Abdominal: Refers to pelvic fin placement posterior to pectoral fin. See Diagram

Adipose fin: A small, fleshy fin without rays, usually on median line of body. See Diagram

Anadromous: Having a life history that begins in fresh water followed by migration to the sea, with a return to fresh water to spawn.

Anal Fin: Fin on undersurface of body along median line, usually just behind anus. See Diagram

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.

Caudal Fin: Median fin at the end of the body (tail fin). See Diagram

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.

Dorsal: On the top surface or "back" of the body. See Diagram

Dorsal fin: Rayed fin on median line of the dorsal surface of body. See Diagram

Epipelagic: Occurring in the open ocean, at or near the surface.

Fin Base: The portion of a fin that attaches to the body.

Finlet: A small, isolated fin, usually without rays. See Diagram

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 Cover: Bony flap covering and protecting gills. See Diagram

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.

Isthmus: Narrow, triangular area at the anterior end of the underside of the body, between the gill openings. See Diagram

Lateral: On the side.

Lateral line: A series of receptors, or scales modified with pores opening to a sensory canal on the side of a fish, which detect movements in the surrounding water. See Diagram

Littoral: Occurring on or near the shoreline.

Maxilla (or maxillary bone): Bone of the upper jaw. See Diagram

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.

Pectoral Fins: Pair of fins attached to the 'shoulder' bones, just behind the head. See Diagram

Pelagic: Living in open water, away from the bottom.

Pelvic Fins: Pair of fins on the lower part of the body, which may be located between the anus and the isthmus. See Diagram

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.

Preopercle (or preopercular bone): Generally most anterior of opercular bones (bones forming gill cover). Spines on preopercle, if present, are significantly anterior to the gill opening. See Diagram

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.

Suborbital Stay: Bony extension (below the skin) from below the eye to the preopercle. See Diagram

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.

Thoracic: Refers to pelvic fin placement about even with or anterior to pectoral fin. See Diagram

Tubercle: A domelike projection from the surface of a fish.

Ventral: On the lower surface of the body or "belly." See Diagram

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.