How to Determine the Gender of a Piranha

There are only two certain methods of sexing piranhas. The best way is to watch for egg-laying, as only female fish deposit eggs. The only other certain way is to dissect the fish after death and identify the reproductive organs. For living fish, there are a few characteristics that allow you to make an educated guess regarding the gender of your piranha.

  1. Size

    • Female piranhas are generally larger than males, but tank size and conditions may also influence the size of your fish, so using size as a guide is a guess at best. Often the female is larger in girth than her male counterparts, but there are times when the food eaten by the fish allows for more growth in one over another. For this reason, you may find you have male piranhas larger than their female tank mates. It is also important to note the age of the fish, being certain they are fully grown before comparisons are made. Depending on the species, piranhas are anywhere from 5 to 18 inches in length.

    Abdomen

    • Observe the belly of your piranha. If your fish are healthy, an enlarged belly may indicate a fish ready to lay eggs. In unhealthy fish, it may be sign of internal atrophy, and is also accompanied by a darkening of skin color. Bellies also enlarge after feeding. The presence of eggs in your tank does not indicate the presence of a male, only of females, as they will deposit unfertilized eggs if the eggs are not reabsorbed by their bodies.

    Pygopristis Denticulataits

    • In one species, Pygopristis denticulataits, the anal fin -- the fin that follows the underline of the fish directly beneath the tail fin -- is bilobed in the male fish. This means it possesses a rounded appearance at the lower end that swells off the main fin structure. It resembles the letter "D." In the female, the structure at the lower part of the anal fin is pointed, or jagged, and may even take on a spiked appearance.

    Warnings

    • Do not overcrowd your piranha tank. Piranha need the right conditions to stay healthy and vigorous. Piranhas are often thought to be skittish, but this is usually only the case when their habitat is overcrowded. Since they often take chunks out of one another during feeding frenzies, piranha are leery of tight spaces and will dart away from one another in too small or crowded a tank. This behavior makes for a busy aquarium, and nervous fish. Well cared for piranhas are not usually aggressive, but make sure to handle piranhas with caution to avoid bites.