Glossary A-K
Abbreviations and Terms
A: number of elements in the anal fin, often comprising spines (Roman numerals) and soft rays (Arabic numerals).
AC: ventral series of photophores posterior to anal-fin origin.
ACA: First section in ventral series of photophores posterior to anal-fin origin.
ACB: Second section in ventral series of photophores posterior to anal-fin origin.
ACC: Third section in ventral series of photophores posterior to anal-fin origin.
Ant: light organ anterodorsal to eye.
AO: ventral series of photophores comprising AOa + AOp.
AOa: ventral series of photophores above anal fin.
AOp: ventral series of photophores just posterior to anal fin.
AOP: preorbital light organ anteroventral to eye.
AR: anal ring.
abyssal: at or near the deepsea floor, usually in excess of 2000 m.
accessory lateral line: branch of the lateral line following the dorsal outline of the body just below the dorsal-fin base.
actiniarians: sea anemones; short, broad tubular invertebrates usually attached to rocky substrates at the base, with the free end encircled by tentacles; members of the cnidarian class Anthozoa and order Actiniaria.
adherent: not easily dislodged; firmly attached; non-deciduous.
adipose: fatty or fleshy tissue.
adipose eyelid: a thick, mostly transparent tissue that partly or wholly covers the eye (Fig. 00, p. 000).
adipose fin: a small fleshy fin, without fin rays, usually situated behind the dorsal or anal fins (Fig. 00, p. 000).
adnate: joined together.
adpressed: pressed against the body.
adult: sexually mature.
aestivate: spend periods of drought in a torpid state, usually in summer, or during long periods of dryness.
aggregate: group together.
alar: wing-like shape, often referring to the region of the pectoral fins or the central portions of the fused pectorals and head in rays.
ammocoete: filter-feeding freshwater larval stage of lampreys with reduced eyes.
amphibians: primitive vertebrates, including frogs and salamanders, living in water and on land.
amphipod: a sand hopper; a small crustacean of the order Amphipoda usually with a compressed shrimp-like body.
amphitemperate: occurring in temperate environments in both the Northern and Southern Hemispheres.
anadromous: migrating from the sea to fresh water to spawn.
anal fin: the unpaired fin situated on the lower surface of the body behind the anus.
anal ring: in the Syngnathidae, last trunk ring bearing the anus.
anal spine: a spine at the origin of the anal fin before the soft rays.
anterior(ly) (adj., anteriormost, comb. antero): relating to the front or head end of the body
anterolateral: to the front and side.
anteroposterior: from the front end to the rear end.
anti-equatorial: away from the equator in both the Northern and Southern Hemispheres.
Antipodean: relating to the region of Australia and New Zealand.
antitropical: those parts of the tropics away from the equator in the Northern and Southern Hemispheres.
anus (adj. anal): external opening of the digestive system for expelling body wastes
apex (pl. apices): the tip or end.
armoured: having an armour-like, rigid and often jointed protective external covering.
articulate (ing): united by a movable joint.
ascidian: a sea squirt; a soft, leathery-bodied invertebrate of the tunicate class Ascidiacea, often resembling simple barrel-shaped sponges.
aspect: view.
assemblage: grouping, often in a relationship sense (eg a group of organisms sharing a common ancestor).
asymmetrical: not symmetrical.
atriopore: the rear opening to the atrium or cavity, through which water is circulated in lancelets
attenuate: slender.
auditory: pertaining to hearing.
austral: southern.
axial muscles: muscles in the axis of the body (running from the front end to the rear end).
axil, axilla: the angle formed by the inner edge of a fin and the body at the fin's point of attachment to the body.
axillary process: a small scale-like or fleshy projection at the axil of the pectoral or ventral fins
axillary tissue: epidermal, and sometimes dermal, scale-like feature on the posterior side of the pectoral- or ventral-fin base.
axis: imaginary line around which a body or structure is symmetrically arranged.
B
BI: section of the lateral line canal from behind the head to the base of the tail.
BL: body length.
Br: 3 photophores on branchiostegal membranes.
BR: series of photophores on branchiostegal membranes.
ballast water: water taken in by a vessel to provide stability when operating in the open ocean, usually held within tanks constructed low in the hull for that purpose.
band (adj. banded): vertical strip of colour.
bar: band.
barb (adj. barbed): secondary backward-projecting point, as on a fish hook; also a serrated, spear-like, structure, as on the tail of some rays
barbel: a fleshy, tentacle-like structure on the head (often near the mouth) of some fishes
basal(ly): at or near the base.
basibranchial: one of several median cartilaginous or bony structures in the branchial arch behind the tongue.
basketwork: arranged in a cross-hatch pattern, resembling the pattern of a wicker basket.
bathypelagic: free swimming in ocean depths of 1000 to 4000m, but not close to the bottom.
batteries: cellular structure capable of supplying electrical discharges.
beak: horny projecting jaws, as in birds.
benthic: living on the sea floor.
benthopelagic: free swimming near the sea floor.
bicuspid: referring to a tooth with two cusps or points.
bifid, bifurcate: divided into two parts.
bilateral: on both sides.
bill: slender, sword-like anterior extension of either the upper or lower jaw.
bilobed, bilobate: having two lobes.
binomial: having two names, or parts to a name.
biogeographical: relating to distributions determined by historical events and biological tolerances of organisms.
bioluminescence (adj. bioluminescent): light produced chemically by living organisms, including fishes, where chemical energy is transformed into light energy.
biomass: total quantity or weight of organisms in a given area.
biotone: area of overlap between adjacent natural ranges of geographical distribution or ‘provinces' around the continent
biserial: occurring in two regular series.
blind side: side of body in flatfishes without eyes.
body depth: maximum vertical distance between the dorsal and ventral profiles of the body unless a specific location is provided (eg. ‘at dorsal-fin origin').
body length: overall length of a fish expressed as ‘fork length', ‘standard length', ‘total length', or as in the billfishes, ‘body length' (direct measurement from the anterior tip of lower jaw to hind margin of caudal fin at its centre
branched ray: a soft fin-ray that is divided into two or more parts at its free (unattached) end.
branchial: pertaining to the gills.
branchial length: distance from the front of the first to the front of the last gill aperture.
branchial slime pores: slime pores in immediate association with the gill aperture or apertures.
branchiostegal ray: each of the bony rays supporting the gill (branchiostegal) membranes behind the lower jaw.
breast: area of the body below the pectoral fins.
brood pouch: pouch in the body where eggs are deposited and held until hatching.
bryozoan: primitive colonial invertebrate with a calcareous skeleton, often resembling an encrustation, or a three-dimensional structure resembling coral.
buccal: inside of the mouth.
buckler: bony plate or plates, sometimes bearing spines, on the bodies of some fishes.
bulbous: swollen or rounded in shape.
bycatch: non-commercial component of fisheries' catch.
C
C: number of principal caudal-fin rays in the caudal fin, frequently equalling the number of branched rays + 2.
Ce: photophore above posterior edge of preopercle.
CITES: the Convention on International Trade in Endangered Species of Wild Fauna and Flora.
cm: centimetre(s).
CO: circumorbital pores.
caecum (pl. caecae): see ‘pyloric caecum'.
canine tooth: a long conical tooth in the mouth, adapted for grasping prey.
caniniform: having the form of canine teeth.
capsule: surrounding envelope, often protective.
carapace: rigid covering encasing the body.
cartilage (adj. cartilaginous): soft and flexible gristle-like material that helps to provide structural support for the body.
caruncle: fleshy outgrowth.
catchment: series of more or less interconnected creeks, streams and rivers draining a geographical area.
caudal: referring to the tail region.
caudal filament: slender prolongation of caudal-fin ray or membrane.
caudal fin: tail fin.
caudal peduncle: part of the body between the end of the anal fin base and the base of the caudal fin.
caudal peduncle depth: least vertical distance between the upper and lower profiles of the caudal peduncle.
caudal peduncle length: oblique distance between the base of the last anal-fin ray and the hind end of the hypural plates.
caudal vertebra(e): those vertebrae in the vertebral column, posterior to the abdominal cavity, which possess a haemal spine (at least on the anteriormost).
Caulerpa: green macroalgae, often with fleshy ‘leafs' in a variety of forms.
cephalic lateralis canal: portion of the laterosensory system on the head, partially surrounded in some areas by superficial elements of the skull.
cephalopod: cuttlefish, octopus and squid; representatives of the specialised mollusc class Cephalopoda.
cheek: area between the eye and the free edge of the preopercular bone.
chevron-shaped: having the shape of a bent bar or inverted V.
chitin (adj. chitinous): horny exoskeleton, like that of insects and crustaceans.
ciliate: having many fine hair-like structures.
circumglobal: around the world.
circumnarial groove: groove around the nostrils.
circumorbital: pertaining to the region immediately encircling each eye, especially the series of bones forming the bony orbital rim and enclosing a portion of the laterosensory canal.
circumtropical: around the world at tropical latitudes.
cirrus (pl. cirri): small, thin, tentacle-like or flap-like appendage that may be subdivided into branches.
clade: natural grouping containing all descendants of a common ancestor.
clasper: specialised structures of male cartilaginous fishes that facilitate breeding, most frequently referring to the tubular modified part of each ventral fin,used to transmit sperm during copulation.
clavus: rudder-like structure replacing the caudal fin at the posterior end of the body in sunfishes (Molidae).
cleithral spine: spine at the dorsal end of the cleithrum, usually prominent when present and directed posteriorly.
cleithrum: major bone providing the basal support for the pectoral girdle and fin.
cloaca: common opening to the exterior for the digestive, reproductive and urinary tracts in some fishes.
cockscomb: crest on the head of a male chicken.
colonial: aggregate, connected or living together in close proximity.
commensal: referring to an organism that lives with or on another but neither at the expense of the other; living in a mutually beneficial relationship.
complete: in entirety, uninterrupted.
compressed: flattened from side to side.
confluent: joined together, continuous.
congener: belonging to the same genus.
conical: cone-shaped.
contiguous: adjoining; next in order.
continental shelf: gently sloping sea bed extending from the shore to a depth of about 200m.
continental slope: rather steeply sloping sea bed extending from the outer margin of the continental shelf to a depth of about 2000m.
continuous: unbroken; often a fin or lateral line that is not divided into two or more parts.
convergent: evolution of similar characters in unrelated taxonomic groups, eg fin-like structures in fishes and whales.
co-occur: occur together.
copepod: small, often tiny crustaceans of the subclass Copepoda having a wide variety of body shapes.
copulatory: associated with the physical act of reproduction of male and female.
coronal pore: cephalic sensory canal pore on the dorsal midline of the head at the centre of the coronal commissure joining the canals dorsoposteriorly on either side above the eyes.
coronal spine: spine(s) above and behind the eyes at the anterior edge of the occipital pit, originating from the posterior ends of the interorbital ridges.
corselet: band of specialised scales encircling the pectoral region of the body.
cosmopolitan: occurring everywhere.
couplet: pair of contrasting statements presented in identification keys that assists the reader in reaching an identification.
countershading: colour pattern of fishes in the water column that are dark-coloured on top but light-coloured on the bottom, effectively to obscure the outline of the fish to predators by blending with the dark sea floor when viewed from above and the light sky when viewed from below.
cranial, cranium: portion of the skull enclosing the brain; neurocranium.
craniate: group including all animals with a cranium (a cartilaginous or bony skull) enclosing a well-developed brain.
crenulate: with the margin in the shape of small, rounded notches.
crescent-shaped: shaped like the moon in the first or last quarter.
crest: elevated ridge, especially on a bone surface.
cross band: band of colour, usually fully traversing the structure.
crossbar: cross band.
cross-section: transverse section.
crown: top part, especially the head; in the form of a monarch's crown.
crumenal organ: small pouch-like structure behind the fourth gill arch in which gill rakers from both sides interdigitate for breaking up food particles.
crustacean(s): each of a group of invertebrate animals that have external skeletons with jointed appendages; includes crabs, prawns, crayfish, etc.
cryptic: camouflaged; well hidden.
cteni: minute spines.
ctenoid (scale): scale in which the hind margin and often the exposed lateral surface bears minute spines, giving the body a rough feeling.
ctenophores: comb jellies; free-swimming invertebrates of the small phylum Ctenophora, usually with a clear gelatinous body somewhat resembling jelly fish (medusoid coelenterates).
cunjevoi: sea squirt, attached invertebrate (ascidian) resembling a leathery sponge.
cusp: small point or projection on a tooth.
cusplet: small cusp, often at the base of a larger central cusp.
cutaneous folds: skin folds or ridges.
cycloid (scale): scale in which the hind margin and exposed lateral surface is smooth.
D
D: number of elements in the dorsal fin, often comprising spines (Roman numerals) and soft rays (Arabic numerals).
Dn: light organ above nostril.
DNA: deoxyribonucleic acid, the blueprint of life in body cells.
DW: disc width.
deciduous: easily shed or rubbed off.
deepsea: away from inshore and surface waters, usually at depths with little or no light from the sky.
degenerate: said of a character or structure that has evolved from an advanced state to a less developed state.
demersal: living on or in association with the bottom of the sea.
dendritic organ: brush-like external organ adjacent to the anus in eeltail catfishes (Plotosidae), which is thought to have a salt-regulating function.
dentary: largest and most anterior of the bones on each side of the lower jaw, on which teeth are based.
denticle: tooth-like scale found on the bodies of sharks and rays.
denticulation, denticulate (adj. denticular): small tooth-like sculpturing.
dentition: teeth in the mouth.
depressed: flattened from top to bottom.
depressible: not fixed in an erect manner, capable of being rotated or bent down.
dermal: pertaining to the skin.
dermal flap: small skin flap.
dermal tentacle: fleshy outgrowth of the skin.
detritus (adj. detrital): debris; broken, often abraded material, most frequently organic.
diagnosis: brief summary of characteristics that define a natural group (eg species, genus, family).
diagonal scale rows: oblique scale rows; regular arrangement of scales in slanted rows along the side of a fish's body usually counted above the lateral line, if present, from the edge of the operculum to the hypural edge.
dichotomous key: identification aid organised as a sequential series of contrasting statements (couplets) pertaining to morphological structures and appearances present in the organism in question.
dichromatic: having two different colour forms.
dimorphic, dimorphism: condition where two different body shapes and/or colourations are present (often between the sexes).
disc: fused head, trunk and paired fins of some fishes, particularly sharks, skates and rays.
disjunct: not continuous, interrupted.
dispersal: moving outwards, frequently away from the place of origin.
distal: remote from the point of attachment.
distensible: capable of projecting or swelling outwards.
diurnally: of the daylight hours.
divergent, diverging: proceeding in different directions.
diverticula: (in relation to pyloric caeca) an outpocketing or blind-ending tube from a cavity or blind sac.
dorsad: in a dorsal direction.
dorsal(ly), (comb. dorso-) adj.: relating to the upper surface or back.
dorsal fin: unpaired fin extending along the midline of the back.
dorsal-fin chambers: series of lymph compartments within the finfold surrounding the notochord on the dorsal side of the body.
dorsolateral: between the back and the middle of the side, the upper area of the side.
dorsoventral(ly): stretching from the dorsal to the ventral surfaces.
dorsum: upper (dorsal) surface of the head or body.
drop-off: place of rapid increase in depth, like an underwater cliff face.
drumming: making low pitched beating sound.
duct: channel or tube for conveying a substance.
E
Ecklonia: macrophytic brown seaweed with fluted leathery fronds.
ectopterygoid: bone forming a forward part of the suspensorium, the ventral edge usually positioned laterally on the roof of the mouth and sometimes bearing teeth.
eddies: whirlpools, especially at the periphery of a major current.
eelgrass: Zostera, a marine green plant with long slender leaves.
efferent branchial duct: tube-like connection from the gill chamber to the gill opening to the exterior.
elasmobranch: pertaining to sharks and rays.
electric organ: structure capable of emitting electrical discharges through the surrounding water and which may be used to stun prey, to repel predators or as a radar-like device to detect objects under conditions of poor visibility.
electrosensory: capable of detecting electrical discharges.
elliptical: relating to an oval shape.
elongated: extended or drawn out.
elver: fully pigmented immature eel, resembling a small version of an adult.
emarginate: having a slightly concave, indented or forked margin or edge.
embayment: body of water enclosed into a bay.
embedded (scales, spine): completely covered with skin.
embryo (adj. embryonic): offspring prior to birth.
encapsulate: enclose in a capsule.
encyst: enclose in a bladder-like sac.
endemic: occurring naturally only in a specific area.
endoskeleton: internal skeleton.
ephemeral: lasting for a markedly brief time.
epidermis (adj. epidermal): outer surface layer of skin.
epipelagic: free swimming in the upper part of the open ocean from the surface to a depth of about 200m.
erectile: capable of being raised, as in the spines of some fishes.
esca (adj. escal): lure or 'bait' on the end of the rod-like dorsal head appendage (illicium) of some anglerfishes and their relatives.
estuary (adj. estuarine): portion of a river where it meets the sea and fresh and salt waters mingle or alternate.
euryhaline: the ability to live in a wide range of water salinity.
euphausid: krill; shrimp-like crustacean that feeds on phytoplankton and often occurs in huge numbers.
evert(ed): turned outward or inside out.
exoskeleton: external skeleton.
extralimital: occurring outside one's normal distribution.
extremity: very end.
eye diameter: maximum diameter of the eye, frequently measured in a horizontal direction.
eyed side: the side of the body in adult flatfishes on which both eyes are located.
eyeshine: seemingly iridescent reflection of light in nocturnally active animals that have a special reflective layer associated with the retina.
F
FL: fork length.
falcate: curved or sickle-shaped.
fang: very elongated, slender canine.
fertilisation: fusion of the male and female reproductive cells.
filament (adj. filamentous): thread-like.
filter-feeder: fish that obtains small particles of food (plankton) by filtering them out of the water, usually with numerous elongated and fine gill rakers.
fimbriate(d): with the margin fringed, bordered with hairs.
fin: structure on the fish's body used for propelling or steering.
fin ray: bilaterally paired supporting element of a fin that is often flexible, segmented and branched.
finfold: ridge on the body that is a precursor of, a remnant of, or a continuation of a fin.
fingerling: young or small fish.
finlet: small fin supported by a ray or rays, situated behind the dorsal and anal fins.
fisher: one who catches fish.
fixed: immovable.
flake: marketing name for cartilaginous fish, primarily sharks.
flange: projecting flat rim or edge, for strengthening or attachment.
flank: fleshy part of the side of the body between the ribs and the caudal peduncle.
flats: bottom with broad level surface at shallow depth.
flattened teeth: compressed teeth; incisors.
flexed: bent.
flexure: curved, bent state.
flotsam: floating wreckage.
fluorescent: producing visible or invisible radiation.
fork length: length of a fish measured from the tip of the snout to the fork of the caudal fin.
forked: bifurcated, with diverging corners, the margin between concave to V-shaped.
fossa: shallow depression or cavity.
frill: ornamental edging.
fringe: ornamental boarder of thread-like skin.
frontal bones: pair of bones forming the roof of the brain case anteriorly.
furrow: groove, long indentation.
fused: joined together as one.
fusiform: spindle-shaped; tapering towards each end.
G
GR: number of gill rakers on the anterior face of the 1st gill arch, often presented as rakers on upper limb + rakers on lower limb, including raker at angle.
gape: widely open mouth.
genital papilla(e): small tube-like structure, situated behind the anus, from which sperm or eggs are released.
gestation: period of embryonic development between conception and birth.
gill: respiratory organ in fishes.
gill aperture: gill opening.
gill arch: bony or cartilaginous arch supporting the gills and gill rakers.
gill cover: bony flap covering the gill chamber; the operculum.
gill membrane: skin on each side of the head, supported by branchiostegal rays, which is periodically drawn across the lower portion of the gill opening to enclose the gill chamber during respiration.
gillnet: fishing net with mesh large enough that fish can push their head through the opening, but not their body, allowing them to be entangled by their gill covers.
gill opening: opening behind the head from the gill chamber to the exterior.
gill raker: one of a number of bony finger-like structures on the anterior edges of the gill arches, for the straining of food or protection of gills; may range from tooth-like bumps to thread-like stiff filaments; rakers usually arranged in parallel inner (mesial) and outer (lateral) rows, the numbers in the lateral row of the anteriormost gill arch reported in descriptions unless otherwise stated.
gill raker rudiment: very short gill raker whose length is shorter than its width.
gill slit: gill opening; usually paired and positioned laterally or ventrally on the head.
girdle: see ‘pectoral girdle' and ‘pelvic girdle'.
glass eel: juvenile eel with the shape of the adult, but not the pigmentation, remaining mostly transparent.
gnathostome: referring to the group of fishes having jaws.
gonad: reproductive organ (ovary or testes).
granular (granulation): having the structure or surface of fine hard particles, eg. sand paper.
gravid: sexually ripe.
gular: pertaining to the region between the chin and isthmus.
gular plate: bony plate below the chin and between the sides of the lower jaw.
gut: lower alimentary tract, intestine.
H
HL: head length.
haemal spine: ventrally directed spine on each caudal vertebra bridging the caudal artery and vein at its base.
handline: simple hand-operated fishing line used without a rod and reel.
haremic: pertaining to a social group containing a dominant male and a number of subordinate females.
hastate: having the form of a triangular spearhead.
head depth: vertical distance between the upper and lower profiles of the head, often at a specified location, eg posterior margin of the eye.
head length: horizontal distance between the most anterior point on the snout or upper lip and the most distant point on the opercular membrane.
herbivorous: feeding on plant material.
hermaphrodite (adj. hermaphroditic): having both male and female organs in the same body, although not necessarily developed functionally at the same time.
heterocercal: referring to a caudal fin in which the upper lobe is noticeably larger than the lower lobe.
high-latitude: latitude well away from the equator.
Holarctic: the entire arctic region (in oceanography).
holdfast: attaching base of seaweed.
holotype: the single specimen designated or indicated as ‘the type-specimen' of a nominal species-group taxon (species or subspecies) by the author at the time of the original publication (or the single specimen when no type was specified but only 1 specimen was present for the description).
humeral: pertaining to the shoulder.
humeral spine: cleithral spine.
hyaline: transparent, clear.
hybridise: produce offspring from different species.
hyoid: non-functional remnant of an anterior gill arch just posterior to the mandibular arch that has given rise to the lower jaw; provides a base for the chin barbel in stomiid fishes.
hyomandibular: major bone forming the rear part of the suspensorium.
hypobranchial: rather short bone providing the internal skeleton for the basal end of the lower limb of anterior gill arches.
hypural: the expanded and sometimes fused lower spine-like elements of the last few vertebrae that support the caudal fin.
I
IC: ventral series of photophores, comprising IP+PV+VAV+AC.
IP: ventral series of photophores anterior to pectoral-fin base.
IUCN: The World Conservation Union (International Union for the Conservation of Nature and Natural Resources).
IV: ventral series anterior to ventral-fin base.
ITAR: inferior tail ridge.
ITR: inferior trunk ridge.
IV: ventral series of photophores anterior to ventral-fin base, comprising IP+PV.
iliac pelvic spines: spine-like processes directed ventrolaterally from the lower end of the pelvic girdle.
illicium (adj. illicial): slender and sometimes jointed appendage (‘fishing rod') on the head of anglerfishes and their relatives.
imbricate(d), imbricating: overlapping, as in roof shingles.
incised: having a notched margin (particularly with regard to a membrane).
incisor (adj. incisiform): tooth with a flattened chisel-like tip, modified for cutting.
incubate: to maintain eggs at optimal environmental conditions for growth and development.
Indo-Pacific: of the Indian and Pacific oceans.
inferior mouth: ventrally positioned mouth, below or underneath the snout.
inferior tail ridge: in the Syngnathidae, the longitudinal ridge on the ventral margin of the tail rings, abbreviated ‘ITAR'.
inferior trunk ridge: in the Syngnathidae, the longitudinal ridge on the ventral margin of the trunk rings, abbreviated ‘ITR'.
infracaudal: on the underside of the caudal peduncle.
infraclass: taxonomic grouping infrequently used between class and order.
infraorbital: pertaining to the suborbital bones.
insertion (of a fin): base of the fin, where it joins the body, usually pertaining to the origin or forwardmost extent.
insular: pertaining to an island.
integument: outer covering of the body.
inter (prefix): between.
interbreed: breeding between species or populations.
interdorsal distance: distance between the dorsal fins.
interdorsal ridge: fleshy ridge on dorsal midline of body between dorsal-fin bases, characteristic of certain sharks.
intermaxillary: between the maxillae on the 2 sides.
internarial space: area between the nostrils.
internasal distance: distance between the nostrils.
internasal flap: fleshy flap extending between the nostrils and partly covering the mouth of some rays and sharks.
internasal valve: fleshy flap of skin between the nostrils of rays.
interopercle: often narrow bone forming the edge of the gill cover posterior to the subopercle and the lower part of the opercle.
interorbit (adj. interorbital): area between the eyes, on top of the head.
interorbital width: shortest distance on top of the head between the eyes measured from the bony orbital rims.
interpelvic: space between the ventral-fin bases.
interspace: intervening space.
intersphenotic: distance between the anterolateral angles of the sphenotic bones that form the dorsolateral surfaces of the cranium immediately behind the orbits.
interspinous: between the spines, as 'interspinous membrane' - the fin membrane supported by spines anteriorly and posteriorly.
intertidal: region between the edges of the high and low extremes of the tide.
intromittent oragan: specialised structure of a male used to deposit sperm into the female.
invertebrate: multi-cellular animals that have not evolved a notochord.
iridescent: showing colours like those of the rainbow.
iris: flat, usually circular coloured membrane behind the cornea of the eye with a circular opening in the centre.
isocercal: pertaining to a perfectly symmetrical tail fin without upper and lower lobes.
isthmus: fleshy area on the underside of the head that separates the two gill chambers.
joint: structure in the body by which two bones are joined together.
jugular: pertaining to the throat.
K
keel: fleshy or bony ridge, usually on the sides, belly, or caudal peduncle.
knot: speed equivalent to 1 nautical mile per hour (approximately 1.85km per hour).
krill: eupahusid, shrimp-like crustacean that feeds on phytoplankton and often occurs in huge numbers.