1. Size and Shape:
- Male black mollies are generally larger than females, with more prominent and sword-like tail fins.
- Females have a rounder body shape, especially when viewed from above.
2. Anal Fin:
- Male black mollies have a modified anal fin that is transformed into a gonopodium. This specialized organ is used for reproduction and can be seen as a tube-like structure near the anal area.
- Females, on the other hand, have a normal fan-shaped anal fin.
3. Coloration:
- Males often display more vibrant and intense coloration compared to females.
- Females might have a duller or less intense color pattern.
4. Behavior:
- Male black mollies are known for their courtship behavior. They may swim actively around females and display their fins to attract them.
- Males may also chase and nudge females as part of the mating process.
5. Mating:
- If your two black mollies are exhibiting mating behavior, you may notice the male chasing and circling around the female.
- During mating, the male will position himself near the female and use his gonopodium to fertilize her eggs.
It's important to note that these physical characteristics can vary depending on the species or variety of black molly you have. Additionally, some species and color morphs may have less pronounced differences between males and females, making it challenging to determine the gender. If you're unsure, observing their behavior and observing any courting or mating behavior can provide additional clues to their gender.