult body size and sexual maturation in male albino, lyretail P.
latipinna; P. orri and P. petenensis, In the former two species, males
were obtained from single broods which matured in only three weeks. These
males total about 1/2 inch long and have not grown since becoming sexually
mature. They are less than half the size of late-maturing vales from the
There is an apparent myth in molly lore indicating that females may
reverse their sex and become large males, This myth probably generated
because late-maturing males are physically similar to females and grow to
large size before beginning to mature. Numbers of large maturing male P.
latipinna have been dissected by the author who has never found any
evidence for an ovary, or degenerating ovary, in any of them, The gonad
was always a developing testis, The sex reversal myth should be laid to
rest as it is not consistent with the observed facts.
To develop good, large-bodied male molly stock for breeding purposes,
molly aficionados should not obtain small males expecting that they will
grow into the large beautiful sailfin varieties. Small males remain sall
throughout life. There are no exceptions! In the author's experiences it
takes three to five months for late-maturing males to commence gonopodial
development. Early-maturing males should be culled from any breeding
stock, Virgin females should always be mated to the best, large-bodied
males available, Another factor in raising large mollies they grow better
if frequent partial changes of the water are made and feedings are quite
frequent......several times per day, but only in amounts that do not
accumulate in the aquarium. The addition of some salt to the water may be
desirable, but not necessary. The two fish depicted (refers to the
illustration again) were raised in a 145-Gallon aquarium with discus!
These are strange aquarium fellows and salt was never added to the water.
Editors notes I have encountered small, early-developing males as well as
large, late-developing males in most of the approximately fifteen strains
of guppies I have kept, all of the aquarium-bred mollies, platies and
swordtails I have had, and also in the molly Species P. mexicana and P.