Fish Reproduction, Mating, Development and Parenting

Home | Category: Reef Fish / Ocean Fish


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baby fish
Most sea creatures reproduce when males release milt (sperm) sperm and females release eggs into the seawater and rely on the sperm and eggs to come together and unite in the water on their own. Sometimes the sperm and eggs meet in the open water and the eggs are fertilized. Sometimes the eggs are laid in long strings that cling to rocks and the sperm fertilizes them there.

Almost all fish reproduce with eggs but some fish hatches from eggs inside their mother's body. The eggs of fish that hatch inside the females body are fertilized inside the body by the male through sexual intercourse.

Breeding fish display by flexing and quivering their fins. In about 60 or so species of fish, the male gets so caught up in his spawning ritual that he is unaware when a rival male quickly slips between him and the female and fertilizes the female's eggs. Schooling fish spawn in a mass orgy set off by "mad dashes and gyrations” by each sex. The nearly invisible eggs released by the females and similarly minute drops of sperm mix in a massive reproductive soup floating in the water.

Many fish breed in mangrove swamps Young fish are called larvae. They feed on plankton and develop into adults in strange ways that are often unique to each species. Most don’t get far before they are gobbled up by predators. Among fishes, males are often the most caring parent.

Male sharks and rays have penis-like claspers which extend out from their pelvic fins and are revealed by beating the tail in one direction and the midsection in the other. These appendages are both a penis and means of holding on to the female during mating. “All male sharks possess two claspers but only one is used in copulation, the choice dependent on which side of the female he grips.” See Sharks

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio

Fish Development

fish egg: A. vitelline membrane B. chorion C. yolk D. oil globule E. perivitelline space F. embryo

According to Animal Diversity Web (ADW): In general, five major developmental periods are recognized in fish: embryonic, larval, juvenile, adult, and senescent. Fish development is known for its confounding terminology, so there are many gray areas within these major categories, and, as with many other animals, many species tend to defy classification into discrete categories. For instance, species in several teleostean families bear live young (viviparous) — Poeciliidae, Scorpaenidae, and Embiotocidae (to name a few), and the young in some families (Salmonidae) seem to emerge as juveniles after hatching (externally) from the egg. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

There are two important developmental characteristics that separate fish from most vertebrates: indeterminate growth (they continue growing throughout their lives) (growing throughout life) and a larval stage. The fact that most fish (although there are always exceptions) are always growing means they constantly change in terms of anatomy, ecological requirements, and reproduction (i.e. larger size means larger clutches, more mates, better defense, etc. in most species). Increased age is also associated with better survivability, As physiological tolerances and sensitivity improve, familiarity with the local environment accrues, and behavior continues to develop. The larval stage is usually associated with a period of dispersal from the parental habitat. Also, the disappearance of the yolk sac (the beginning of the larval stage according to most researchers) marks a critical period in which most individuals die from starvation or predation. /=\

Among some species, before they are born sex is determined by temperature. After birth they continue growing throughout their lives. The development and life cycle of some species is characterized by neoteny (juvenilization, delaying or slowing of the physiological development) and paedomorphism (retention in adults of infantile or juvenile characteristics). The life cycle of some species is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. /=\

Fish Reproduction

Most fishes reproduce continually throughout their lifetime (iteroparity), although some such as Pacific salmon and lampreys spawn only once and die shortly thereafter (semelparity). Fertilization occurs externally in the great majority of species, however in some mouthbrooding species (incubation occurs inside mouth for the purpose of protection, mostly among cichlids), fertilization occurs inside the mouth. In a few families, such as clinids , surfperches , scorpionfishes , liverbearers , eggs are fertilized internally. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

Anatomy of larval fish, YSL stands for yolk syncytial layer

Fish engage in seasonal breeding and year-round breeding. They can be semelparous. This means that offspring are all produced in a single group such as a litter, clutch, after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction. Fish can also be are iteroparous. This means that offspring are produced in groups such as litters multiple times in successive annual or seasonal cycles. /=\

Some fish engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body. Others engage in internal reproduction in which sperm from the male fertilizes the egg within the female. Most fish are are oviparous, meaning that young are hatched from eggs. Some are viviparous, meaning they give birth to live young that developed in the body of the mother. Others are ovoviviparous, meaning that young are produced from eggs that hatch within the body of a parent. /=\

Fish employ both sexual and asexual reproduction. Some are parthenogenic, meaning development takes place in an unfertilized egg. There are simultaneous hermaphrodites in which individuals have sex organs of both sexes and can produce both sperm and eggs even in the same breeding season. They are also sequential hermaphrodites in which individuals change their sex at some point in their lives and typically produces eggs and sperm at different stages their lives. Some are are protandrous (the condition of hermaphrodites that have male organs and sperm before female organs and eggs). They are also protogynous (the condition of hermaphrodites that have female organs and eggs before male organs and sperm). /=\

Fish Mating System

According to Animal Diversity Web (ADW): Fish exhibit quite a variety of mating systems . The four major types, along with a few examples, are: 1) monogamy — maintains the same partner for an extended period or spawns repeatedly with one partner (damselfishes , hawkfishes , blennies); 2) polygyny — the male has multiple partners over each breeding season (sculpins , sea basses , sunfishes , darters); 3) polyandry — female has multiple partners over each breeding season (anemonefishes); and 4) polygynandry or promiscuity — both males and females have multiple partners during the breeding season (herrings , sticklebacks , wrasses , surgeonfishes). Polygyny is much more common than polyandry, and usually involves territorial (defend an area within the home range), males organized into harems (males breed exclusively with a group of females), as in numerous cichlid species and several families of reef fishes (parrotfishes , wrasses and damselfishes , tilefishes , surgeonfishes and triggerfishes). [Source: Nicholas White, Animal Diversity Web (ADW) /=]

There are also "alternative mating systems ," which include alternative male strategies, hermaphroditism, and unisexuality. Alternative male strategies usually occur in species with large males dominating spawning, such as salmon , parrotfishes and wrasses . In this situation, smaller males attempt to 'sneak' fertilize the eggs of females as peak spawning is occurring; the smaller males release gametes simultaneously in the vicinity of the spawning pair. Hermaphroditism in fish involves individuals containing ovarian and testicular tissue (synchronous or simultaneous), as in the black hamlet, as well as individuals that change from one sex to another (sequential).

Sequential hermaphrodites most commonly change from being female to male (protogynous), as in parrotfishes , wrasses and groupers . A much smaller number of ray-finned fish, such as anemonefishes and some moray eels , change from being male to female (protandrous). Finally, unisexuality (egg development occurring with or without fertilization) can also occur in a variety of forms, and usually involves some male involvement, although at least ones species (Texas silverside) appears to utilize true parthenogenesis — females produce only female offspring with no participation from males. In most cases, however, there is at least some male involvement, either simply to commence fertilization (gynogenesis) or to produce true female hybrids (hybridogenesis).

The mating systems above do not necessarily represent discrete categories and, as with development, the discussion ignores much of the complexity and variety within each system. For instance, one unisexual species, which is actually part of a "species complex" (Mexican mollies), the Amazon molly , uses the sperm from two other bisexual species within the complex (shortfin molly and sailfin molly) to activate development of the eggs; only genetic material from the female lineage is retained. This means that the unisexual females are actually parasitizing bisexual males of these other species. Also, many species exhibit a combination of major and alternative mating systems. For instance, hermaphroditism is known among some polygynous wrasses and parrotfishes (among others). /=\

They are monogamous (having one mate at a time), polyandrous (with females mating with several males during one breeding season) and polygynous (males having more than one female as a mate at one time). They are also cooperative breeders (helpers provide assistance in raising young that are not their own) and polygynandrous (promiscuous), with both males and females having multiple partners. /=\

Fish Courtship and Mating Rituals

During courtship fish exhibit a wide range of complex behaviors, reflecting their evolutionary heritage and the particular environments they inhabit. For instance, pelagic (living in the open ocean, far from land) spawners tend to have more elaborate courtship rituals than benthic (living on or near the bottom of the sea) spawners. Some of the behaviors include sound production, nest building, rapid swimming patterns, the formation of large schools, and many others. In addition, fish frequently change color at specific points in their reproductive cycle, either intensifying or darkening depending on the species, release pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species), or grow tubercles (tiny bumps of keratin) on the fins, head or body. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

One of the more peculiar mating behaviors among ray-finned fish is found in deepsea anglerfishes (superfamily Ceratioidea). Many female deepsea anglerfishes are essentially "passively floating food traps"; quite a useful adaptation in the dark, barren waters of the deep sea. However, this makes it quite difficult to locate a mate. Finding a female, therefore, is the sole purpose of many males, which are dramatically smaller than females (from three to 13 times shorter) and unable to feed as they lack teeth and jaws.

With good swimming capabilities and olfactory organs, they are guided to females by pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species) (a unique chemical odor). After finding their mate, males attach themselves to females with hooked denticles, and in some species (Haplophryne mollis) the tissue between the two fuses; the males become permanently attached and receive nourishment from the female while the testes develop. /=\

Fish Parenting

Among most species of fish there is no parental involvement in the rasing of young. In many cases eggs are laid and young are born and their parents leave them to fend for themselves immediately after they hatch or before. There are some examples of parental care provided by both females and males.[Source: Nicholas White, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web (ADW): While a surprising number of fish families exhibit parental care, it is not common, occurring only in approximately 22 percent. Unlike mammals , most parental care is the responsibility of males (11 percent), with seven percent the sole responsibility of females and the rest carried out by both sexes. Not surprisingly, virtually no pelagic (living in the open ocean, far from land) spawners, which release their gametes into the water column, exhibit parental care. However, among the fishes that do exhibit parental care, there is considerable diversity. /=\

Some of the most extensive parental behaviors are found in cichlids. Many cichlids brood the eggs in the mouth and, although rare, the free-swimming young of some species also rush into the parent’s mouth for protection. Quite an elaborate form of parental care is found in spraying characin. At peak spawning, males and females of this species make simultaneous leaps out of the water, touching and briefly adhering to the underside of overlying vegetation (a leaf). Each time, a fertilized egg is stuck to the underside of the leaf, usually a dozen or so. Then, to keep the leaf moistened, the male, correcting for the refraction of the water surface, sprays the eggs at one- to two-minute intervals by splashing with his tail. After keeping this up for two to three days (!), the newly hatched young fall into the water. Several tidal species utilize similar methods to keep eggs from desiccating as the tide goes out. Two such methods include coiling the body around the eggs (pricklebacks and gunnels) and covering the eggs with algae (temperate sculpins and wrasses). /=\

correlation between parenting involvement and traits of different groups of fish

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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