This section describes and defines many of the terms and traits, or characters, that are used to refer to fishes in the key and elsewhere on this site. Links to these definitions are provided in the key.
Anadromous: Having a life history that begins in fresh water followed by migration to the sea, with a return to fresh water to spawn.
Anterior: Toward the head end of the body.
Bathypelagic: Living in very deep water, but not directly on the bottom.
Benthic: Living on or near the bottom.
Catadromous: Having a life history that begins in the sea followed by migration to fresh water, with a return to the sea to spawn.
Cirrus ( pl. cirri): A small, fine flap of skin.
Compressed: Body flattened from side to side.
Confluent: Refers to fins that are joined to (continuous with) each other. There may or may not be a notch, but there is no true separation between confluent fins.
Ctenoid scales: Posterior margin of scales have fine teeth.
Cycloid scales: Scales have smooth edges (compare with ctenoid scales).
Depressed: Flattened from top to bottom.
Dextral: Occurring on the right side of the body.
Diploid: Having two chromosome sets. This is the case in most animal cells. Contrast with haploid, tetraploid.
Epipelagic: Occurring in the open ocean, at or near the surface.
Fin Base: The portion of a fin that attaches to the body.
Gas Bladder: Sac in the body cavity between the vertebral column and the gut. It may or may not be connected to the respiratory tract. In different fishes, it can function in maintaining buoyancy, aid in respiration, or help produce sound. Also called the swim bladder.
Gill: The main respiratory organ of most fishes.
Gill Rakers: Projections on the gill arches, often used in filter-feeding.
Haploid: Having only one chromosome set, as in most animal gametes (egg and sperm cells). Contrast with diploid, tetraploid.
Hermaphroditism: Both male and female sex organs present in a single individual. Both organs may occur at the same time (bisexuality) or at different times (sequential hermaphroditism).
Heterocercal: Describes the caudal fin of a fish in which the vertebral column extends into the dorsal lobe of the caudal fin, which usually causes the dorsal lobe to be longer.
Homocercal: Describes the caudal fin of a fish in which all of the principal rays of the caudal fin attach to the last vertebra (the hypural plate). Homocercal caudal fins are usually symmetrical.
Hypural Plate: Modified last vertebra, to which caudal fin rays attach.
Intertidal: The zone between the high- and low-water marks of the tide; also organisms living in this zone.
Lateral: On the side.
Littoral: Occurring on or near the shoreline.
Median Fin: Unpaired fins located along the median plane of the body (dorsal, anal and caudal fins).
Midwater: Occurring in the middle stratum of water, rather than on the bottom or at the surface.
Monotypic: Containing only one species.
Naked: Having no scales.
Notched: Indented, especially in a fin (the rays are shorter).
Oblique: Slanted; neither horizontal nor vertical.
Opercle (or operculum, or opercular bone): The main bone forming the gill cover in higher fishes. Spines on the opercular bone, if present, are directly anterior to, or along the edge of, the gill opening. See Diagram
Oviparous: Laying eggs (sometimes in egg cases) which develop free of the mother's body.
Ovoviviparous: Reproductive system in which fertilized eggs are retained in the mother's body during development. The mother gives live birth, but most of the embryo's nourishment comes from a yolk rather than from a placenta.
Paired Fins: Fins occurring in (usually symmetrical) pairs, with one on each side of the body. The pectoral and pelvic fins are paired fins.
Pelagic: Living in open water, away from the bottom.
Pharyngeal: In the pharynx, the respiratory part of the throat. In some species, pharyngeal bones have teeth.
Photophore: A specialized, well-defined light-producing organ, modified from a mucous gland. Photophores in some fish actively secrete glowing substances; in others they sustain colonies of bioluminescent bacteria.
Pore: Tiny opening in the skin.
Precaudal Pit: In sharks, a cavity just anterior to the caudal fin.
Protrusible: Refers to a mouth which can be thrust forward and out to help the fish catch prey.
Pelvic axillary process: A fleshy appendage at the base of the pelvic fin.
Ray: A bony element (see spine or soft-ray) that supports a fin membrane.
Rhomboid: Having the shape of a rhombus, an angled four-sided figure with opposite sides equal.
Scute: A relatively large, hard plate.
Sinistral: Occurring on the left side of the body.
Soft-Ray: A type of ray that is flexible, segmented, and often branched. See ray, spine.
Spine: A hard, unbranched ray (may be in a fin or isolated). Also a bony projection, usually on the head.
Spiracle: In sharks, skates and rays, a vestigial gill slit visible as an opening posterior to the eye.
Standard Length: The standard scientific measure of a fish's length; found by measuring from the most anterior part of the snout, lip or chin to the end of the last vertebra.
Subtidal: The zone just below the low-water mark of the tide (never exposed even at low tide); also organisms found in this zone.
Swim Bladder: See: gas bladder.
Tetraploid: Having four chromosome sets. Contrast with diploid, haploid.
Tubercle: A domelike projection from the surface of a fish.
Vestigial: Refers to a body part which was functional in an ancestor but has become reduced or nonfunctional in a descendant.
Viviparous: Having live birth. Usually refers to species in which the embryo receives nourishment from maternal tissue rather than yolk, but also sometimes used to describe ovoviviparous species.