How do you tell if a fish is girl or boy?

Most fish do not have distinct genders, so they are not classified as either male or female. Instead, most fish species have only one sex, with individuals being either male or hermaphroditic (possessing both male and female reproductive organs). Some species of fish, however, do have separate sexes, with distinct male and female individuals. In those cases, there can be various ways to distinguish between them, such as:

1. Coloration: In many species of fish, males and females have different coloration patterns or intensities. For example, male guppies are often more brightly colored than females, and male swordtails have longer and more elaborate sword-like tail fins.

2. Body Shape: Some fish species exhibit sexual dimorphism, meaning that males and females have different body shapes. For instance, male bettas (Siamese fighting fish) have longer, more flowing fins than females, while female mollies tend to have a rounder body shape compared to males.

3. Genital Openings: In certain fish species, the location and shape of the genital openings can help distinguish males from females. In livebearer fish such as platies and swordtails, males have a gonopodium, which is a modified anal fin used for fertilization, while females have an ovopositor. In egg-laying species, the genital opening of females is typically larger and more rounded compared to males.

4. Behavior: During mating season, male fish may display specific behaviors to attract females. For example, male cichlids may engage in elaborate courtship dances or build nests to prepare for spawning. In some clownfish species, the largest female is the dominant individual and changes sex to become a male when the previous male dies or is removed.

It's important to note that these characteristics can vary among different species of fish, so it is always best to consult reliable sources or experts to accurately determine the sex of a particular fish species.