The guppy, one of the most common fish in the world, is common for a reason. If you've ever had a batch of guppies, then chances are you've also had a batch of baby guppies, probably even several. Prolific breeders as they are, what are the stages of pregnancy that lead to such copious birthing?
Discovered in Trinidad by Robert John Guppy in 1866 (though described earlier in America by Peters in 1859), the common guppy has risen to prominence as possibly the most popular aquarium fish in the world. Reaching a maximum size of about 6 centimetres for females and 4 centimetres for males, the guppy has been introduced all over the globe with a variety of taxonomic names. Exhibiting some of the most striking colours of all freshwater fish, guppies are often sought for their natural beauty, and have become commonplace in the home aquarium because of their inherent availability, which has been spurned by their continuous breeding. Guppies are livebearers by nature, meaning they give birth to live fry (baby guppies) in place of laying eggs. The cycle of guppy pregnancy has been studied intently over the years, and as a result can be broken down relatively simply.
Being livebearers, guppies literally get pregnant. Many fish lay eggs, which are then fertilised externally. Guppies, on the other hand, are inseminated by males. The female utilises some of the sperm and stores the rest, which she can use for five or six pregnancies henceforth if she so chooses. The gestation period now begins, which will last about 30 days.
The Gravid Patch
Upon examination of back of the female guppy's stomach, where it meets the tail, you should start to notice what is called the gravid patch (or "gravid spot" in some circles). This is a slight discolouration that will slowly begin to darken as the guppy progresses through her pregnancy and is the primary indicator of just how far along she is. At first, the patch will be a yellowish tinge, but will turn brown, even deep orange, as time goes by. This patch is where the fertilised eggs are stored and growing, and the darkening is actually the eyes of the baby guppies as they develop and the orange tinge is the jellylike eggs.
Eventually, the female guppy's stomach will begin to noticeably plump up. The jelly and fry in her womb will become even darker, and as she grows (and the fry grow), the eyes of the babies will begin to emerge from the orange. When her labour nears, her chest will protrude like her stomach and her behaviour will become more solitary.
Finally, after a month or so, the female guppy will give birth to anywhere from five to 30 fry, though there are extreme circumstances where she'll only give birth to one or two all the way up to over 100. Soon after birthing, she'll return to her previous physical state from before her pregnancy. Sometimes, male guppies will attempt to mate with her right away and/or eat her fry. It's usually recommended that the fry are kept in a separate aquarium to avoid being eaten (and many will die regardless). If they're lucky and survive, they'll reach maturity in about 2 months, ready to start cycles of their own.