Temperature and light are important. The optimum temperature for fastest development of the brood inside of a female is between 77 F and 80 F and development will take approximately 4 weeks in the average live-bearer. At 68 F, the interval lengthens to some 35 days or more. It is important that live-bearers be kept in good bright light to simulate sunlight In dull light, the development also lengthens, and cool conditions plus dull light conditions will sometimes stop reproduction completely since this simulates winter conditions.
Spotting the Condition and the “Gravid Spot”
In most live-bearers, the pregnant mother swells unmistakably and also presents the well-known “gravid spot” which is a dark spot near the base of the anal fin caused by the stretching of the peritoneal wall. The female has a normal-shaped anal fin.
Slightly forward of this, inside her body appears a dark area which is known as the gravid spot. It is, in fact, equivalent to the womb, but unlike mammals, the egg is not attached to the mother’s body and fed by her directly.
Each egg contains an embryo and is well furnished with nutritive elements, provided by the mother's system, on which the developing embryo feeds during its development. However, recent studies show that there is a symbiotic relationship between the mother and the developing fry with an exchange of fluids, the total extent of which is still under investigation.
The Eyes of the Fry and Signs of Impending Birth
As the eggs incubate, the eyes of the fry are sometimes visible through the thin walls of the gravid spot. To accommodate the developing eggs, the mother’s body expands, becoming deeper and broader. A few days before delivery, she develops a bulge below the gills, her outline becoming fairly square in this region, while the gravid spot has enlarged its area.
The Birth Process of New Fry
When the young are perfectly formed they lie in a semicircular position and are delivered, usually tail first, one at a time, over a period of hours. On birth, the fry falls a few inches through the water, but quickly straightens out and, if strong enough, makes for cover among the plants. If these are not nearby, the fry sink to the bottom and take refuge in the sand, rocks, or whatever other covers there may be. They lie motionless for a short time while gathering strength.
Why It's Ok to Remove the Male
It is unnecessary, and in fact, inadvisable to have the male parent in the spawning tank. Once the female is impregnated, she can give birth to as many as five or more broods of young at intervals of four to six weeks. The number of each brood may vary from 12 to 15 babies from a young mollie to as many as 150 young from a large swordtail.
Size of the Fry
The majority of live-bearers are born about 1/4-inch long, and all their fins are formed in the normal shape. Most look like a miniature of the adult fish at birth. They are not only much larger than the newly hatched young of egg-layers but are also capable of swimming and looking for food and protection. Most newly born live-bearers are large enough not to require infusoria as the first food. In a few hours after birth, live-bearer fry can eat baby brine shrimp, daphnia, and even finely ground dry food. In fact, many will do just fine eating the left over’s in a community tank as they survive hiding in the plants until they are large enough to emerge a month or so later. Many an aquarist has been surprised by new fish that suddenly appear in their tank this way!
Record Deliveries by Live-Bearers
The largest single delivery of fry by a guppy, as far as we know, was recorded by Paul Hahnel, the well-known Guppy breeder. Mr. Hahnel reports a brood of 170 young from one female, although only 120 of them lived to maturity. Dr. Myron Gordon found 168 embryos in a large wild-caught Platy fish. Swordfish mature at six to eight months of age while guppies are recorded as delivering their first brood when only 90 days old! Records kept of a female mollie show that she gave birth to 570 young, through multiple births, in one year.
A Model for Optimal Offspring Size in Fish, Including Live-Bearing and Parental Effects. The American Naturalist.