10-12-2011, 02:21 AM
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The fun thing about the Big Four live-bearer breeds: Mollies, platties, guppies, and swordtails, is that when they mate, the sperm is delivered in 'packets' by the male. The 'packets' can be stored within the female for 6-9 months after the initial mating. Whenever the female is comfortable, she can decide to use one of the packets or not and get pregnant again.
As for the gravid spot, the spots is essentially the eyes of the fry developing within the mother. The spot will tend to get darker as the pregnancy progresses. Typical pregnancies last for about a month, but can be longer or shorter depending on water temp, stress, etc. Just be sure that the mothers don't get too stressed, otherwise they will miscarry the fry, and they will be re-absorbed into her body.
That's all I got for ya! I don't have any experience with guppies, only platies, so I can only give general livebearer information. :)