How Live Bearers Breed

The two types of tropical fish that hobbyists will have in their tanks, are those that lay eggs, and those that birth live young. These are called viviparous fish, or fish whose eggs are fertilized internally, mature, and result in "fry", or live babies.

In the tropical fish world, there are four basic groups of live bearing fish: platys, swordtails, guppies and mollies. Males are easily identified in some strains such as the swordtail, by the elongated bottom tip on their tail. However, males will also have a gonopodium, which is an anal fin that is modified to use for inserting in the female, to deposit sperm that fertilizes the eggs internally.

The most successful colonies of livebearers are those that have adequate numbers of females for each male, so that they are not constantly chasing and stressing only one or two mates. Sometimes having more than one male can be a problem, unless you have a large tank that allows for two males and 5-7 females of any live bearing species.

Once bred, females will produce live fry in about a month, sometimes a few days sooner, if the tank environment is quite warm. Hobbyists that want to keep some of their young fish, will need to provide for their safety, either with floating weeds, or specially made fry "traps", which are compartments where the female is placed as she gives birth. These come in a couple of styles that are made of either netting or plastic with holes in the side for circulation. They are hung on the inside of the tank, with the female in the top level, and a slot where newborns can fall through into a second chamber, safe from the mother and other adults.

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