I had some regular mollies in a tank of mine recently. They bred far too often for me. Everytime I turned around, the females were pregnant again....and again.....and again. The gravid spot is most visible on fish that aren't "dalmation" mollies or black mollies. They can also remain pregnant and deliver multiple batches of babies after mating just once (or so I've been told. Everytime I looked in the tank, the male was busy having his way with all the ladies so I never personally tested this.....).