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Fish Guide

Ichthyology terms

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Ichthyology uses several terms that are unique to the science.

Contents: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


  • Abdomen: belly
  • Abdominal: pertaining to the belly
  • Actinosts: a series of bones at the base of the pectoral rays.
  • Acuminate: tapering gradually to a point
  • Acute: sharp, pointed
  • Adipose fin: a small fleshy fin which lacks fin rays.
  • Adnate: joined together
  • Adpressed: pressed against the body
  • Anal fin: the fin on the median line behind the vent
  • Anal fin origin: the most anterior point of the anal fin base
  • Ankylosed: grown firmly together
  • Anterior: relating to the front portion
  • Antrose: turned forward
  • Approximate: placed close together
  • Asperity: roughness of surface
  • Asymetrical: without symmetry
  • Attenuate: tapering to a point, usually in reference to a tail
  • Axillary: pertaining to the axilla or upper angle of the pectoral fin
  • Axillary process: an enlarged, pointed scale projecting from the insertion of the pectoral or pelvic fin.)


  • Barbel: an elongated fleshy projection, usually about the head
  • Basal: pertaining to the base, at the base
  • Base: the part of a projection, (commonly a fin), which is connected to the body
  • Bathypelagic: living at a depth between 1000 m and 4000 m, but well off the bottom
  • Benthic: bottom-dwelling, living on the sea bed
  • Benthopelagic: pertaining to fishes that swim just above the seabed at depths below about 200 m (the edge of the continental shelf)
  • Bicuspid: having two cusps or points
  • Bifid: cleft in two
  • Bifurcate: forked, or divided into two parts or branches
  • Branchial: pertaining to the gills
  • Branchial aperture: the gill opening
  • Branchiostegals, Branchiostegal ray(s): bony rays supporting the gill membranes behind the lower jaw
  • Bristle: a stiff hair-like projection
  • Buckler: a bony shield
  • Bycatch: species other than the target species that are caught incidentally in a trawl


  • Caniniform: shaped like a canine tooth, conical in form
  • Carapace: a horny or bony covering encasing the body
  • Carinate: keeled, having a ridge along the middle line
  • Caruncle: a fleshy outgrowth
  • Caudal: pertaining to the tail
  • Caudal fin: the tail fin
  • Caudal peduncle: the region of the body between the end of the anal fin and the base of the caudal fin
  • Ciliated: fringed with eyelash-like projections
  • Cirri: small, thin appendages, often subdivided into branches.
  • Cirrus: singular of cirri (see above)
  • Claspers: the external reproductive organs of male sharks, rays and chimaeras
  • Coalesced: grown together
  • Compressed: flattened laterally
  • Confluent: joined together
  • Conical: cone shaped, with a cylindrical base and a pointed tip
  • Corselet: a scaly covering behind the pectorals of some fishes
  • Countershading: body colouration which is dark above and lighter below
  • Crenulate: having the edge slightly scalloped
  • Cutaneous: pertaining to the skin
  • Ctenoid: rough-edged
  • Cycloid: smooth-edged


  • Deciduous: temporary, falling off
  • Demersal: living on or near the sea bed
  • Dendritic: resembling a tree or shrub
  • Denature: the "unfolding" of a protein resulting in a lessening of its biological properties. In the case of some fish toxins, denaturing with hot water can lessen painful symptoms.
  • Dentate: with tooth-like projections
  • Denticle, denticulate: a little tooth, having an edge with small projecting teeth, the placoid scales of cartilaginous fishes
  • Depressed: flattened from top to bottom
  • Dermal: pertaining to the skin
  • Dewlap: a fold of loose skin
  • Disc: the flattened head and body of various fishes such as stingrays, which also commonly includes the pectoral and ventral fins
  • Distal: remote from the point of attachment
  • Dorsal: pertaining to the back
  • Dorsum: the upper (dorsal) surface of the head or body
  • Dorsal fin: the fin on a fish's back
  • Dorsal fin origin: the most anterior point of the dorsal fin base


  • Elasmobranchs: cartilaginous fishes - sharks, rays and allies
  • Electrocyte: a cell that generates electricity
  • Elongate: extended, drawn out
  • Emarginate: with the margin slightly hollowed
  • Endemic: restricted to a particular region, for example endemic to Australia
  • Entire: with a smooth margin
  • Epibenthic: referring to organisms living on the bottom surface
  • Epipelagic: referring to organisms living in the region between the surface and 200 m depth
  • Erectile: capable of being raised or erected, often referring to spines
  • Esca: the lure or "bait" on the end of the illicium of some anglerfishes and relatives
  • Estuarine: living in estuaries


  • Falcate: scythe-shaped, long, narrow, and curved
  • Falciform: curved like a scythe
  • Filament: a slender or thread-like structure
  • Filiform: thread-like
  • Fimbriate: fringed at the margin
  • Finlet: a small fin, positioned behind the dorsal or anal fins, that is supported by a ray or rays
  • Fluviatile: living in rivers
  • Free rear tips (of fins): the posterior tip of the fin that is closest to the most posterior point of the fin base
  • Frontal ridge: a ridge running along the top of the head along the midline
  • Furcate: forked
  • Fusiform: tapering towards both ends


  • Gill arches: the bony arches to which the gills are attached
  • Gill cover: or bony flap that covers the gills, the operculum
  • Gill filaments: a series of projections along the posterior edge of the gill arch, the site of gas exchange
  • Gill membranes: membranes covering the gill openings, attached to the branchiostegals
  • Gill opening: opening behind each operculum, leading to the gills
  • Gill rakers: a series of appendages along the anterior edges of the gill arches
  • Gills, Branchiae: organs for breathing the air contained in water
  • Glossohyal: the tongue bone
  • Gonopodium: a modification of the anal fin of males of certain live-bearing species (i.e. family Poeciliidae), that is used to transfer reproductive products to the female during mating.
  • Gravid: Sexually ripe
  • Gular region: pertaining to the region behind the chin and between the sides of the lower jaw


  • Head length: the distance from the tip of the snout (or upper lip) to the most posterior point of the opercular margin
  • Homology: similarity of features based on common evolutionary descent
  • Hyperostosis: a condition resulting in enlargement of areas of bone.
  • Hyaline: translucent or transparent
  • Hypural joint: the joint between the caudal fin and the last of the vertebrae
  • Hypural plate: the flattened bony plate at the posterior end of the vertebral column, formed from parts of the posterior vertebrae.


  • Ichthyoplankton: fish eggs and larvae
  • Illicium: a "fishing rod-like" appendage on the head, usually a modified dorsal fin spine
  • Imbricate: overlapping, like the shingles on a roof
  • Incised: with a notched margin (often referring to fin membranes)
  • Incisors: the front or cutting teeth
  • Inferior: pertaining to the lower side (usually of the head)
  • Interdorsal: between the dorsal fins
  • Interorbital: the space between the orbits
  • intromittent organ: a structure to facilitate sperm transfer in some internally fertilizating species
  • Iris lappet: a fleshy flap or lobe-like structure in the eye, short and rounded, simple or multiply branched.
  • Isthmus: the fleshy projection of the body separating the gill openings


  • Jugular: pertaining to the throat
  • Juxtaposed: placed near together


  • Kidneys: organs involved in excretion and regulation of water balance


  • Lanceolate: spear-shaped, gradually tapering toward the extremity
  • Labial: pertaining to the lips
  • Labial furrows: shallow grooves around the lips
  • Labial papillae: small fleshy projections around the lips
  • Lateral: at or toward the side
  • Lateral line: a series of muciferous tubes forming a raised line along the side of the body
  • Leptocephalic: tallness and narrowness of the skull - in fish usually refers to "elongate highly compressed transparent, ribbon-like larval stage" (source FishBase glossary)
  • Liver: a digestive and storage organ
  • Longitudinal series (scales): the number of scale rows above the lateral line from the first pored lateral line scale to the caudal fin base.
  • Lunate: shaped like a crescent moon, with long upper and lower lobes


  • Maxilla, Maxillary: the upper jaw, or pertaining thereto
  • Maxillae, Maxillaries: the hindmost bones of the upper jaw; preceded by the premaxillaries
  • Median, medially: pertaining to the middle
  • Median fins: fins located on the median line of the fish; the dorsal, anal and caudal fins
  • Mediolateral: between the middle and the sides
  • Mesocoracoid: a bone of the pectoral arch or shoulder girdle
  • Mesopelagic: refers to the region of the open ocean between 200 m and 1000 m in depth
  • Mesopelagic fishes: fishes that live in the mesopelagic zone
  • Midwater: the middle stratum of water, well below the surface and well above the seabed. See also mesopelagic.
  • Midwater fishes: fishes that live in the midwater
  • Molars: blunt and rounded grinding teeth
  • Morphology: form and structure of an organism
  • Muciferous: producing or containing mucous or slime
  • Myotomes: blocks of lateral trunk muscles


  • Nape: upper surface of the body behind the head and before the dorsal fin
  • Nasal: pertaining to the nostrils
  • Nasoral: between the nostrils and mouth
  • Nictitating membrane: an inner eyelid
  • Notochord: a rudimentary of embryonic spinal column
  • Nuchal: pertaining to the nape


  • Obsolete: faintly marked, scarcely evident
  • Obtuse: blunt
  • Occipital: pertaining to the posterior part of the skull
  • Ocellus: an eye-like spot
  • Ocular: pertaining to the eye
  • Odontode: a dermal tooth
  • Oesophagus: the gullet
  • Opercule, operculum: the bony flap that covers the gills
  • Opercular: pertaining to the operculum
  • Opercular spine: spine projecting from the operculum (view the fish figure)
  • Orbit: the eye socket
  • Origin: the most anterior point of a fin base
  • Osseus: bony
  • Ovate: egg-shaped


  • Palate: the roof of the mouth
  • Palatines: the bones on each side of the palate
  • Papilla (papillae): a small fleshy projection(s)
  • Papillose: covered with papillae
  • Pectoral: pertaining to the breast
  • Pectoral fins: the anterior or uppermost of the paired fins, which correspond to the anterior limbs of the higher vertebrates
  • Pectoral girdle: the bones to which the pectoral fin is attached
  • Peduncle: usually referred to as the caudal peduncle, the region of the body between the end of the anal fin and the base of the caudal fin
  • Pelagic: living on or in the open seas
  • Pelvic girdle: the bones to which the ventral fins are attached
  • Pelvic fins: paired fins behind or below the pectoral fins
  • Pharyngeal bones: bones behind the gills in the oesophagus or gullet
  • Pharyngeal teeth: teeth within the pharynx
  • Pharynx: the back part of the throat, into which the gill slits open
  • Photophore: a circular light-producing organ on the surface of a fish
  • Posterior: towards the hind end of the fish
  • Postorbital: behind the eye
  • Precaudal: anterior to the tail portion
  • Premaxillaries: two bones forming the front portion of the upper jaw
  • Preocular spine: spine positioned above and in front of the eye
  • Preopercle, preoperculum: the bone between the cheek and the gill cover
  • Preopercular spine: spine projecting from the preopercule (see preopercle) (view the fish figure)
  • Preorbital: the area under and in front of the eyes
  • Protractile: capable of extending forward
  • Protrusible: capable of extending forward, often referring to the jaws of fishes
  • Proximal: nearest
  • Pseudobranchiae: small gills developed on the inner side of the gill cover
  • Pseudoclasper: stiff ossified lobes or prongs in the tip of the intromittent organ
  • Pterygiophore: an internal cartilage or bone that supports a median fin ray or spine
  • Pyloric caecae: finger-like pouches connected with the alimentary canal (the gut)


  • Ramus: one branch or one half of the jaw
  • Ray: a jointed rod which supports a fin
  • Retrose: turned backward
  • Rostrum: a projecting snout or beak
  • Rugose: rough


  • Scalation: pertains to the pattern/arrangement/presence of scales
  • Scute: any external horny or bony plate
  • Serrate: notched like a saw
  • Setae: bristles or hairs
  • Soft dorsal: the posterior part of the dorsal fin which is composed of jointed rays
  • Spatulate: shovel-like; having a broad, flat and rounded shape
  • Spine: a sharp projecting point; an unjointed support in the anterior portions of the dorsal and anal fins
  • Spinous, spiniform, spinate: spine-like or composed of spines
  • Spinous dorsal: the anterior part of the dorsal fin supported by spines
  • Spiracles: respiratory openings behind the eyes in sharks and rays
  • Standard length (SL): the length of a fish measured from the tip of the snout to the posterior extremity of the hypurals, the expanded bones at the end of the backbone that support the caudal fin
  • Submarginal: almost at the edge
  • Suborbital: below the eye
  • Superior: above or on the upper surface
  • Supracleithrum: the bone forming a connection between the back of the skull and the pectoral girdle
  • Supralateral: above the side
  • Supramaxillary: a supplemental bone lying along the upper edge of the maxillary
  • Supraocular: positioned above the eye
  • Supraorbital: above the eye
  • Supraorbital tentacle: a flap or filament of skin positioned above the eye
  • Suprascapular: a bone uniting the shoulder girdle with the skull
  • Suture: the line of union of two bones or plates
  • Symphysis: point of junction of the two sides of the jaw
  • Symmetrical: similarly arranged on both sides
  • Swimbladder: a sac filled with gas, lying beneath the backbone


  • Teleost: a member of Teleostei, an infraclass containing most of the bony fishes
  • Terminal: at the end
  • Tessellated: marked with little checks or squares, like tiles
  • Thoracic: pertaining to the chest
  • Thorax: the chest region, just behind the head
  • Total length: the length from the tip of the snout to the tip of the tail
  • Transverse: crosswise
  • Trilobate: with three lobes
  • Tricuspid: with three cusps or points
  • Truncate: terminating abruptly, as if cut off square
  • Trunk: the region of a fish between the head and tail, or the last gill slit and vent
  • Tubercle: a small, usually hard excrescence or lump
  • Tubiform, tubuliform: resembling a tube
  • Type locality: the location from which the type specimen was collected.


  • Undulated: waved
  • Urogenital papilla: a papilla through which the urinary waste and gametes leave the body


  • Vent: the external opening of the alimentary canal, the anus
  • Ventral: pertaining to the abdominal or lower surface
  • Ventral fins: paired fins behind or below the pectoral fins
  • Vertical fins: fins on the median line of the body; the dorsal, anal, and caudal fins
  • Vestigial: reduced, or very poorly developed
  • Villiform teeth: small, slender teeth forming velvety bands
  • Vomer: a bone forming the front part of the roof of the mouth
  • Vomerine teeth: teeth on the vomer (see above)

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