1 . Fishes can become sexually mature at various ages, depending on species. Several factors influence sexual maturity, including age, gender, and size.
n 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.
b. Some bony fishes become sexually mature shortly after birth. The western mosquitofish (Gambusia affinis) becomes sexually mature within a year.
c. Most bony fishes become sexually mature between one and five years. Most bony fishes are in excess of 8 cm (3 in.) before reproducing.
d. 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.
2. In general, species of a small maximum size begin reproducing at an earlier age than those with a large maximum size. Age and associated size are major factors in determination of adulthood.
1 . In most species of bony fishes, sperm and eggs develop in separate male and female individuals. Fertilization can be either internal or external. 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.
2. Some bony fishes are hermaphrodites: a single individual produces both sperm and eggs. A fish may be a sequential hermaphrodite or a synchronous hermaphrodite.
a. Sequential hermaphrodites are born one sex and change sex sometime during the course of life.
Like most wrasses (family Labridae), sheephead are born female and have the potential to transform into males later in life. Shown here is an adult male California sheephead (Semicossyphus pulcher).
(1) Most wrasses (family Labridae) are sequential hermaphrodites. They 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. In many wrasse species, 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.
(2) Anemone fishes (family Pomacentridae, subfamily Amphiprioninae) are born male and change into females later in life.
(3) Some damselfishes (family Pomacentridae) begin life as males and change into females. In some species, females can revert back to males. Some seabasses (famly Serranidae) change from female to male, and are capable of reverting back to female.
b. Synchronous hermaphrodites have both sperm- and egg-producing organs at the same time. In a few species, self-fertilization is possible.
3. Some species are unisexual. In unisexual species, there is no fusion of sperm and egg. The sperm is necessary to trigger the egg cell to develop into an adult, but the sperm cell ultimately degenerates and does not take any part in heredity. The resulting young always are females, with no trace of paternal characteristics. Thus, unisexual species are entirely female. They mate with males of related species to produce female offspring. Poecitia formosa is an example of a unisexual species. Always female, P formosa mates with male P mexicana or P latipinna.
4. Depending on the species, bony fishes may have internal or external fertilization.
1 . Various factors may influence bony fish breeding.
a. Changes in the duration of sunlight, or photoperiod, can stimulate some species of bony fishes to begin reproduction.
b. Temperature change may trigger the maturity of bony fishes in temperate and subpolar areas.
c. Other factors that may affect reproduction are presence of the opposite sex, currents, tides, moon stages, and presence of spawning areas.
2. 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.
a. Some bony fishes may spawn many times a year.
b. Many bony fishes reproduce once a year until they die.
c. 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.
3. Diadromous fishes must have access to both marine and freshwater systems to complete their life cycle.
Bony fishes show at least three types of embryonic development: oviparous (egg layers), ovoviviparous (egg retainers), and viviparous (live bearing).
1 . In oviparous development, the female releases eggs. The developing embryo is nourished by a yolk sac.
a. Bony fish eggs generally are spherical. Most are 0.4 to 3.0 mm (0.02-0.1 in.) in diameter (Bond, 1979).
b. Depending on the species, parents (male and/or female) may scatter, hide, guard, or brood eggs.
(1) Some bony fishes lay eggs that drift through the water column. Some bony fish eggs have oil droplets that help them float.
(2) Some bottom-dwelling bony fishes produce eggs that sink and remain on the ocean bottom.
(3) Some eggs may be sticky or may have tendrils that allow them to attach to plants and other living or nonliving materials in the environment.
(4) Some species brood eggs in their mouth or on their skin, fins, or gill areas.
2. Some fishes that have internal fertilization are ovoviviparous. The female retains fertilized eggs in her body. The developing embryo is nourished by a yolk sac formed prior to fertilization, and there is no nutrient connection between the mother and the developing embryos. One example of an ovoviviparous fish is the seahorse (family Syngnathidae). In seahorses, it's the male that is responsible for incubating fertilized embryos. 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.
3. Viviparous development occurs in some fishes with internal fertilization. Fertilized eggs are retained and develop within the female's ovary or uterus. The developing embryo has a nutrient connection with the mother.
1 . There is great variation in the development stage at which offspring are released. Depending on whether fertilization is internal or external and which type of embryonic development occurs, bony fishes may release eggs or sperm, larvae, or even sexually mature adult fishes.
2. Gestation periods vary among species and between individuals within a species. Since bony fishes are cold-blooded, gestation time may vary within a species. The rate at which the embryo develops depends on the water temperature. The gestation period may be as short as a few days or as long as several months.
3. 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 mola (Mota mola) may produce more than 28,000,000 eggs in a single season. Guppies (family Poeciliidae), which bear their young live, often produce less than 25 young at a time.
4. Within a species, the number of offspring a female produces varies according to many factors including age, size, food availability, species, season, and water temperature.
1 . Many species give no care to their eggs or young. Parental responsibility only includes getting eggs and sperm in the vicinity of each other.
2. Some species hide their eggs.
3. Some species guard their eggs. A male jawfish (family Opisthognathidae) broods fertilized eggs in its mouth.
A male jawfish (family Opisthognathidae) broods fertilized eggs in its
mouth.
4. Some ovoviviparous and viviparous bony fishes bear live young that can protect themselves at birth. Very little, if any, parental care is needed after hatching.
5. Some species care for their young after they have hatched. Male bowfins (family Amiidae) fiercely guard their young (Bond, 1979). 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).
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