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Swordtail or Platy?

Swordtails and platies are highly variable and will easily hybridize to create interesting varieties.

By John Dawes |

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Aquarium Fish InternationalSwordtails and platies are quite rightly among the most popular aquarium fish in the world. No community aquarium is complete without one or the other, or both. Most aquarists believe that they have kept a true swordtail (Xiphophorus hellerii); platy, southern platyfish or moonfish (X. maculatus); or sunset, variegated or variatus platy (X. variatus). But most aquarists have actually kept hybrids between these species — not “pure” swordtails and platies.

Take a close look at any platies or swordtails you may have in your tanks or those that you see in aquatic shops. Some platies appear more elongated than they should be. Indeed, some may even look a little like swordless swordtails. Conversely, some swordtails may look somewhat shorter than they should.

Faced with this situation, it is easy to think that these slight or more apparent departures from the original body shapes are down to selective breeding in captivity, and this is true — but only to a point. For example, every batch of newborn “pure” platies or swordtails will exhibit some variation in body shape. If we select, say, only the most elongated platies from these batches and breed from them, subsequently selecting just the most elongated offspring — and so on — we can eventually end up with true-breeding elongated platies. The same (but in the opposite direction) applies to swordtails. This type of intense artificial selection (as opposed to natural selection) carried out in a protected aquarium environment accelerates the pace of evolution far beyond its natural rate and often goes in directions other than those that would occur in nature.

Want to read the full story? Pick up the May 2012 issue of Aquarium Fish International today.

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