In this group we find many of the most popular aquarium fishes such as guppies, platies, and swordtails. The colourful livebearers we keep in our aquaria bear little resemblance to the livebearers that exist in nature, but are “fancy” (more colourful, longer finnage) forms that have been cultivated in large breeding facilities, principally in Asia but also elsewhere. Their natural habitat extends from North America (eastern United States) and the Caribbean to South America (south to northeastern Argentina and Uruguay). They are hardy fishes that are suited to community aquaria shared with other fishes, and they are therefore usually regarded as “beginners’ fishes”, but they are also of interest to specialists and in many countries there are special aquarium clubs for people that are interested in livebearers.
They prefer well-lit aquaria with relatively dense vegetation and appreciate some floating plants in a corner. There is no need to detail decor here since the general rules discussed in the first section of this book largely apply in the case of these fishes. Water chemistry is not of major importance, but they prefer slightly alkaline to acid water since the water is alkaline in the natural habitat and in Asia where most of them are bred. The most interesting feature of these fishes is that they give birth to live fry. When the fry are born they are fully developed and immediately swim to the water’s surface, fill their swimbladders with air, and begin to search for food. If there is sufficient cover many of the young will survive, otherwise they are likely to be prey for other fishes.
They often adapt well to most types of aquariums, but they clearly prefer well-lit aquariums with relatively dense vegetation and also appreciate having some floating plants in a corner. The filter must also be effective and relatively powerful because they prefer clean and oxygen-rich water.
Water values don't really matter, but they prefer slightly alkaline water to acidic water, since the water in their natural habitat, and also the water where most of them are grown, is alkaline. Some of these species also appreciate having a little salt in the water.
In their natural environment, they mostly feed on small invertebrates, but in aquariums they take whatever is offered. If you give them a varied diet of frozen food, flakes and granules, it is quite sufficient, although, like all other fish, they naturally appreciate live food. Some species such as black mollyn (Poecilia spenops) also eat algae and if there is not enough algae in the aquarium, they should be given a vegetable feed.
They fit into most community aquariums, although some of the live feeders can take some very small fry.
One reason for the incredible popularity of these fish is surely that they are so easy to breed and, of course, that they give birth to live fry. When they are about half-grown, in the males, the anterior anal-fin rays are transformed into a rotatable mating organ (gonopodium). During reproduction, a so-called sperm packets (spermatophores) to the female's genital opening from where they continue to the oviduct. There they are stored and are sufficient for fertilization of several rounds of eggs. The sperm can actually be viable for up to 15 months. In the females, new eggs are then formed every three or four weeks and the sperm reach the eggs in the order that the sperm packets have entered the female's genital opening. However, there are some exceptions to this.
Then the embryos develop in a fairly normal way. When the female is about to give birth, the egg shell bursts during expulsion. When the fry are born, they are fully developed and swim directly towards the surface of the water, fill the swim bladder with air and start looking for food. If there is enough protection in the aquarium, e.g. in the form of floating plants and java moss (Vesicularia dubyana), many of them will manage, otherwise there is a great risk that they will be eaten by other fish.
If you want to grow a little more rationally, you do it in a special breeding aquarium where you place the pregnant female (you can see that she is pregnant because she is very round) in a so-called litter box When the young come out, they fall out through the cracks in the birthing box and the "cannibalistic" mother does not have time to eat them.
As we mentioned earlier, they are hardy and unassuming fish and this of course also means that they rarely suffer from diseases. Sometimes, however, some of the Asia-grown fish can be quite delicate, perhaps because that they were grown in a completely different environment than what we have in our aquariums, often in very salty and/or very warm water. If these fish are not kept in quarantine and slowly adapted to our conditions by the importer, they may have difficulty adapting to the aquarium. This generally applies to all mass-farmed live-breeders, but especially to guppies and mollys.