Principles of Genetics and Breeding

14.1.12. Other domesticated fishes

Most fishes in culture are still only exploited captives but a few are on the threshold of becoming domesticated, most for the aquarium. The best-known example is the small viviparous guppy whose males attain 3 cm standard length and females 5 cm.

Guppies originate from Trinidad and Venezuela but were released worldwide for mosquito control. Populations now live in Africa, India, North America, Italy and even in the warm springs of Hungary and Slovakia. Wild forms are smaller than the domesticated varieties.

The colouration of the wild guppies varies according to habitat and many different coloured populations were described. Endler (1980) explained the reason for males being much smaller than females and the colour polymorphism in P. reticulata.

Natural populations in Trinidad and north-eastern Venezuela are so polymorphic that no two males are alike. The colour patterns in a particular population represent a balance between selection for crypsis by predators and selection for conspicuousness by sexual selection. Black melanic spots are voluntarily increased in size during courtship, but reduced in size and intensity at other times. Carotenoid spots (red, orange and yellow) genetically decrease in size and frequency with increased predation and may be direct indicators of male physical fitness.

This predisposition to colour variations facilitated artificial selection in captivity where the multitude of forms and colours far exceeds that found in nature.

Guppies were bred with short fins, long fins and into those that have the natural olive-grey colour replaced by another colour. Because of inbreeding the short-fin morphs had to be severely culled to avoid vertebral deformities or stunted growth. In guppies, several forms has been sacrificed for colour.

The long-fin fish have problems with mating; males have difficulty inserting the gonopodium and to transfer spermatophores. Some breeders anaesthetize the females to give the males a chance to inseminate them.

The various colour forms are then crossbred, but in some, where both sexes have elongated fins only artificial insemination enables reproduction.

There are standard breeds, like triangle (the caudal fin is triangular), fantail, veil tail, banner tail, lyre tail, top sword, bottom sword, double sword, spear tail, round tail and spade tail.

None of the long-tailed and most colourful fish would survive in the wild and, therefore, can be considered truly domesticated.

Other livebearers common in the aquarium hobby are the swordtails and platies of the genus Xiphophorus, that include 17 species from Central America.

Swordtail males have elongated lower caudal-fin rays; platies have none. Most of the morphs in aquaria have resulted from hybridization and artificial selection of three species: X. helleri, X. maculatus and Xiphophorus variatus.

Natural hybrids are observed only rarely because the different species rarely live in sympatry or are separated in the wild by behaviour.

The same applies to fancy mollies of the genus Poecilia. The popular black molly was obtained by crossing Poecilia sphenops and Poecilia latipinna. The black Yucatan molly Poecilia petenensis was introduced into the cross. Crossing further with Poecilia velifera produced the high-finned molly.

‘The progenies of wild specimens bred in captivity greatly resemble their parents. The first apparent differences are usually in size. The bred captives often remain like their ancestors in appearance and behaviour for years. The genera Xiphophorus and Poecilia are among these very variable fishes, even in the wild, that develop many regional and colour variants’.

The most popular fish in the aquarium trade, the neon tetra was discovered by chance in one of the Amazon tributaries near the border of Peru and Colombia.

Initially, it seemed impossible to breed these fish in captivity; but once this was mastered, in the 1950s, the species became the most popular aquarium fish of all. Not long after, neon tetra became domesticated by selection for a long fin as well as for golden, diamond-head, albino and xanthic morphs and aberrations.

The same reasoning applies to the discus, a cichlid from Amazonia, in which selection of some colour aberrations formed the basis for many aquarium forms. The majority of these were achieved by hybridization of two known species, Symphysodon discus, and Symphysodon aequifasciatus, with some contribution of the questionable sub-species Symphysodon discus willischwartzi, Symphysodon aequifasciatus axelrodi and Symphysodon aequifasciatus haraldi. Each occurs in the wild in many colour varieties, especially the green discus, S. aequifasciatus.

These cichlids are unique in having a discoidal shape strongly laterally compressed, similar to the marine butterfly fishes of the genus Chaetodon.