03-18-2007, 08:50 PM
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THere are about 5 species of Mollies. The ones that you find in the trade are almost all hybrids. The three most common strains are Polecilia sphenops, which is pure freshwater and does not have the sailfin (though some hybrids have a lyre tail), P. latipinna which des have the sail on the mails only (though compared to P. sphenops, the P latipinna females have a more sil like dorsal) and requires either brackish or very hard water, and P velifera which is very similar to P. latipinna but larger.
Insofar as I know, all the common livebearers (Mollies, Platies, Guppies, Swordtails, and mosquitofish) can be sexed easily by looking at the anal fin (and I believe the uncommon ones can as well). The female has a fin. The male has, to put it delicately, a male member. This is called the gonopodium, and is absolutely charateristic. Once you have that memorized, there is no other item you need to sex livebearers. That second picture ion Julie's post illustrates it perfectly.