…of Weird & Fun Fish-Related Terminology

Admittedly, some of the language and terminology used in this blog is a little bit technical. For this, I make no apology, but have included this handy-dandy glossary to aid with confusion, which I will try to update whenever I add a “big word” to a post. Any one of many resources out there on introductory genetics will aid in further clarification should one require a crash-course on the topic. Happy learning!


albinism – an albino fish lacks black pigment in the skin due to reduced production of melanin. Albino fish typically have red eyes.

allele – a genetic variation at a given locus on a chromosome that can result in different physical traits. For example, in angelfish, the traits for dark (D), marble (Dm), gold marble (Dgm) and gold (dng) all form alleles and only one can exist at each locus.

autopepsia – “self-digestion.” Where stomach acid creates ulcers in the gastric mucous membrane of the digestive tract.

autosomes – non-sex-chromosomes. An autosomal trait is on a chromosome that is not sex-linked and will be passed on to both males and females with an equal probability.

axanthism – an axanthistic fish lacks yellow pigment in the skin due to reduced production of pteridine. This is the opposite of xanthism.


back-crossing – mating daughters to fathers and/or sons to mothers in order to preserve or enhance a specific (recessive) trait within a strain of fish.


cf. – from the Latin confer meaning “bring together”, cf. is used between genus and species names when one fish resembles another that is already described, but has a difference of sorts. This is for when a fish species is first discovered, which may be either a variant or a new species altogether, requiring more study to be sure. In this case, the literal meaning is “similar to”. For example, Vieja cf. gregthecrazyfishguyi would describe a species/variant of a Vieja species similar to Vieja gregthecrazyfishguyi.

chromatophores (or chromophores) – pigment cells, responsible for the giving a fish its colour, which include erythrophores, leucophores, melanophores, xanthophores, etc.

chromosome – a single piece of coiled DNA that contains the genes of an organism. Humans have 23 pairs of chromosomes (2n = 23) – 22 pairs of autosomes and a pair of sex-chromosomes. Different fish species have different numbers of chromosomes. For example, cichlids can have 2n = 32 to 2n = 60 chromosomes.

cichlid – Really? Do I really have to put this one in here? Ugh. All right. It’s pronounced “sick-lid” (/ˈsɪklɨd/) and not to be confused with a certain type of chewing gum! Cichlids are a huge family of fishes, with the number of species among various genera somewhere in the four-figure thousands. The defining trait of cichlids is that they all have a unique tooth structure, comprised of the lower pharyngeal bones (bones within the pharynx – the back part of the throat) that have been fused together, that augment their regular teeth and allow for more efficient processing of food.

co-dominant – co-dominant genes are neither dominant, nor recessive, but are expressed to a degree that is dependent on the number of copies of the gene present. For example, variegation of skin pigment can be a co-dominant trait: zero copies of the gene result in the standard solid colouration, one copy results in some white blotches, while two copies produce a very blotchy appearance.

conspecific – “of the same species”

cross-breeding – the mating together of two different species of fish to produce offspring that are a hybrid of the two. This practice can have consequences in that pure strains can be diluted if these hybrid fish go unlabelled as such and are allowed to mate with wild-type fish. Creating hybrids carries the responsibility of ensuring that all future owners of the offspring know exactly what they are receiving.

cyanophores – atypical cells that are responsible for the vibrant blue colouring of a fish. See iridocyte.


detritus – waste; poop; debris. Particles of dead organic material that accumulate in your tank, formed from the decay of leftover food, excrement, fallen plant leaves, scales, organisms, etc.

dimorphism – sexual dimorphism occurs when the males and females of the same species display differences in colour, size, fins, presence of a nuchal hump, etc. In contrast, species where the males and females are indistinguishable from one another do not display sexual dimorphism.

DIY – do it yourself.

dominant – a gene is dominant if it is masks or overrides the expression of other genes. Fish carrying either one or two copies of a dominant gene will typically look identical as far as the trait the gene controls is concerned. Dominant genes are represented by capital letters. See recessive.


erythrism – an erythristic fish will appear excessively red, due to either its diet or a genetic mutation that increases the production of red pigments in relation to other colours.

erythrophores – cells that are responsible for the red colouring of a fish.

expressivity – within a given individual specimen carrying a particular gene, expressivity is the degree to which that gene is expressed. For example, if an individual is known to carry the gene for a given trait, but to a lesser degree as is known to be possible, the gene has a lower expressivity. Quantitative: To what extent is the trait expressed? See penetrance.


F1 – first filial generation. Similarly, F2, F3, etc. Fn represents the nth filial generation.

F-word – is Fl*werh*rn! It’s a really dirty word! I draw the line at Blood Parrots. 😉

filial – denotes the generation or generations after the parental generation.

fishes – is used to indicate plural when multiple fish species are involved. So, “one fish, two fish, red fish, blue fish” is still correct when referring to counts and characteristics of fish, but “the Amphilophus genus contains large and aggressive fishes” would be correct when referring to the A. citrinellus, A. labiatus, A. robersoni, A. trimaculatus, A. zaliosus, etc. species as a collective. However, I’m still undecided whether I have a hundred or so fish or fishes!

fry – are young fish. This is a plural noun that is also used as the singular: one fry, two fry, big fry, small fry. Fries are chips, not fish! (Oh, that’s clever!)


gene – a unit of heredity, used to hold the information to build and maintain the cells of fish and pass genetic traits to offspring.

genera – plural of genus.

genotype – classification by genetic type (two fish with the same genes will have the same visual appearance). See phenotype.

genus – a taxonomic grouping of related species.

GH – general hardness of water, measured in either degrees or ppm (1 °GH = 17.848 ppm). This measurement tells you how high the dissolved mineral content of your water is as a result of how many calcium (Ca2+) and magnesium (Mg2+) (cat)ions are present. See KH.


heterospecific – “of differing species”

hererozygous – when a gene locus contains two different alleles of a gene, i.e. Aa, Bb, D/+, D/dng, D/Dm, etc.

homozygous – when a gene locus contains two of the same alleles of a gene, i.e. AA or aa, BB or bb, D/D or dng/dng, etc, etc.

hybrid vigour – when two different species are cross-bred, offspring can be genetically superior when they inherit desirable traits from each parent. This phenomenon is strongest for the first cross only and vigour is reduced with each subsequent hybrid generation.


inbreeding depression – the reduced fitness or vigour of offspring resulting from breeding of related individuals. Some species are more resilient to inbreeding depression than others, remaining unaffected after several generations.

incompletely-dominant – Genes that show incomplete dominance are neither dominant, nor recessive, but will be expressed in an additive way, blending traits together. For example, a dark-orange fish crossed with a pale-orange fish may result in offspring that have a medium-orange hue.

iridocyte (or iridophore) – cell responsible for the iridescence, reflective silver or blue sheen in a fish.


KH – From the German “karbonathärte“, the carbonate hardness of water, measured in either degrees or ppm (1 °KH = 17.848 ppm). This measurement tells you how alkaline your water is as a result of how many carbonate (CO32-) and bicarbonate (HCO3) (an)ions are present. This is not truly a measurement of hardness, but of the alkalinity or acid buffering capacity (how stable the pH of your water is). Since aquarium literature usually connects KH with GH and pH so closely, and KH is uses the same units as GH, the term KH has been adopted, however, “carbonate content” might be a more accurate term.

kok – See nuchal hump.


leucism – a leucistic fish has reduced amounts of all colours of pigment in the skin. Leucistic fish typically have black eyes.

leucophores – cells that are responsible for the white colouring of a fish. These cells add a reflective white shine and do not account for colourless (i.e. leucistic) fish.

LFS – “local fish store”

line-breeding – the crossing of filial members of a species of fish, usually done in order to bring out a desirable trait or characteristic.

loci – plural of locus.

locus – “place”. In genetics, it is the specific location of a gene on a chromosome. Only one gene may exist in one place, so genes that share the same locus are mutually-exclusive and form alleles. For example, a fish may either be fully-coloured or albino.


macro-melanophores – cells that are responsible for large black blotching on a fish.

male growth superiority – the tendency for the males of a spawn to grow faster than their sisters, sometimes to the extent that the proportion of females is reduced due to male aggression or the females’ inability to compete for food.

media – plural of medium. Note that mediums and medias are both not words and should be struck from one’s vocabulary.

medium – besides the usual definitions of the word, a filter medium is a substance through which aquarium water is passed to aid in mechanical, biological or chemical filtration and conditioning. Examples include carbon, sponge, BioMax, filter wool/floss, a water-softening resin-containing pillow, peat moss, alder cones, etc. Collectively, these different items in your filter are known as filter media. If you’re referring to only one substance, it is a filter medium.

melanism melanistic fish have and increased amount of black pigment in the skin due to the over-production of melanin and appear black. This condition is the opposite of leucism and albinism.

melanophores – cells that are responsible for the black colouring of a fish, contain melanin.

micro-melanophores – cells that are responsible for tiny black spotting on a fish.


nuchal hump – a deposit of fatty tissue on the forehead of a fish. These are mainly found on cichlids and typically on males of the species, although females of some species can have smaller, less pronounced humps. Nuchal is the adjective form of the Latin noun nucha (pl. nuchae) which means nape or back of the neck and is pronounced  “nyoo-kal” (/ˈnyukəl/).

null allele – an allele that has no expression. For example, a sex-linked trait for red colouration can have a null allele on one sex chromosome and full-expression on the the other.


out-crossing – strengthening a line of fish that possesses a desirable trait by crossing it with a wild type of the species and then crossing their offspring (or offspring from a similar crossing) together in order to obtain a broader genetic base and prevent inbreeding depression.

outbreeding depression when two different species are cross-bred, offspring can be genetically inferior when they inherit undesirable traits from each parent. Hybrids showing this phenomenon are often unfit to survive and will not provide strong genes to their offspring.

ovipositor – egg laying tube.


P0 – (subscript naught) original parental generation.

papilla – a small fleshy projection, process or outgrowth, similar to a nipple.

papillae – plural of papilla.

penetrance – within a given population of specimens carrying a particular gene, penetrance is the percentage representing how often a gene is expressed. For example, if half of a spawn expresses a trait, but all of the fish are known to carry the gene, the gene has 50% penetrance. Qualitative: Is the trait displayed or not? See expressivity.

pH – power of hydrogen. The logarithmic scale, ranging from zero-to-fourteen, that is used to measure the acidity/alkalinity of a solution. Pure water should have a pH of 7.0, which is considered neutral. Below 7.0 is an acid and above is a base. Keep in mind that a 1.0 decrease in pH actually represents tenfold increase in acidity!

phenotype – classification by appearance (two fish that look the same may have differing genes). See genotype.

piebald – a form of leucism that results in irregular patches of white mixed with a fish’s normal colouration.

polygene – A number of genes that are collectively responsible for certain characteristics.

ppm – part(s) per million. 1 ppm = 1 mg/L


recessive – a gene is recessive when it can be masked in the presence of a dominant gene. Recessive genes are only expressed when a fish carries two copies of the gene, one inherited from each parent, otherwise the dominant gene is expressed instead. Recessive genes are represented by lowercase letters.

reciprocal cross – used to determine if gender plays a role in how a particular trait is passed on. For example, if a spotted male is initially crossed with a plain female and their children observed, the reciprocal cross performed next would be a plain male crossed with a spotted female. If there are differences in the two batches of offspring produced, parental gender plays a role in inheritance; if they are the same, however, parental gender does not play a role.

ROreverse osmosis is the active process (as opposed to the passive process that is forward osmosis) of pressure-forcing a solution through a membrane in order to remove any dissolved solutes and end up with pure solvent. For aquarium use, we pass tap water through a RO unit to produce pure water, removing chemicals, phosphates, copper, nitrogen compounds, silicates, chlorine, chloramines, etc. This RO water will (theoretically) have pH 7.0, 0 °GH, 0 °KH and will be missing everything good for fish as well as everything bad.


school – Schooling fish organise themselves in a tightly synchronised manner, such that they swim in the same speed and direction. See shoal.

sex-linked – a genetic trait is sex-linked if it is contained within one of the sex-determining chromosomes (W, X, Y or Z) and can have varied expression between males and females.

shoal – Shoaling fish group together in a small community, swimming and foraging in close proximity, but do not swim in as organised a fashion as in a school. A shoal is often the more correct term for the groups of fish kept in aquaria.

sp. – abbreviation of species, a single member of a particular genus.

strain – stock, line, variety and strain all indicate a group of fish that have been selectively-bred for a particular favourable trait or traits.


var. – variant/variety/variation. These specimens visually differ from members of their species, but still carry the same scientific name.


W-chromosome – the female chromosome of a female-determined sex model. WZ represents the female gender. Atypically, WX and WY females are also possible. WW females are possible through the masculinization of females mated to regular females.


X-chromosome – the female chromosome of a male-determined sex model. XX represents the female gender.

xanthism – a xanthistic fish has excess amounts of yellow pigment or reduced amounts of darker pigments that allow the yellows to shine through more prominently.

xanthophores – cells that are responsible for the yellow colouring of a fish.


Y-chromosome – the male chromosome of a male-determined sex model. XY represents the male gender. Atypically, WY and ZY males are also possible. YY males are possible through the feminization of males mated to regular males.


Z-chromosome – the male chromosome of a female-determined sex model. ZZ represents the male gender. Atypically, ZX and ZY males are also possible.

Revised: 2013-10-10

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