Most people think that the "sword" on a swordtail fish is its tail. But in fact, it is a male sexual organ called gonopodium, and therefore a female swordtail fish has no "sword" at all!
Swordtail fish originally came from Central America. Wild swordtail fish is actually olive green in color with red or brown lateral stripes along its dorsal. However, the swordtail fish has been widely bred for its unique tail and this has produced many color varieties, such as black, red, or sliver. The color variations and patterns are especially pronounced on its tail.
Since only the male swordtail fish bears a significant tail, most of the colorful species you see in aquariums are male. However, the swordtail fish is also well-known for another aspect among aquarists – they are live bearers!
Swordtail fish gives birth to live fish fry instead of laying eggs. This is act of giving birth to live fry is a wonder to watch and amazes both aquarists and the layman observer alike. Besides their sword-like tail, swordtail fishes are also popular aquarium fish for this reason.
The swordtail fish is often recommended to new aquarists because they are considered hardy fishes that can survive well even in less than optimum conditions. Their giving birth to live fry also means that swordtail fish fry are much bigger and more robust at birth. They are easy to feed and handle, and therefore easier to breed as well.
Swordtail fishes are omnivorous, and they generally will survive well on anything you feed them, including plants, flakes, insects and annelid worms.
However, this hardiness has led to swordtail fish becoming a nuisance species in many of its non-native countries. Swordtail fish populations have been established in regions of South Africa as well as along the east coast of Australia.
It is not known how these feral populations came about, but they are likely to have been a result of aquarium swordtail fishes being released into the wild after they owners have gotten tired of rearing fishes.
For better or worse, the swordtail fish remains as one of the most popular tropical aquarium fish. Numerous inbreeding for color perfection have reduced the vigor of swordtail fishes, but more recently, the wild type swordtail has seen a resurgence in aquarium tanks for their ability to survive well in community tanks.
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