Swordtail Breeding
The swordtail is also known as Xiphophorus helleri. Wild swordtails reach lengths of about six inches, but aquarium breeding has developed a smaller fish that is more suitable to the home tank. In the male swordtail, the lower part of the caudal fin develops into a long sword-like tail that serves as a positive identification for the sex of this fish. Since it is native to waters in and near Mexico, this fish is sometimes called the "Mexican swordtail."
The swordtail is subject to the same care and treatment as all livebearers, and as its temperament is much like that of the guppy, it is an ideal fish for the community tank. The swordtail has another feature that adds to its desirability, that is, its ability to be used in hybridization experiments. (Dr. Myron Gordon and the author used them extensively in cancer research as well as in hybridization work.) X. helleri has been successfully crossed with the platies, or "moons" as they are commonly called.
The platies (also Xiphophorus) are hardy fish and are also very desirable for the community tank. One of the many different hybrids that has been developed is the "red Helleri," the final result of a cross between a reddish moon and a swordtail.
It is remarkable how these fish react to their environment. One case is known where a female swordtail was kept in a small tank with seyeral betty females—the bettas are a rather ferocious bunch, although the female is less so than the male. The bettas immediately chased the swordtail around, and for a few days it looked as though they would kill her. Then, suddenly, they left her alone, and they got along very well. After a month of this rough environment the female was put into a tank with a male swordtail to mate. The female immediately started for the male and in 20 minutes she had killed him. After a few days another male swordtail was put into the tank and the same thing took place. The female never accepted a mate.
Two lessons about fish can be derived from this case: first, they are quick to respond to their environment; and second, they have potentially many different behavior patterns. Together with a group of swordtails, this female would probably have been all right, but by placing her in a rough-and-tumble environment she lost all her timid traits and the more savage instincts in her were brought out. Many fish of a timid family get very savage in a savage environment, and the reverse statement is also true. So keep an eye on all new fish that are introduced to the community aquarium.
The X. helleri is an interesting fish to watch, especially when the male is "driving" the female. He will swim in front of her and then, with his sword-like tail erect, swim backward in a quivering courtship display. The male constantly swims back and forth about a female, always eager to mate. An interesting fact about the female is that at an old age she may lose her female characteristics and develop the secondary characteristics of the male, growing a sword-like tail and sometimes trying to court other females. This process of sex reversal occurs regularly in certain species of fish. In the swordtail, however, it is apparently an abnormal occurrence, and the vast majority of such transformed females is infertile. By injecting hormones into normal female swordtails, scientists have been able to change them to look and act like males, but these, too, are never functional.
