Hybrid Aquarium Fish: Origins, Health and Conservation Concerns

How hybrid aquarium fish such as blood parrots and flowerhorns arose, what sources report on their fertility and deformities, and why hybrids raise concerns for pure lines.

What a hybrid aquarium fish is

A hybrid aquarium fish is the offspring of two different species, produced deliberately on fish farms or by accident when related species share a tank. Some hybrids, such as the blood parrot cichlid and the flowerhorn cichlid, exist only in captivity and are sold under their own common names. Others, including many platies, swordtails and Endler's livebearers, are hybrids sold under the name of one parent species. Hybrids are a separate topic from colour strains produced within a single species by selective breeding; guppy colour genetics, line breeding and inbreeding are covered in their own guides.

Blood parrot cichlid

According to Wikipedia, the blood parrot cichlid was first bred in Taiwan around 1986, and its parentage is usually given as Amphilophus citrinellus × Vieja melanurus. The exact cross is not settled: a 2016 physiology study in SpringerPlus describes the fish as Cichlasoma synspilum ♀ × Cichlasoma citrinellum ♂. The fish reaches about 20 cm and, per Wikipedia, can live 10–15 years.

Mouth: a beak-shaped mouth that cannot close fully; the fish crushes food with its throat muscles, and the narrow opening makes it vulnerable to malnutrition (Wikipedia).
Body: compressed vertebrae and a deformed nuchal hump; some individuals have swim-bladder deformities that cause awkward swimming, and the irises are often deformed (Wikipedia).
Fertility: males are generally infertile, although successful spawnings have been recorded; females are mostly fertile and lay eggs on hard surfaces. Wikipedia reports that some farms inject males with hormones to raise fertility.
Colour: the natural colours are red, yellow and grey; other colours come from injected dye, which Wikipedia notes can shorten life expectancy.

The so-called heart parrot is made by cutting off the tail of a young fish. Wikipedia reports that enthusiasts generally consider this inhumane and that most retailers have stopped selling such fish. A Practical Fishkeeping ethics debate (2018) adds that tail mutilation and tattooing occur in some regions, but not in the UK, where they are illegal.

Flowerhorn cichlid

Wikipedia dates flowerhorn breeding to 1993 in Malaysia. The fish first appeared for sale there in the late 1990s. Red devil (Amphilophus labiatus), trimac (A. trimaculatus) and blood parrot cichlids were crossed to produce it. The head protuberance, called the kok in the trade, is a nuchal hump; males usually develop a larger one than females. Wikipedia states that most flowerhorn males are sterile, and that breeders have to wait 8–10 months for sexual maturity before testing a male's fertility. Reported health problems include hole-in-head disease, ich and digestive blockages. Wikipedia also records tattooed flowerhorns and the use of hormones to enhance colour.

Hybrid livebearers

According to Wikipedia, the platies commonly sold in shops are not pure Xiphophorus maculatus. They are hybrids between X. maculatus and the green swordtail, X. hellerii, which are closely related and interbreed. The trade labels males with a sword-shaped tail as swordtails and the others as platies. Decades of selective breeding have fixed the expected body shape within each captive strain. Seriously Fish adds that some ornamental forms of the green swordtail come from artificial hybridisation with X. maculatus and X. variatus.

In a 2009 Practical Fishkeeping feature, Endler's livebearers (Poecilia wingei) are described as mostly hybrids with the guppy (Poecilia reticulata). The crosses probably date from the early 1980s, when the fish reached German aquarists. These hybrids breed readily, producing about 20 fry per brood roughly every 23–24 days.

Hybrids beyond cichlids and livebearers

Seriously Fish reports that Betta imbellis has been hybridised with B. smaragdina, B. mahachaiensis and B. splendens for the ornamental trade, which has produced colour forms not found in nature. For the rainbowfish Melanotaenia trifasciata, the same source states that locality morphs hybridise freely and should never be mixed in one aquarium; they are often sold with the collection locality added to the scientific name.

Conservation concerns and the pure-line debate

The main conservation concern is that hybrids sold as, or mixed with, pure species dilute captive lines of wild species. Wikipedia lists the dilution of genetic lineages among the controversies around flowerhorns. Seriously Fish states that introduced ornamental forms and other Betta species are harming the genetic integrity of some wild Betta imbellis populations. In the scientific literature this process is sometimes called genetic pollution, meaning uncontrolled gene flow into wild populations. Wikipedia notes that some researchers prefer the neutral term genetic mixing, because hybrids are not necessarily less fit than their parents.

The Practical Fishkeeping debate sets out both sides. Opponents argue that man-made hybrids ignore bloodlines and the animal's long-term health, lack transparent genetic records, and are made only for profit and human taste. Supporters argue that the fish appear unaffected by their features in aquaria, are hardy and interactive, and may suit captivity better than some wild-caught cichlids.

Labelling

Hybrids are often sold under the name of one parent species, as with shop platies and most Endler's livebearers, so a trade name does not guarantee a pure species (Wikipedia, Practical Fishkeeping).
Seriously Fish notes that aquarists often label wild Betta imbellis with their collection locality to keep records accurate and preserve pure bloodlines.
Locality morphs of Melanotaenia trifasciata are often sold with the collection locality in the name; keeping such forms in separate tanks prevents accidental crosses (Seriously Fish).