School of fish


Sexual Maturity

Several factors influence sexual maturity, including age, gender, and size.

Fishes become sexually mature at various ages, depending on species. In general, small species begin reproducing at an earlier age than large species.

  • Some bony fishes are sexually mature at birth. Males of the dwarf perch (Micrometrus minimus) can spawn immediately after birth. Although female dwarf perch receive sperm soon after they're born, they do not bear young for up to a year.
  • Some bony fishes become sexually mature shortly after birth. The western mosquitofish becomes sexually mature within a year.
  • Most bony fishes become sexually mature between one and five years.
  • It may take ten years or more for some bony fishes to become sexually mature. The eels (family Anguillidae) become sexually mature at 10 to 14 years of age, and the sturgeons (family Acipenseridae) may take up to 15 years to mature.


Reproductive Modes

In most species of bony fishes, sperm and eggs develop in separate male and female individuals. Males and females may look similar, or they may look very different. Male/female differences may include size, coloration, external reproductive organs, head characteristics, and body shape.

Some bony fishes are hermaphrodites: a single individual produces both sperm and eggs.

  • Sequential hermaphrodites are born one sex and change sex sometime during the course of life. For example:
    • Some damselfishes (family Pomacentridae) begin life as males and change into females. In some, females can revert back to males.
    • Some seabasses (family Serranidae) change from female to male, and are capable of reverting back to female.
    • Most wrasses (family Labridae) are born female, grow into sexually mature females, and have the potential to transform into functional males later in life. In many of the wrasses, sex change correlates with social hierarchy and social behavior: social structure includes a large dominant male and many smaller, subordinate females. Removing the male from the group triggers the largest female to begin transforming into a male.
  • Synchronous hermaphrodites have both sperm- and egg-producing organs at the same time. In a few species, self-fertilization is possible.

A few species are unisexual: there is no fusion of sperm and egg. A sperm cell is necessary to trigger an egg cell to develop, but the sperm cell ultimately degenerates and does not contribute genetic material. The resulting young always are females. Thus, unisexual species are entirely female. They mate with males of related species to produce female offspring. Poecilia formosa is an example of a unisexual species. Always female, P. formosa mates with male P. mexicana or P. latipinna.


Reproductive Behavior

Various factors may influence bony fish breeding.

  • Changes in the duration of sunlight (called photoperiod) can stimulate some species of bony fishes to begin reproduction.
  • Temperature change may trigger breeding in temperate and subpolar areas.
  • Other factors that may affect reproduction are presence of the opposite sex, currents, tides, moon stages, and presence of spawning areas.

Reproduction is generally cyclic in bony fishes. The duration of cycles may be as short as four weeks or as long as many years. Some species spawn continuously throughout the spring and summer.

  • Some bony fishes may spawn many times a year.
  • Many bony fishes reproduce once a year until they die.
  • Other bony fishes may reproduce only once during their lifetime. Pacific salmon (family Salmonidae) reproduce only once during their five-year lifespan, then die soon after.

Diadromous fishes must have access to both marine and freshwater systems to complete their life cycle.


Fertilization and Embryonic Development

Some species release unfertilized eggs and sperm. Young develop from eggs that are fertilized in the water.
Some species have internal fertilization; these species mate. For species with internal fertilization, there is great variation in the development stage at which offspring are released: fertilized eggs, larvae, juvenile fish, or even sexually mature adults.
Oviparous bony fishes release eggs, and the developing embryo is nourished by a yolk sac.
  • The eggs of a bony fish generally are spherical. A soft membrane protects the egg. Most are 0.4 to 3.0 mm (0.02-0.12 in.) in diameter.
  • Some bony fishes produce and "scatter" their eggs. Some eggs drift through the water column. Some have oil droplets that help them float. Some bottom-dwelling fishes produce eggs that sink and remain on the ocean bottom.
  • Some bony fish eggs are sticky or have tendrils that entangle them among plants and other living or nonliving materials in the environment. In some cases parents protect their eggs until the embryos develop and the young swim free.
  • Some parents brood eggs in the mouth or on the skin, fins, or gill areas.

In ovoviviparous fishes, one parent (usually female) retains the fertilized eggs in her body, and the developing embryo is nourished by a yolk sac formed prior to fertilization. There is no nutrient connection between the parent and the developing embryos.

  • Examples of ovoviviparous fishes are the seahorses (family Syngnathidae). In contrast to most other animals, it's the male seahorse that incubates fertilized eggs. The female seahorse deposits eggs into a pouch on the male's abdomen. The male releases sperm into the pouch, fertilizing the eggs. The embryos develop within the male's pouch, nourished by their individual yolk sacs. After the embryos have developed, the male gives birth to tiny seahorses.

In viviparous fishes, the female retains the fertilized eggs in her ovary or uterus, and the developing embryo is nourished by connection with the mother.

Fish larvae develop from hatched embryos. A transitional stage, larvae of many species look and behave differently than adults. Fish larvae are free-living organisms that feed on plankton, bacteria, or organic debris.

Gestation periods vary greatly among species, ranging from just a few days to several months. Within a particular species, water temperature affects the rate at which an embryo develops.

The number of offspring is inversely related to the chance a single egg has to reach maturity and reproduce.

  • In general, species whose eggs have little chance to reach maturity lay the most eggs. The common ocean sunfish, which "scatters" unfertilized eggs, may produce more than 28,000,000 eggs in a single season. Guppies (family Poeciliidae), which mate and bear their young live, often produce less than 25 young at a time.
  • Within a species, the number of offspring a female produces varies according to many factors including age, size, food availability, season, and water temperature.


Parental Care

Many species give no care to their eggs or young.

Some species hide or guard their eggs.

Some species, such as the jawfish (family Opisthognathidae) brood fertilized eggs. A male jawfish broods fertilized eggs in its mouth.

Some bony fishes bear live young that can protect themselves at birth. Very little, if any, parental care is needed after young are released.

Some species care for their young after they have hatched. Male bowfins (family Amiidae) fiercely guard their young. Some species make elaborate nests and provide parental care to the developing fishes. Sticklebacks (family Gasterosteidae) construct elaborate nests to care for 30 to 100 fry (juvenile fish).