Matt Clarke answers some of the most frequently asked questions on livebearers...
What are livebearers?
Most fish lay eggs, but some give birth to live young. There are two different types of livebearers - viviparous and ovoviviparous. In ovoviviparous fishes the eggs hatch internally and are born fully-formed, receiving nutrition from the egg yolk. In viviparous fishes the fry recieve nourishment from the mother.
About 15 of the approximately 480 fish families known exhibit some form of livebearing. This includes half of all sharks and rays, as well as the livebearing tooth-carps, such as Guppies, Platies, Mollies and Swordtails, and a number of unusual species.
How do I sex livebearers?
Male livebearers, like Guppies, Platies, Mollies and Swordtails, have a modified anal fin called a gonopodium, which is used to internally fertilise the female. Females lack gonopodia and have a triangular anal fin instead, so they're easily sexed.
In some species, such as Phallichthys fairweatheri, the gonopodium may be as much as 50% of the total body length.
Hormone treated female Guppies may have gonopodia. These are useless for breeding and should be avoided.
Are they best kept in pairs?
Male livebearers can be somewhat over-amorous, so it's less stressful for the females to keep at least two females per male, rather than just a pair.
How long is the gestation period?
It varies according to species, but most common livebearing tooth-carps can give birth to between 20-200 fry every 4-6 weeks.
Most livebearers reach sexual maturity quickly. Male Guppies can breed after two months; females at three months. So the tank can soon become overpopulated if you have lots of females.
I keep getting fry, but there are no males in my tank. How can this be possible?
Many livebearers produce packets of sperm called spermatozeugmata and impregnate the female with these. Spermatozeugmata can remain in the folds of the ovaries and uterus of female livebearers for up to a year in some species, allowing the female to fertilise several successive batches of eggs from a single spawning.
Chances are, if you buy female livebearers, that they've already been impregnated by any males in the tank, so even if you buy all females you still may become overrun with fry. If you don't want fry, don't keep females.
How do I feed the fry?
Livebearer fry are usually larger than the fry of egg laying species, and are normally able to take fine foods straight away, unlike the fry of egg layers which need to absorb the yolk sac before they will eat. Most newborn fry will take liquid or powdered fry foods for livebearers, or live or frozen brine shrimp nauplii. They grow quickly and within a couple of weeks can be weaned on to crumbled flake or growth foods.
Should I use a breeding trap?
Most experts agree that breeding traps aren't ideal for livebearers and that they may cause gravid (pregnant) females undue stress.
You can raise a reasonable number of fry in a community tank if you plant heavily with bushy plants like Cabomba, and ensure there are no fish present large enough to eat the fry. For bigger broods you'll need a separate tank.
I've been told that Guppies are easy to keep. Why do mine always die?
A decade or two ago, Guppies were tough, disease-resistant and easy to keep. Today, things are quite different and they can be hard to keep, disease-prone and sensitive to pollution. Some shops won't stock them any more, and very few recommend them to new fishkeepers.
It's likely that continued inbreeding, the use of hormones and incorrect use of antibiotics have caused genetic and health problems, making many strains much weaker.
The fins of my Guppies keep getting shredded. What's wrong?
Male Guppies have long fins that may be nipped by other fish, particularly by some barbs and tetras. Check the compatibility of any fish before buying them.
While fin-nipping is common, shredded fins are sometimes a sign of Guppy disease, which is thought to be caused by the ciliate parasite Tetrahymena.
Treatment is often difficult, but anti-parasite treatments, especially those containing some copper, can be be effective.
Adding aquarium salt at a dose of 2-3 g/litre can help reduce salt loss through the open wounds. Good water quality is vital.