See gender. Also used to refer to the process of distinguishing the gender of an individual; sexing.

The ratio of males to females in a given population. Often used to describe the same ratio in a batch of captive-bred fishes.

The process of distinguishing the gender of an individual.

Difference in colour pattern between males and females of a single species.

Phenotypic difference between males and females of a single species. Used in species profiles here on SF to refer to differences in morphology or colour pattern, but officially also refers to behavioural differences.

Behavioural symptom that indicates deteriorating water quality or disease in which an individual appears listless with fins normally clamped to the body and displays an abnormal, 'shimmying' swimming action. Particularly typical in poeciliid livebearers.

A group of fishes, normally of the same species, that remain close to each other for some reason. Differs from a school in that individuals are not necessarily swimming together in the same direction. Also used to refer to a shallow area in a water body.

One of two or more individuals having one or both parents in common; a brother or sister.

One of two or more very closely-related species that are reproductively isolated from one another but almost indistinguishable in morphologically and/or behaviourally. May occur sympatrically and even interbreed but are incapable of producing fertile hybrid offspring. Sometimes referred to as cryptic species.

Sand containing a high concentration of silicon dioxide. Noted in aquaria for its purity, delicate appearance and lack of effect on water chemistry. Also known as silver sand.

Any of a largegroup of chemical compounds containing silicon, oxygen, and at least one metal. Most rocks and minerals are silicates.

See silica sand.

Unbranched and unsegmented soft fin ray.

A dichotomous or otherwise sequential identification key. See dichotomous key.

Having a wavy margin with strong indentations.

See sister group.

In cladistics, a monophyletic group more closely related to the group in question than any other, i.e., the closest outgroup. Sister groups normally share an immediate common ancestor.

In vertebrates, the internal structure composed of bone and/or cartilage that supports the soft tissues and protects the internal organs.

In vertebrates, the thin, membranous layer of external tissue forming the protective covering of the body.

Subdominant male individual of certain fish species that attempts to sneak into the nest of a spawning pair and fertilise deposited eggs. Typically smaller in size and/or possessing reduced colouration compared with a normal male. Also known as a sneaky male.