swordtailArticle Free Pass swordtail, (Xiphophorus helleri),
popular tropical fish of the live-bearer family Poeciliidae
(order Atheriniformes). The swordtail is an elongated fish,
growing to about 13 centimetres (5 inches) long and
characterized, in the male, by a long, swordlike extension of
the lower tail fin lobe. The fish is closely related to the
platy and, like its relative, has been used extensively in
genetic and medical research. Originally green with a red stripe
on each side, the swordtail has been bred in many colour
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