Black Molly Breeding Guide

How to breed the black molly (Poecilia sphenops var.): sexing by gonopodium, hard alkaline conditioning, internal fertilization, gestation and care of free-swimming live fry.

Overview

The black molly is a melanistic ornamental strain derived from the short-finned molly, Poecilia sphenops, a livebearing poeciliid native to Mexico, Guatemala and Honduras. Commercial black mollies are mass-produced in fish farms rather than collected from the wild. Like all poeciliids it reproduces by internal fertilization and gives birth to free-swimming live young, which makes it one of the more accessible egg-free fish to breed.

Sexing

Males are smaller and slimmer, while females grow larger and become more rounded; in P. sphenops females are roughly a centimetre longer than males, and males stop growing once the gonopodium is fully developed. The defining male trait is the gonopodium, a modified anal fin used to transfer sperm. Mature females show a darkened gravid area near the vent.

Conditioning

Mollies require moderately hard or harder water with a basic pH and weaken in soft or acidic conditions; Seriously Fish gives 21-28 °C, pH 7.0-8.5 and 15-30 dH for P. sphenops. The diet is omnivorous, consisting of zoobenthos and detritus in nature, and captive fish should receive a good proportion of vegetable matter such as blanched spinach to keep adults in condition.

Mating & Gestation

Mollies lack elaborate courtship; males approach females and attempt copulation directly. After internal fertilization the female carries developing embryos. In P. sphenops gestation is roughly four weeks to about eight weeks depending on conditions, and broods of up to about 120 fry are not uncommon. Keeping several females per male reduces constant pursuit of any single female.

Birth & Fry Care

Newly born fry are relatively large and will accept brine shrimp nauplii or powdered flake from birth. Because adult mollies may eat fry, a heavily planted tank or moving the gravid female to a separate tank until she gives birth improves survival. Adding vegetable matter to the diet supports healthy growth.

Common Challenges

Soft or acidic water is the most common cause of decline, so maintaining hard alkaline conditions is essential; mollies do better in slightly brackish water and tolerate even full marine conditions. Predation of fry by adults and overcrowding from frequent broods are the other recurring issues.