Do Fish Have Vents?

Two small openings on the underbellies of fish allow them to pass waste and engage in reproduction. Known as vents, these openings are barely discernible to the human eye, particularly on small fish. Found between the anal fin and the pelvic fin, the vents can be circular or slitlike and can be visible as a singular opening or dual openings.

  1. Anal Vent

    • Carnivorous fish have shorter digestive tracts than herbivorous or omnivorous fish, who need more intestine to break down fibrous matter. In fish with two vents, the anal opening is situated closer to the head between the pelvic fin and the reproductive opening. The anus looks similar in male and females of the same species. In fish with only one opening, the anus is found within the cloaca, a common opening for reproductive and elimination purposes.

    Ovipositor

    • The vent of a female fish who lays large eggs -- such as a mouthbrooding cichlid -- will be significantly larger than her anal opening and larger than the similarly positioned opening on the male of the species. Fish who lay smaller eggs can present more difficulty to determine whether the vent is an ovipositor designed for laying eggs. Females may have rounder abdomens near the vent due to eggs within the abdomen.

    Male Vents

    • Male fish release sperm through the reproductive vent to fertilize the female's eggs. In mouth-brooding fish, such as some species of African cichlids, the female will collect eggs in her mouth, mistaking spots on the male's anal fin for missed eggs. As she attempts to pick them up, the male will release his milt into her mouth to fertilize the eggs. Among open brooders who lay their eggs on the bottom of a habitat, a male will swim over the eggs to fertilize them.

    Sexing Fish

    • It is possible to tell the genders of fish with two vents by turning them ventral-side up with head away from you. In females who lay larger eggs, such as mouth-brooding fish, you may readily notice that the reproductive opening appears larger and rounder than the same opening on the male. Some species can be much more difficult to differentiate; you may need to look for pigmentation or other characteristics with a magnifying lens.