Temperature and light are critical factors. The ideal temperature for the quickest development of the brood within a female is between 77 and 80 degrees Fahrenheit, and development in the average live-bearer takes around 4 weeks. The interval lengthens to 35 days or more at 68 F. It is critical to keep live-bearers under strong light to replicate sunshine. The development lengthens in dim light, and chilly temperatures combined with dim light might cause reproduction to cease altogether, simulating winter conditions.
Spotting the Condition and the “Gravid Spot”
The pregnant mother grows noticeably in most and also displays the well-known "gravid spot," a dark patch around the base of the anal fin caused by peritoneal wall stretching. The female's anal fin is typical in form.
A black region within her body, known as the gravid spot, appears somewhat front of this. Although it is similar to the womb, unlike mammals, the egg is not linked to the mother's body and is not nourished directly by her.
Each egg contains an embryo and is well-supplied with nutrients from the mother's system, which the developing embryo feeds on during its growth. Recent research suggests that the mother and growing fry have a symbiotic interaction involving fluid exchange, albeit the full scope of this association is yet unknown.
The Eyes of the Fry and Signs of Impending Birth
The eyes of the fry may occasionally be seen through the thin walls of the gravid area when the eggs incubate. The mother's body grows to make room for the growing eggs, becoming deeper and wider. She develops a bulge below the gills a few days before birth, her form becoming rather square in this location, and the gravid spot has extended its area.
The Birth Process of New Fry
When the young are fully developed, they lie in a semicircular configuration and are delivered one by one over several hours, generally tail first. The fry falls a few inches through the water at birth, but rapidly straightens up and seeks protection among the plants if strong enough. If they aren't available, the fry drop to the bottom and seek safety in the sand, rocks, or other available cover. They remain stationary for a brief period of time while they build power.
Why It's Ok to Remove the Male
The presence of the male parent in the spawning tank is superfluous and even harmful. After being fertilized, the female can have up to five or more broods of young at four to six week intervals. Each brood can include anywhere from 12 to 15 kids in a small to up to 150 young in a huge swordtail.
Size of the Fry
The majority of live-bearers are born with fins in the regular shape and are around 1/4-inch long. At birth, most appear like miniatures of adult fish. They are not only considerably larger than newly hatched egg-layer young, but they can also swim and search for food and safety. Most newborn live-bearers are large enough that infusoria is not required as the initial feeding. Live-bearer fry may devour baby brine shrimp, daphnia, and even finely ground dry food within a few hours after birth. Many will survive lurking in the plants until they are large enough to emerge a month or two later, and will happily eat the leftovers in a communal tank. Many an aquarist has been taken aback by new fish that emerge out of nowhere in their aquarium!
Record Deliveries by Live-Bearers
Paul Hahnel, the well-known Guppy breeder, holds the record for the biggest single delivery of fry by a guppy. Mr. Hahnel claims that one female had a brood of 170 children, but only 120 of them survived to adulthood. In a huge wild-caught Platy fish, Dr. Myron Gordon discovered 168 embryos. Swordfish mature between six to eight months, while guppies have been known to give birth to their first brood at the age of 90 days! In one year, a female mollie gave birth to 570 young through several births, according to records.