Guppies have got to be one the most often bought fish on the market. They are cheap, easy to breed, and come in more shapes and colors than most any other fish. They have their own society dedicated to the breeding and showing of them and new strains are constantly being developed. They also make good fish food.
But this article isn't just another "How to Breed a Guppy" article. I'm going to try to get a little more in depth on the reproductive system of this most fancy of livebearers.
To start off, most people can tell a girl guppy from a boy guppy, right? Just like in humans. Sort of. Contrary to popular belief, the male's gonopodiun is not a hollow tube. The gonopodiun on the male is composed of nine long fin rays crowded together. The third and longest of these rays is equipped with a hood and a hook pointing backwards folding over- the gonopodiun. These long fin rays have the ability to swing foreword and form a temporary· groove which passes tiny packets of sperm to the female.
"Packets of sperm" You say, "What packets? I thought milt just sort of floated around". Well, unlike egg laying fish, the male guppy does not eject milt into the environment (obviously) but instead manufactures sperm in tiny globes called spermatophore. When the gonopodiun swings forward, these globes move through the sperm duct, along the gonopodiun and are transferred to the females genital pore (her unmentionables). From there they move to the oviduct (ovary) and the globes burst open releasing the sperm and allowing them to disperse into the folds of the ovary to wait for the eggs to be formed. The male can transfer as much as 3000 packets of sperm – not all of which are used in only one hatch (God forbid).
The walls of the ovary are lined with germinal epithelium, which are tiny cells that divide, acquire a yolk, and become unfertilized eggs. This process in a fish is known as ovoviviparism, which means that the eggs and unborn young take their nutrition from a yolk and not from the mother fish. Most Poeciliids reproduce this way but not all livebearers do. For instance, Goodeid young take their nutrition direct from the mother fish via an umbilical chord. This process is known as viviparism.
The female guppy is able store the male's sperm for several months - which explains how your fish keep having batches of babies even after all the males have been removed from the tank (Goodeids, on the other hand must mate and be fertilized for each batch of young born). They will not, however, form any new eggs until all the young from the previous batch are gone. It takes about 5-6 days for the unfertilized eggs to be completely formed and once fertilized, 22-24 days before the young are born. Not all livebearers operate this way, either. Heterandria formosa will carry young and eggs at all stages of development at the same time and will give birth to two or three young a day for' about a month before taking a break for a couple of weeks.
The guppy is one of the easiest fish to breed (which is why it’s only worth five BAP points) but the challenge can come from developing your own strain or improving on an established one. Just remember to be extra firm with your culling. I’m sure anyone with an Oscar would be glad to take them off your hands.
Fishkeeper's Guide to Livebearing Fishes, Peter W. Scott. Tetra Press. Getting to Know the Guppy. Mike Schadle. Aquarium Fish magazine, Feb.1 989. Fancy Publications, Inc.
Guppies Across the Generations, Mark F. Batell. Tropical Fish Hobbyist. Oct. 1990. T.F.H. Publications.