Fish Reproductive Strategies

How fish reproduce: egg scatterers, cave and substrate spawners, bubble nesters, mouthbrooders and livebearers, plus sex change and the main forms of parental care.

A framework for classifying fish reproduction

Fish reproduce in a wider variety of ways than any other vertebrate group. FishBase classifies each species along several axes: the reproductive mode (separate sexes, protandry, protogyny, true hermaphroditism or parthenogenesis), where fertilization takes place (in open water, in the oviduct, in the mouth or in a brood pouch) and the reproductive guild proposed by the ichthyologist E. Balon. The guild system first sorts fish by the kind of care they give and then by what happens to the eggs.

• Nonguarders: open-substratum egg scatterers, which leave the eggs in the water column or on rocks, gravel, sand or plants; and brood hiders, which deposit eggs in hidden places such as caves, rock gaps, gravel depressions or even inside live invertebrates.
• Guarders: clutch tenders, which guard eggs on the surface, on the underside of objects or on open substrate without building a nest; and nesters, which deposit and often guard eggs in a nest made of mucus bubbles, gravel, sand, holes, plants or other material.
• Bearers: external brooders, which carry eggs on the body (in a pouch, the mouth, the gill cavities or on the fins); and internal live bearers, in which eggs are fertilized and develop inside the female.

In the ornamental trade the practical split is simpler. University of Florida extension material divides cultured freshwater fish into egg-laying species and live-bearing species, because the two groups need different breeding methods and equipment.

Egg scatterers

Most cyprinids (barbs, danios, rasboras) and tetras breed by group spawning. Males and females release eggs and sperm together over gravel, plants or woody debris, and the parents give no further care. Rainbowfishes also spawn in groups over vegetation, where the adhesive eggs stick and are left unattended. Corydoras catfish spawn over plants or other surfaces and leave their sticky eggs alone as well.

Because these fish give no care, adults often eat any eggs they find. The species account for the zebrafish (Danio rerio) on Seriously Fish describes it as an egg scatterer with no parental care. In aquaria, breeders protect the eggs with mesh that lets eggs fall through but keeps adults out, with fine-leaved plants or with artificial grass matting, and they remove the adults after a few days. The same source reports that zebrafish eggs hatch in about 24–36 hours and the fry become free-swimming a few days later.

Substrate and cave spawners

Many fish lay eggs on a firm surface and then guard them. In cavity spawners, pairs lay in caves or crevices and defend and clean the eggs. Guarding parents commonly fan water over the eggs to keep them supplied with oxygen, remove dead eggs and defend the site until the eggs hatch. Depending on the species, this care lasts from a few days to several months.

UF/IFAS notes that all cichlids give some form of parental care. Many are biparental substrate spawners, in which both parents tend eggs laid in a nest. Loricariid catfish (plecos) usually breed in caves, under rocks or in holes, and the male guards the adhesive egg mass and may protect the young for a short time.

Bubble nests

Many labyrinth fish (gouramis and paradisefish) build nests of bubbles at the water surface, and the male maintains the nest and cares for the eggs and fry. In the Siamese fighting fish (Betta splendens), the male builds the nest before spawning and wraps his body around the female. Each embrace releases about 10–40 eggs, and the male picks up sinking eggs in his mouth and places them in the nest. Incubation takes about 24–36 hours, and the larvae stay in the nest for another 2–3 days. Not every labyrinth fish nests: a few species are mouthbrooders, and the female kissing gourami (Helostoma temminckii) simply releases masses of floating eggs. Some armoured catfish, such as Callichthys and Hoplosternum, also build bubble nests that the male guards.

Mouthbrooders

Mouthbrooding (oral or buccal incubation) means a parent holds the eggs, and often the young, in its mouth for an extended period. It can be maternal, as in most mouthbrooding African and South American cichlids; paternal, as in arowanas, some bettas, sea catfish, cardinalfish and jawfish; or biparental, as in the cichlid genus Xenotilapia. A brooding parent cannot feed normally. Some cichlids eat now and then, but mouthbrooders come out of brooding underweight and need time to recover. The cuckoo catfish exploits this behaviour: it slips its own eggs among those of a mouthbrooding host and eats the host's eggs, so that its young develop in the host's mouth.

Livebearers

More than 97% of known fish species lay eggs, so live-bearing is the exception. In ovoviviparous fish such as the guppy, fertilization is internal and the eggs develop inside the mother with little or no direct nourishment from her. In viviparous fish the embryos are fed inside the parent: some by eating other eggs or tissues, others through a placenta-like structure, as in surfperches, splitfins and the male pouch of seahorses and pipefish.

In poeciliids (guppies, mollies, platies, swordtails), males have a gonopodium, an anal fin modified to fertilize the female internally. Mating is promiscuous, a female may carry sperm from several males, and the young are born live.

Sex change and hermaphroditism

Most hermaphroditic fish are sequential, meaning they change sex during their lifetime. Protogyny (female to male) is the more common form and occurs in many groupers, parrotfishes and wrasses. When the dominant male of a harem is removed, the largest female can turn male within a few days. Protandry (male to female) is rarer. Anemonefish (Amphiprion) are the best-known protandrous fish: the largest fish in a group is the female, the second largest is the breeding male and the smaller fish do not breed. If the female dies, the breeding male becomes female and the largest non-breeder becomes the new breeding male.

The size-advantage hypothesis explains which way sex changes. Where one male controls a harem, small fish do better breeding as females and large fish as males, which favours protogyny. In monogamous pairs, a larger female lays more eggs while a small male can still fertilize them, which favours protandry. A few fish are simultaneous hermaphrodites. Hamlets carry both ovaries and testes and take turns releasing eggs and sperm, and the mangrove rivulus usually fertilizes itself.

Parthenogenesis

The Amazon molly (Poecilia formosa) was the first vertebrate shown to reproduce without fertilization. It is gynogenetic: the eggs need sperm from a male to start developing, but the male contributes no genes to the offspring.

Species-level breeding details such as water conditions, triggers and fry foods are covered in the individual species breeding guides.