Most teleost species are
oviparous, having
external fertilisation with both eggs and sperm being released into the water for fertilisation.
Internal fertilisation occurs in 500 to 600 species of teleosts but is more typical for
Chondrichthyes and many tetrapods. This involves the male inseminating the female with an
intromittent organ. Fewer than one in a million of externally fertilised eggs survives to develop into a mature fish, but there is a much better chance of survival among the offspring of members of about a dozen families which are
viviparous. In these, the eggs are fertilised internally and retained in the female during development. Some of these species, like the
live-bearing aquarium fish in the family
Poeciliidae, are
ovoviviparous; each egg has a
yolk sac which nourishes the developing embryo, and when this is exhausted, the egg hatches and the larva is expelled into the
water column. Other species, like the splitfins in the family
Goodeidae, are fully viviparous, with the developing embryo nurtured from the maternal blood supply via a placenta-like structure that develops in the
uterus.
Oophagy is practised by a few species, such as
Nomorhamphus ebrardtii; the mother lays unfertilised eggs on which the developing larvae feed in the uterus, and intrauterine
cannibalism has been reported in some
halfbeaks. There are two major reproductive strategies of teleosts;
semelparity and iteroparity. In the former, an individual breeds once after reaching maturity and then dies. This is because the physiological changes that come with reproduction eventually lead to death. Salmon of the genus
Oncorhynchus are well known for this feature; they hatch in fresh water and then migrate to the sea for up to four years before travelling back to their place of birth where they spawn and die. Semelparity is also known to occur in some eels and smelts. The majority of teleost species have iteroparity, where mature individuals can breed multiple times during their lives. For species whose sex is determined by genetics, it can come in three forms. In monofactorial sex determination, a single-locus determines sex inheritance. Both the
XY sex-determination system and
ZW sex-determination system exist in teleost species. Some species, such as the
southern platyfish, have both systems and a male can be determined by XY or ZZ depending on the population. Multifactorial sex determination occurs in numerous
Neotropical species and involves both XY and ZW systems. Multifactorial systems involve rearrangements of sex chromosomes and autosomes. For example, the
darter characine has a ZW multifactorial system where the female is determined by ZW1W2 and the male by ZZ. The
wolf fish has a XY multifactorial system where females are determined by X1X1X2X2 and the male by X1X2Y. Some teleosts, such as
zebrafish, have a polyfactorial system, where there are several genes which play a role in determining sex. Environment-dependent sex determination has been documented in at least 70 species of teleost.
Temperature is the main factor, but pH levels, growth rate, density and social environment may also play a role. For the
Atlantic silverside, spawning in colder waters creates more females, while warmer waters create more males.
Hermaphroditism Some teleost species are
hermaphroditic, which can come in two forms: simultaneous and sequential. In the former, both spermatozoa and eggs are present in the gonads.
Simultaneous hermaphroditism typically occurs in species that live in the ocean depths, where potential mates are sparsely dispersed. Self-fertilisation is rare and has only been recorded in two species,
Kryptolebias marmoratus and
Kryptolebias hermaphroditus. In deep-sea
anglerfish (sub-order Ceratioidei), the much smaller male becomes permanently attached to the female and degenerates into a sperm-producing attachment. The female and their attached male become a "semi-hermaphroditic unit".
Mating tactics courting a female There are several different mating systems among teleosts. Some species are
promiscuous, where both males and females breed with multiple partners and there are no obvious mate choices. This has been recorded in
Baltic herring,
Guppies,
Nassau groupers,
humbug damselfish, cichlids and
creole wrasses.
Polygamy, where one sex has multiple partners can come in many forms.
Polyandry consists of one adult female breeding with multiple males, which only breed with that female. This is rare among teleosts, and fish in general, but is found in the clownfish. In addition, it may also exist to an extent among anglerfish, where some females have more than one male attached to them.
Polygyny, where one male breeds with multiple females, is much more common. This is recorded in
Sculpins,
sunfish,
darters,
damselfish and cichlids where multiple females may visit a territorial male that guards and takes care of eggs and young. Polygyny may also involve a male guarding a
harem of several females. This occurs in coral reef species, such as damselfishes, wrasses, parrotfishes,
surgeonfishes,
triggerfishes and
tilefishes. Courtship in teleosts plays a role in species recognition, strengthening pair bonds, spawning site position and gamete release synchronisation. This includes colour changes, sound production and visual displays (fin erection, rapid swimming, breaching), which is often done by the male. Courtship may be done by a female to overcome a territorial male that would otherwise drive her away.
Sexual dimorphism exists in some species. Individuals of one sex, usually males develop
secondary sexual characteristics that increase their chances of
reproductive success. In
dolphinfish, males have larger and blunter heads than females. In several minnow species, males develop swollen heads and small bumps known as
breeding tubercles during the breeding season. Dimorphism can also take the form of differences in coloration. Again, it is usually the males that are brightly coloured; in
killifishes,
rainbowfishes and wrasses the colours are permanent while in species like minnows, sticklebacks, darters and sunfishes, the colour changes with seasons. Such coloration can be very conspicuous to predators, showing that the drive to reproduce can be stronger than that to avoid predation. Males that have been unable to court a female successfully may try to achieve reproductive success in other ways. In sunfish species, like the
bluegill, larger, older males known as parental males, which have successfully courted a female, construct nests for the eggs they fertilise. Smaller satellite males mimic female behaviour and coloration to access a nest and fertilise the eggs. Other males, known as sneaker males, lurk nearby and then quickly dash to the nest, fertilising on the run. These males are smaller than satellite males. Sneaker males also exist in
Oncorhynchus salmon, where small males that were unable to establish a position near a female dash in while the large dominant male is spawning with the female.
Spawning sites and parental care males (red belly) build nests and compete to attract females to lay eggs in them. Males then defend and fan the eggs. Painting by
Alexander Francis Lydon, 1879 Teleosts may spawn in the water column or, more commonly, on the substrate. Water column spawners are mostly limited to coral reefs; the fish will rush towards the surface and release their gametes. This appears to protect the eggs from some predators and allow them to disperse widely via currents. They receive no
parental care. Water column spawners are more likely than substrate spawners to spawn in groups. Substrate spawning commonly occurs in nests, rock crevices or even burrows. Some eggs can stick to various surfaces like rocks, plants, wood or shells. " male seahorse Of the oviparous teleosts, most (79 percent) do not provide parental care. Male care is far more common than female care. Male territoriality
"preadapts" a species to evolve male parental care. One unusual example of female parental care is in
discuses, which provide nutrients for their developing young in the form of mucus. Some teleost species have their eggs or young attached to or carried in their bodies. For
sea catfishes,
cardinalfishes,
jawfishes and some others, the egg may be incubated or carried in the mouth, a practice known as
mouthbrooding. In some African cichlids, the eggs may be fertilised there. In species like the
banded acara, young are brooded after they hatch and this may be done by both parents. The timing of the release of young varies between species; some mouthbrooders release new-hatched young while other may keep then until they are juveniles. In addition to mouthbrooding, some teleost have also developed structures to carry young. Male
nurseryfish have a bony hook on their foreheads to carry fertilised eggs; they remain on the hook until they hatch. For seahorses, the male has a brooding pouch where the female deposits the fertilised eggs and they remain there until they become free-swimming juveniles. Female
banjo catfishes have structures on their belly to which the eggs attach. In some parenting species, young from a previous spawning batch may stay with their parents and help care for the new young. This is known to occur in around 19 species of cichlids in
Lake Tanganyika. These helpers take part in cleaning and fanning eggs and larvae, cleaning the breeding hole and protecting the territory. They have reduced growth rate but gain protection from predators.
Brood parasitism also exists among teleosts; minnows may spawn in sunfish nests as well as nests of other minnow species. The
cuckoo catfish is known for laying eggs on the substrate as mouthbrooding cichclids collect theirs and the young catfish will eat the cichlid larvae.
Filial cannibalism occurs in some teleost families and may have evolved to combat starvation.
Growth and development Teleosts have four major life stages: the egg, the larva, the juvenile and the adult. Species may begin life in a pelagic environment or a
demersal environment (near the seabed). Most marine teleosts have pelagic eggs, which are light, transparent and buoyant with thin envelopes. Pelagic eggs rely on the ocean currents to disperse and receive no parental care. When they hatch, the larvae are
planktonic and unable to swim. They have a yolk sac attached to them which provides nutrients. Most freshwater species produce demersal eggs which are thick, pigmented, relatively heavy and able to stick to substrates. Parental care is much more common among freshwater fish. Unlike their pelagic counterparts, demersal larvae are able to swim and feed as soon as they hatch. At the juvenile stage, a teleost looks more like its adult form. At this stage, its
axial skeleton, internal organs, scales, pigmentation and fins are fully developed. The transition from larvae to juvenile can be short and fairly simple, lasting minutes or hours as in some damselfish, while in other species, like salmon,
squirrelfish, gobies and flatfishes, the transition is more complex and takes several weeks to complete. At the adult stage, a teleost is able to produce viable gametes for reproduction. Like many fish, teleosts continue to grow throughout their lives. Longevity depends on the species with some gamefish like
European perch and
largemouth bass living up to 25 years.
Rockfish appear to be the longest living teleosts with some species living over 100 years. == Shoaling and schooling ==