swordtail

The swordtail (Xiphophorus hellerii) is a freshwater fish in the family Poeciliidae. It is widely kept in home aquariums and is notable for the male’s elongated lower tail fin, or sword. Swordtails are part of the genus Xiphophorus, which also includes platies.

Native to streams and rivers of southern Mexico through Central America, swordtails inhabit warm, slow-flowing waters with aquatic vegetation. In the wild they feed on plant matter, small invertebrates, and algae.

They are livebearing, with males sexually dimorphic and the sword-shaped extension on the tail. Females may store sperm and give birth to multiple broods. Gestation lasts about 28–30 days, with brood sizes typically ranging from 20 to 40 juveniles. Young become sexually mature in a few months.

In captivity, swordtails prefer temperatures of 22–28°C and a pH around 7.0–8.4, with moderately hard water. They are peaceful community fish but males may harass females or other males. A planted tank with hiding places and regular water changes supports health and reproduction. They breed readily in well-managed aquaria.

Feeding is omnivorous: they accept flake foods, micropellets, and live or frozen foods such as brine shrimp and daphnia. In breeding setups, providing a separate fry‑rearing tank helps protect young.

Beyond aquaculture, Xiphophorus species have long been used in genetic and cancer research, notably in studies of melanoma in hybrids. In their native range, habitat loss and release of aquarium stock can affect local populations.