breeding and being able to tell male mollies from female

Question
QUESTION: i have 2 mollies in my fish tank. one is a white molly and the other is a dalmation molly. i cant tell if they are a boy or a girl, the dalmation follows the white one around but there was a mysterious bump on its belly that formed, so we put it in our breeder but after a while, the bump was either smaller or it was gone?? so we took it back out of the breeder, should we have done that? why did this happen and how can you tell the diference between boys and girls??

ANSWER: You look for the gonodopodium, the male sex organ.

Copy and paste this into your browser.

http://www.bio.txstate.edu/~gabor/caito/f2.jpg

See how the bottom fish has that spear like organ next to the bottom fins? That is the gonodopodium and it is a male. The top fish does not have this organ, instead it just has a fan-like anal fin. That is the female.

-Nick

---------- FOLLOW-UP ----------

QUESTION: the white fish has the spearlike organ but it just looks like another fin? is that what its sopposed to look like? also the dalmation doesnt have this organ, but it has three big anal fins, does this mean that it is the girl? and should we put it back in the breeder?

ANSWER: Yes the gonodopodium looks just like another fin. It is a fin pointed backwards.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Black_Molly_3_Male.jpg/

And yes, if it does not have that spear it is a female. The females just have a fan-like anal fin, as shown below.

http://www.fishdeals.com/livebearers/mollies/home.jpg

-Nick

---------- FOLLOW-UP ----------

QUESTION: thank you for answering my questions, you have been very helpful! but we still arent sure whether or not my dalmation molly is pregnant or not? if she is pregnant, how long will it take her to release the babies? the bump on her belly is fairly small compared to other pictures ive seen of pregnant mollies. also is it normal for the dalmation molly to follow around my white molly who is a boy?


Answer
You're very welcome!

It is impossible to tell how far along she is without a pic, but I can tell you that the gestation period is usually 4-6 weeks. She will release all the babies in 30min-4 hours. There are a ton of variables, and it is impossible to tell over the internet. You just have to get to know your fish.

And yes, that is normal. They almost form pairs, like cichlids do. Almost. They will follow each other around, sleep in the same spot sometimes, forage together, just part of the relationship.

-Nick