Despite all these devices, most breeders prefer the more natural method of having plants in abundance to provide shelter for the young, and removing the mother at the earliest chance. Moving the young is not to be recommended at so early a stage. If the mother is supplied with more live food then she can eat, she is unlikely to destroy many of her own young; hence the addition of mosquito larvae or Daphnia. If the young have to be moved, do not use a net. Either siphon them off as gently as possible or, better still, ladle them out with a soup ladle or teacup. Mollies will usually not eat their young unless they are hungry, so that if this species is well fed young will be present in plenty without further precautions.

The best plants for young livebearers are masses of Myriophyllum, Ambulia, Nitella, Utricularia (bladderwort), or even algae. They allow the young to dive in for protection but are too dense for the adult to follow with any ease. Young born prematurely may still have a visible bulge formed by the yolksac and will be small. They are often poor swimmers and are likely to die off rapidly. Sometimes the addition of a little salt to the water helps, about a teaspoon to the gallon, making roughly a 0.1 % solution, or even more. Young mollies or guppies are quite happy in a 1% salt solution; if bred from marine-acclimatized
parents, they can take a 3 % solution.

Feeding the Young

Livebearer young are quite large, as young fishes go, and can be fed dry or other prepared food straight away. If they are given only prepared food, growth will be poor, but a mixture of live and dry food is quite satisfactory. The influence of a few feeds of young Daphnia on the subsequent growth rate of newly dropped platys is quite remarkable, and this early feeding of live food is very important for good development. Later it matters much less, although the fishes will still do better with a good proportion of live foods.

They should be fed several times daily, and kept at not lower than 75°F. Young fishes can take high temperatures and thrive in them and they do not like cool water permanently, although they can take chills remarkably well. They like to keep their bellies full and should go around looking like fishes stuck onto a small football for the first few weeks. They will not overeat, no matter how they look. If much dry food is used, and of course it must be suitably small in size, scavengers should be present. Snails are the easiest to install, but Cory dor as will not eat the young fishes as long as there are left-overs to clear up.

Suitable first live foods are mikro worms, newly hatched brine shrimp, shredded earthworm, sifted Daphniae, newly hatched mosquito wrigglers or shredded white worms. Sifted Daphniae are those which have been passed through a fine mesh, eliminating the adults or nearadults. Actually, if adult Daphniae are used in reasonable numbers—so as not to overcrowd the tank and compete seriously for oxygen—their young are a continual source of food. Suitable dry foods include any fine powder food, such as dried shrimp finely ground, fine cereals, and liver or egg powder.

These can be followed a week or so later by larger live foods and coarser dry foods, although the smaller sizes will still be eaten. To the diet may be added tubifex worms, larger larvae of various species, and adult Daphniae.

It is believed by many that it is particularly important to feed mollies little and often, although no critical experiments have been reported on this point. The young should therefore be fed up to 6 or 8 times a day, if possible, and a comparison made with the effects of the same amount of food given, say, twice a day to another batch from the same brood. Only such experiments can settle these questions, but to be of value they must be carefully made on identical batches of fishes housed exactly alike, so that effects of the feeding regime can be identified with reasonable certainty and not be confused with other variables.

Strains

The platys, the swordtails, and the mollies exist in many varieties, and the first two cross readily, so readily that it is doubtful whether the great majority of so-called platys and swords are in fact pure. Most of them are hybrids of one type or another. Some of the hereditary factors (genes) concerned in the size, color, or configuration of these fishes have been studied in detail, while others have not. Some characteristics are clear-cut, present or absent, whereas others are the result of the combined action of several genes and exist in all sorts of grades and shades. Thus, red and green in the swordtail are mutually incompatible colors—a fish is either red or it is green, and both may be produced by the same parents. Moreover, the  redness of the red or the greenness of the green are not affected by the fact that they have come from a parent of another color, although the intensity may be affected by other factors, some genetic and some environmental. Thus it may be true that a really good red swordtail can be bred only from red stock, because only then could factors tending to intensify the redness be properly concentrated and observed. However, this is all conjecture, and observation on the point is needed.

A good practical example is the wagtail platy. This fish carries two factors which together produce the black tail and fins characteristic of its type. Whenever these two factors come together in any sort of platy, wagtail must result. The factors are that producing the "comet" appearance, and another "E," which shows no visible effect on its own, but which, when present with Comet, causes wagtail. American wagtails, the home of origin of the strain, have a considerable amount of black on the fleshy part of the tail as well as on the finny part.

This is regarded as undesirable by British aquarists, who have succeeded in altering the appearance of wagtails by selective breeding so that many of the modified stock have no black on the base of the fin. This was achieved by selecting genes that affect the actions of the two "main genes" which must be present for wagtail to show at all. Clearly, these genes with small, modifying affects could be selected only in the presence of the wagtail feature itself, and thus the particular British standard wagtail must be bred from wagtail stock.

In general, therefore, really fine specimens and really worth-while strains, whether for color or other features, should be kept separate. The fishes do not distinguish between such differences, and any platy will mate with any other platy of opposite sex, as will males of the other species with any female of their own kind.

Hybrids

Platys and swordtails hybridize readily, and most hybrids are fertile. Other species, such as the mollie and the guppy, do not produce hybrids nearly so readily. Thus it is with platy-swordtail crosses that the aquarist is mainly concerned. When placed with a mixture of platys and swords, a fish tends to mate only with its own kind, but, if it has no choice, it will mate with the other species. Thus, for hybridizing, it is best to place a mature male of one species with developing young of the other, whereupon the females will be impregnated by the adult male before their brothers have a chance to catch up. Naturally, these young males are removed as they mature, for they will interfere with the crossing later on if left around.

First-cross hybrids of the platy-swordtail varieties are large, fine- looking fishes. They usually grow bigger than either parent, exhibiting a phenomenon called "hybrid vigor," which is little understood. They are much more uniform than later generations, and, if they come from a mating of fairly pure lines of parents, they will be very uniform indeed. They are deep bodied, with short swords in the males. When mated back to either parent stock, fish of any desired degree of platy or swordtail "blood" can be produced. The process has been very extensively employed, sometimes by accident, in preparing   aquarium varieties of the two species, and it is now suggested that they be not differentiated for classification purposes.

Breeding Outside

In temperate zones, many of the livebearers can be bred in summer in garden pools. The mollies are particularly suited to this and produce extra-fine specimens as long as the water is reasonably warm. In a good pool, they will survive at 60°F. quite comfortably but will not breed unless at about 65° to 70°F. It is a custom in Australia to place mollies out in the Spring and to net out the proceeds in the Fall. A small pool of some 450 to 500 gallons will yield 1,000 young from a stocking which contained perhaps 10 adults 6 months previously. It must be protected from predators like birds and even cats, but smaller enemies such as insect larvae fail to keep up with the mollie production line. Swordtails, platys, and guppies can be bred similarly but do not take a chill as readily as the mollie (except Xiphophorus variatus, which can go down to 50°F.). However, there is not the need for outdoor breeding as with the mollie, which produces better fishes when pondbred. It is even alleged that sail-fin mollies produce a worth-while crop of sail-fin young only when bred out of doors, but of this the author has no first-hand experience.

To an extent, all the tropicals can be acclimatized to lower temperatures outside than they can stand inside, but they must not be suddenly subjected to them. When the fishes are brought in again, any necessary rise must be gradual, and the best plan is to use water from the pond in which the fishes have been kept, gradually replacing it with other water, if desired.
 
Culling Livebearer Young

If livebearers are being bred for particular qualities and colors, they must be separated very early from potential sources of cross-breeding. The scheme will vary according to circumstances, but, if an adult male of the desired type is not placed with the young so as to catch the females early, they must be segregated as soon as it is possible to tell sex, and a careful watch kept for the development of male characters in their supposed sisters. By this method, the stock can be housed in two tanks, one with males and the other with females and undeveloped males, with vigilance as the password!

As soon as the young of the best quality can be selected, which should be possible within 2 months, the rest are discarded and appropriate matings are made. It is best to keep several goodlooking pairs, and to progeny-test them—i.e., keep them and their offspring until you can see which lines of fishes you want, and then go on breeding from the parents, discarding the others of the same generation. Some- times, the best parents are not the best lookers, and a fine strain of fishes may have grandparents of only second-rate appearance.

An alternative but space-consuming method used by some is to pair off all reasonable looking fishes in small receptacles, ignoring sex until you can see which are which, and then to keep chopping pairs around if it becomes clear that certain jars contain two of the same sex, and also culling.

Forced Growth

Rapid growth is generally best, producing vigorous, well-proportioned young. Excessively rapid growth is inadvisable, as the fishes often do badly and even die when placed on a more normal diet and perhaps in less favorable conditions. It is therefore, advisable to give plenty of room and food (especially live food) and to keep the tank warm and clean. Consult p. 60 for the numbers of young per aquarium and keep a safe margin below this if you desire rapid growth. Feed often and use aeration if there is any suspicion of overcrowding; if much dry food is used, place snails in the tank and pay strict attention to cleanliness. Remember that a few good feeds of live food early in development are more important than they are later on. However restricted your live food sup- ply may be, do not stint the young livebearers for the first few days. On the other hand, even with livebearers, which will take dry food so readily, do not feed exclusively on live food unless you are virtually sure that there will always be plenty available to them. Accustom them to some dry food even though there is ample live food, or they may suffer a bad set-back if a switch-over becomes necessary. This is particularly important if your young fishes are to pass into strange hands.

Increased length of illumination daily will keep the fishes eating longer, as long as food is supplied, but once again some regard must be paid to likely future conditions. Any great change may cause harm; therefore, if you have been forcing growth to any marked extent, slow down the pace for a week or so before the fishes are due for a change, especially if they are going to be shipped elsewhere.