See sneaker male.

The projecting mouthparts, jaws, or part of the head anterior to the eyes.

Distance between the tip of the upper jaw and anterior margin of the orbit.

Segmented fin ray which is flexible and usually branched.

Water containing low concentrations of mineral salts, especially calcium or magnesium.

Name often given to the Amazon River main channel from its confluence with the Rio Negro upstream to the border of Brazil and Peru. Often used interchangeably with ‘Amazon’ to describe this stretch.

Abbreviation for species.

The eggs of fishes and other aquatic organisms; to deposit and fertilise such eggs.

The act of releasing and fertilising eggs in fishes.

Migration of fishes to a spawning site or area, occurring in certain fishes. Variable in extent.

Particular substrate type required by a given egg-laying fish species for the purpose of spawning.

Evolutionary formation of new biological species.

Taxonomic rank into which a genus is divided, but otherwise difficult to delineate precisely with a single definition satisfying all eventualities still lacking to date. There exist a number of concepts of which the most popular in vertebrate taxonomy include the biological species concept and phylogenetic species concept.

Diverse group of closely-related, biologically distinct species occurring in an isolated area.

Latin term meaning a species of doubtful identity requiring further investigation; a questionable species.

Scientific name of a species; binomen.

New species, usually given after the species name in a description instead of the author. Abbreviated to sp. nov. or sp. n.

The number of species present in a given sample, habitat, or otherwise specified geographical region.

In zoological nomencature, the second name in a binomen and in a trinomen. Has no nomenclatural function when used alone, though this is often done in aquarium literature or when referring to a species in passing if the genus being referred to is already clearly known. Also known as specific name.

Ratio of the density of a liquid to the density compared with pure water. Often used to deduce the salinity of aquarium water and measured using a hydrometer or refractometer.