Swordtails

The popular aquarium varieties of the swordtail are far removed in appearance from their drabber-looking wild relatives, although both types give birth to live young.

The name “swordtail”was given to these fish due to the elongated swordlike extension on the lower part of the tail (caudal) fin of many males. This feature is also reflected in their scientific name, which comes from the Greek word xiphos, meaning “sword.” Females have rounded tails. As with other livebearers, female

swordtails also grow larger than males. In the wild, these fish are quite plain in color—often greenish overall. Captive-bred strains are much more variable in coloration, frequently being bright orange, sometimes with black markings.The fin shape has also been affected by domestication,with the variety known as lyre swordtails having a symmetrical extension, in the shape of another sword, at the top of the dorsal fin.

Unusual Breeding Habitats

Another difference between the sexes is that the male’s belly (anal) fin has developed into a tubelike structure called a gonopodium. This provides a means of fertilizing the eggs internally in the female’s body. They will then develop there,with live young being produced. The number of offspring that a female produces depends on her size,with larger individuals having bigger broods of up to 200 young.

Unfortunately, swordtails have no maternal instincts, and particularly in aquarium surroundings, the tiny young will be eaten by their mother unless they can be separated by a partition or placed in another tank.