Sex and Growth in Male Mollies

di H. J. Crier

First 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 same brood!

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. velifera.