Albino, Leucistic and Other Colour Morphs in Aquarium Animals

How albinism and leucism differ genetically, what is known about vision and light sensitivity in albino fish, and how morphs arise in corydoras, bristlenose catfish and axolotls.

Overview

Colour morphs are heritable variants that differ from the wild type in pigmentation. In fish and amphibians, colour comes from several types of pigment cell (chromatophores), including melanin-containing melanophores and yellow-to-orange xanthophores; the separate guide on fish coloration and chromatophores describes these cells. A 2020 study of rainbow trout in PLOS ONE describes albinism as the most common colour variation reported in fish. Several aquarium morphs described below are line-bred, meaning that a spontaneous mutant was bred on to fix the trait.

Albinism and leucism: the genetic difference

Albinism is a congenital loss of melanin pigmentation. A common route is disruption of the TYR gene, which encodes tyrosinase, an enzyme needed to make melanin, although many genes can produce albinism; in mice about 100 genes are known to affect it. Albino animals keep their pigment cells but those cells make little or no melanin. Without pigment in the iris, blood vessels show through and the eyes appear red or pink.

Leucism is a partial loss of pigmentation caused by defects in how pigment cells differentiate or migrate from the neural crest to the skin. Because all pigment-cell types share that precursor, leucism reduces every pigment type, not only melanin. The eyes usually keep their normal colour, because the pigment cells of the retinal pigment epithelium do not come from the neural crest. Genes implicated in leucism when mutated include c-kit, mitf and EDNRB.

In species that also carry xanthophores, an albino can look pale yellow rather than white, because only melanin is missing. Rainbow trout (Oncorhynchus mykiss) show this: yellow-albino fish lack melanin, while a white-albino form produced by crossing yellow-albino and cobalt blue lines lacked melanophores and also had fewer xanthophores.

Inheritance: species examples

Swordtail (Xiphophorus hellerii): a spontaneous albino phenotype was mapped to a recessive mutation in the oca2 gene (Pigment Cell & Melanoma Research, 2026).
Mexican tetra (Astyanax mexicanus): mutations in oca2 underlie albinism in the blind cave populations.
Rainbow trout (Oncorhynchus mykiss): cobalt blue was inferred to be dominant over wild type and co-dominant with a dominant yellow-albino line, producing a fourth phenotype, white-albino. Cobalt blue and white-albino fish had lower body weight and a smaller pituitary gland than wild-type and yellow-albino fish.
Axolotl (Ambystoma mexicanum): four pigmentation genes give leucistic (pale pink body, black eyes), xanthic (grey body, black eyes), albino (pale pink or white body, red eyes) and melanistic (dark body without gold speckling) animals. White animals with pink eyes in the pet trade are often double homozygous mutants for both the albino and leucistic genes.

These inheritance patterns apply to the species and strains studied. The same colour name in another species may have a different genetic basis. Guppy colour inheritance is covered in the separate guide on guppy colour genetics.

Vision and light sensitivity in albino fish

Albinism affects eye development in vertebrates, and Wikipedia reports photophobia and refraction problems in albino animals. Fish studies give species-specific evidence. Juvenile albino rainbow trout kept outdoors avoided exposing their eyes to direct sunlight, whereas pigmented fish did not. After 4 months outdoors, albinos had severely shortened or missing rod outer segments, the light-sensing parts of rod cells, although cones stayed intact. When moved indoors to dim daylight of 10 to 30 lux, the rod outer segments regrew to about 95% of normal volume within 34 days (Visual Neuroscience, 1997). A follow-up study found that rods in albino trout die under full daylight and are only partly replaced (Visual Neuroscience, 2006).

In surface-dwelling Mexican tetras, larvae with engineered oca2 albinism were outcompeted by pigmented siblings when hunting prey in the light, but outcompeted them in darkness (Journal of Experimental Biology, 2025).

Health and behaviour differences

Viability: in laboratory medaka, only 29 of 800 albino embryos survived to adulthood; albino channel catfish have lower survival and lay smaller egg masses with poorer hatchability (Wikipedia, citing published studies).
Behaviour: juvenile albino European catfish (Silurus glanis) were less aggressive than pigmented fish and tended to keep more distance from one another (PeerJ, 2016).
Stress: under short-term stress, albino European catfish showed higher cortisol and less pupil dilation than pigmented fish (PLOS ONE, 2020).
Social exclusion: in established groups of pigmented European catfish, an albino individual was kept at a greater distance from the group (PLOS ONE, 2015).

Morphs in common aquarium species

Corydoras aeneus: Seriously Fish lists selectively bred albino and long-finned strains. A melanistic (black) form first appeared in the early 1990s as a few dark fry in a captive-bred batch in Germany and was line-bred to fix the trait. It is sometimes sold under invalid species names but is an aquarium-bred fish. Short-bodied mutants and artificially dyed fish are also sold; dyeing is not a genetic morph and is covered in the guide on dyed and tattooed fish.

Bristlenose catfish (Ancistrus sp. '3', also called Ancistrus cf. cirrhosus): this commercially bred fish is of uncertain origin, and some consider it a hybrid. Line-bred piebald, albino, xanthic and long-finned forms exist.