Larval Fish ID - Larval Fish & Egg Taxonomic Keys

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  1. Morphology of a teleost larva Diagrammatic representation of morphology of a teleost larva.

    Diagrammatic representation of morphology of a teleost larva.

    Auer, N. A. (ed.). 1982, Identification of Larval Fishes of the Great Lakes Basin with Emphasis on the Lake Michigan Drainage.
    Great Lakes Fishery Commission, Ann Arbor, MI 48105. Special Pub. 82-3, p. 8.

  2. Measurement locations for notochord and head depth/width Illustration of the measurement locations for notochord length and head depth and width.

    Illustration of the measurement locations for notochord length and head depth and width.
    Tenera Environmental. Length-Specific Probabilities of Screen Entrainment of Larval Fishes
    Based on Head Capsule Measurements, San Luis Obisbo, 2013 [using diagram from Moser, CALCOFI, "jacksmelt"]

  3. YSL & Larva diagnostic features Diagrammatic representation of typical yolk-sac larva and post yolk-sac larva

    Diagrammatic representation of typical yolk-sac larva and post yolk-sac larva (redrawn from original drawing by Alice J. Mansueti in Mansueti and Hardy 1967).
    Wallus, R., T. P. Simon, and B. L. Yeager. 1990. Reproductive biology and early life history of fishes in the Ohio River drainage. Volume 1: Acipenseridae through Esocidae. Tennessee Valley Authority, Chattanooga, Tennessee, USA. p.8.

  4. Embryo features Embryo features
  5. Rock Bass larva with features Rock Bass larva with features

    Photo courtesy of USGS Great Lakes Science Center, 2011

  6. Walleye YSL with features Walleye YSL with features

    Photo courtesy of Stacey Ireland, USGS Great Lakes Science Center, 2012



abbreviation for anal fin
gut region
Abbreviate heterocercal
tail in which the vertebral axis is prominently flexed upward, only partly invading upper lobe of caudal fin; fin fairly symmetrical externally
horny fin supports (threadlike fibers) which are the precursors of fin rays or spines (lepidotrichia)
attached or joined together, at least at one point
Adhesive (egg)
referring to eggs, those which stick to each other or to a substrate after water hardening, unless otherwise noted; adhesiveness of entire egg capsule may or may not persist after attachment
Adhesive organ
a suctorial organ, located anteriorly on snout, which produces a viscid secretion allowing larva to attach to objects
Adipose fin
fleshy, rayless fin posterior to (behind) the dorsal fin (between dorsal and caudal fins)
Adnasal pore
pore at the end of the short, ascending branch of the infraorbital canal, which lies just behind the anterior nostril; not always present (eels)
joined to; grown together
sexually mature as indicated by production of gametes
Air bladder
see: swim bladder
a term applied to juvenile catfish, trout, and salmon after yolk absorption; exhibiting no post yolk-sac larval phase
wing-like; usually referring to the shape of the pectoral fin
having separate and mutually exclusive areas of geographic distribution
referring to fishes that ascend rivers to spawn
pertaining to the anus or vent
Anal fin
fin (usually single but double in some gadiforms) on the ventral margin of the tail; unpaired median fin immediately behind anus or vent
Anal fin origin
anterior-most point at which the anal fin attaches to the body
Angle of jaw
bony prominence posterior to gape of jaw; region of the junction of the angular, articular, and quadrate bones
the bone at the angle of the jaw or that region; used in reference to pigmentation at, or near, the angular bone
rudimentary form of an anatomical structure or organ; primordium
towards the front; cephalad
anterior bone to which branchiostegal rays attach: formerly ceratohyal
angled forward; usually referring to direction of a spine
orifice and surrounding tissue at the terminus of gut; external orifice of the intestine; vent
main blood vessel lying below the spinal chord that supplies blood from the heart
a bone of the lower jaw between the dentary and angular bones (some consider it part of the angular); used in reference to pigment anterior to the angle of the upper jaw
Auditory vesicle
sensory anlage from which the ear develops; clearly visible during early development
Axillary process
enlarged accessory scale attached to the upper or anterior base of pectoral or pelvic fins



body depth at anus
body length
a strip of pigment that contrasts with the adjacent background pigment or unpigmented area; it may be on any part of the head, body, or fins and may be oriented in any position; a vertical band occurring laterally on the head or body is referred to as a bar and a horizontal band is referred to as a stripe
a vertical band of pigment on the lateral surface of the head or body; often occurring in a series
slender sensory projection on the lips or chin; barbel length measured from the point of attachment on the head to the tip of the barbel
three median bones on the floor of the gill chamber, joined to the ventral ends of the five gill arches
a large complex of rivers or lakes with a single outlet to the ocean (e.g., Great Lakes)
bone or process that supports the pelvic fin
zone below 1,000m depth in the open ocean
living on or in the bottom (substrate)
having or ending in two points; a tooth with two points
cavity of the blastula; segmentation cavity
specifically, early embryonic tissue composed of blastomeres; more generally, embryonic tissue prior to formation of embryonic axis
early embryo of teleosts consisting of a disc-or cap-like mass of cells on the yolk; embryo-forming area of egg prior to cleavage
individual cells forming the early embryo of teleosts (during cleavage)
opening formed by and bordered by the germ ring as it extends over the yolk
stage in embryonic development which represents the final product of cleavage stages, characterized by formation of the blastocoel
Body depth
the vertical distance from the dorsal margin of the body to the ventral margin of the body measured at the base of the pectoral fin where it attaches to the body; fins or fin bases are not included in the measurement
Body depth at anus
vertical distance of body at anus
Body length
general term used to indicate the size of a larva; equivalent to notochord length (distance from the tip of the snout to the tip of the notochord) in preflexion and flexion stage larvae and equivalent to standard length (distance from the tip of the snout to the posterior margin of the hypural plate) in postflexion larvae. In beloniform fishes, body length measured from the end of the opercle to the caudal base because of allometry and because the beak is often broken
Branched ray
soft ray with two or more branches distally
Branchial arches
bony or cartilaginous structures, supporting the gills, filaments and rakers
Branchial pores
pores in branchial region of lateral-line canal (eels)
Branchial region
in petromyzontids, area between the anterior margin of the first gill opening and the posterior margin of the last
Branchiostegal membrane
membrane connecting the elongated bones which ventrally support the gill membranes
Branchiostegal rays (branchiostegals)
bony rays supporting the membranes which close the gill (branchial) cavity under the head; ray-like bony elements attached to the hyoid arch, extending under the gill openings and connected by a membrane
the undifferentiated bump or protuberance that appears at the initial formation of the paired fins
Buoyant egg
an egg which floats free within the water column; pelagic



abbreviation for caudal fin
caudal peduncle depth
finger-like outpouchings at boundary of stomach and intestine
composed of, containing, or characteristic of calcium carbonate
brush-like; referring to teeth of uniform length in patches or bands
fleshy outgrowths; modified dorsal fin rays in ceratiids
fishes which go to sea from rivers to spawn
Caudal fin
median fin at the posterior end of the fish; tail fin
Caudal peduncle
narrow part of the tail between the posterior end of the anal or dorsal fin and the base of the caudal fin; caudal peduncle length measured from the insertion of the posterior-most ray of the anal or dorsal fin (whichever is most posteriad) to the insertion of the anterior-most caudal fin-ray
Caudal vertebrae
the posterior group of vertebrae extending from the centrum bearing the first haemal spine to the urostyle
Cement glands
discrete or diffuse structures which permit a larva to adhere to a substrate
the body of a vertebra
belonging to the head
see: antero-hyal
lateral surface of head between eye and opercle, usually excluding preopercle
the earliest developmental form of myomeres in larvae; describing the angle formed by the epaxial and hypaxial portions of the myosepta
after water hardening, the outermost membrane of a fish egg
Choroid fissure
a cleft in outer layers of the eye visible in early larvae; indentation at the ventral margin of the eye marking the invaginated borders of the optic cup in larval fish; apparent in young fish as a trough-like area below lens
Choroid tissue
mass of primordial vascular tissue of various shapes lying below the eye; usually associated with narrow eyes; often pigmented; its length measured along its longitudinal axis from the interface with pigmented portion of the eye to the tip of the choroid mass
pigment-bearing cell; frequently capable of expansions and contractions which change their size, shape, and color
generally small, dermal, flap-like or tentacle-like process on the head or body
Cleavage stages
initial stages in embryonic development where divisions of blastomeres are clearly marked; usually include 1st through 6th cleavages (2-64 cells)
Cleithral symphysis or junction
where the ventral ends of the cleithral bones meet
large bone of support for the pectoral fins; elongate vertical bone in pectoral girdle at the junction of the head and body of the fish; one of the first bones to form in the body; demarcates the junction of the head and body; clearly visible in many fish larvae
belonging to the body cavity
condition of the gut where it is twisted or convoluted
laterally flattened
coming together to form one
top of the head
Ctenoid scale
scales having small, needle-like projections on the posterior margin; bearing cteni
Cycloid scale
scales with evenly curved free border, without cteni



D or DF
dorsal fin
dorsal fin origin
Dash (pigment)
an elongate or streak-like melanophore
referring to scales that are easily rubbed off and thus not firmly attached
Demersal (egg)
referring to an egg which rests upon the substrate as a result of deposition or settling; at the ocean bottom; used in reference to life stages from eggs to adults; may be free or attached
highly branched; usually used in reference to a melanophore
major bony element of the lower jaw, usually bearing teeth
greatest depth of body, excluding median fins (taken just behind gill opening, at or behind anus, or elsewhere at greatest depth)
flat cup-like structure formed from modified pelvic fin rays (also pectoral fin rays in some fishes) used for holding onto the substrate or structures on the substrate (e.g., gobiesocids, cyclopterids, gobioids); its diameter (P2DL) is measured longitudinally from one margin to the opposite margin; in echeneids, the disk is on the top of the head and formed from modified dorsal fin spines; also refers to the disk-shaped head of ogcocephalids where disk width (DW) is the greatest transverse dimension of the disk
remote from the point of attachment; opposite to proximal
occurrence of species in the region (e.g., Great Lakes region and within the Lake Michigan drainage)
upper part of body; opposite of ventral
Dorsal fin
fin or fins on dorsal margin of the body; median, longitudinal, vertical fins located on the back
Dorsal fin origin
point where first dorsal ray or spine attaches to body
uppermost part of body; dorsal body margin; opposite to ventrum
a group of lakes or streams within a basin (e.g., Lake Michigan drainage)



eye diameter
Early embryo
stage in embryonic development characterized by formation of embryonic axis
Early life history
the early phase in life, spanning the developmental stages from egg to juvenile
Egg capsule
outermost encapsulating structure of the egg, consisting of one or more membranes; the protective shell
Egg diameter
in nearly spherical eggs, greatest diameter; in elliptical eggs given as two measurements, the greatest diameter or major axis and the least diameter or minor axis
Egg pit
the pit or pocket in a redd (nest) into which a trout female deposits one batch of eggs
description of fertilized, water hardened eggs, unless otherwise noted; diameter, color, adhesive properties, buoyancy, yolk and oil globule characteristics and incubation period
stage of eels after glass eel stage and prior to adulthood, when they become pigmented
caudal fin possessing a slight notch or indentation, but not definitely forked
organism at an early stage of differentiation and growth; refers to the stage of development between fertilization and hatching
Embryonic axis
primitive differentiation of the embryo; an elongate thickening of blastodermal tissue
Embryonic shield
thickened area of the germ ring representing the future longitudinal axis of the embryo
the act of leaving the substrate and beginning to swim; swim-up
Engyodontic stage
early stage of anguilliform larvae (preceding euryodontic stage) characterized by few needle-like teeth, the upper and lower jaws equal in length, no nasal capsule, an undifferentiated finfold, no hypurals, the notochord tip straight, and the head and preanal region of the body relatively large
the portion of the body above the horizontal myoseptum; the part of the myomeres above the lateral midline (horizontal septum)
zone at the interface with the sea bottom; refers to organisms living in contact with the sea bottom
movement of embryonic cell mass over the surface of the yolk; the germ ring marks the boundary of the advancing sheet of cells
see: postero-hyal
zone from the surface to 200m depth in the open ocean
modified vertebral elements which lay above the vertebrae and support part of the caudal fin
red or orange pigment cells
the lure at the tip of illicium in most lophiiform fishes; commonly luminous in deepsea species
alimentary tract between pharynx and stomach
Ethmoid pore
pore in the ethmoid canal, included in the count for supraorbital pores (eels)
Euryodontic stage
advanced stage of anguilliform larvae (following engyodontic stage) characterized by a relatively smaller head and preanal region, three series of relatively short, broad teeth, a relatively shortened lower jaw, the formation of nasal capsules, fins, and hypurals, and the flexion of the notochord
Eye diameter
horizontal distance of the iris of the eye; in larvae with round eyes, the diameter of the pigmented part of the eye, usually measured through the horizontal midline; in larvae with oval or elliptical eyes, the horizontal dimension is given first, followed by the vertical dimension
Eye length
the longer axis of an oval or elliptical eye measured through the midline from the pigmented margin of one side of the eye to the other side
Eye stalks
movable peduncles of varying length bearing the eyes; eye stalk length measured from the point where the stalk attaches to the head to the point of attachment at the eye
Eye width
the shorter axis of an oval or elliptical eye measured through the midline from the pigmented margin of one side of the eye to the other side



fork length
scythe-shaped; referring to an anal fin; deeply concave as a fin with middle rays much shorter than anterior and posterior rays
average number or range of ovarian eggs per female, unless otherwise noted
Fin elements
fin spines, rays, and supporting bones (pterygiophores)
Fin insertion
posterior-most point at which the fin attaches to the body
Fin origin
anterior-most point at which the fin attaches to the body
Fin rays
segmented cartilage or bony structures which support fin membranes
median fold of skin surrounding the body within which the dorsal, anal, and caudal fins develop; the part of the ventral finfold in which the anal fin develops is referred to as the anal finfold
small fin-like structures posterior to the dorsal and anal fins
First-feeding larvae
larvae that have used all or most of their yolk and are capable of capturing prey
stage (or the process) when the urostyle bends dorsally concurrently with the development of principal rays and hypural bones in the caudal fin; also, the process of notochord flexion; preflexion refers to the stage prior to the initial bending of the notochord tip and postflexion refers to the stage after the completion of the notochord flexion
Focal point
location of a fish maintaining a stationary position on or off the substrate for at least a 10-second period
a gap or space between bones in the roof of the skull covered only by a membrane
an opening through a bone
anterior region of the developing brain that includes the olfactory lobes
anterior part of the primitive alimentary canal from which the esophagus and stomach develop
Fork length (FL)
distance from the most anterior point on the snout to the end of the shortest central caudal fin ray
a fold of skin that limits movement of the upper jaw
Frontal bones
large paired (fused in some species) bones that form the top of the cranium anteriorly; they often from a ridge over each orbit (supraocular crest) that may bear one or more spines
Frontal pore
a single median pore found in the frontal canal, present in some families (eels)



location of the gall bladder
greatest depth
Ganoid scales
diamond- or rhombic-shaped scales consisting of bone covered with enamel
the border of the mouth
Gargaropteron stage
pelagic stage of the chiasmodontid genus Kali, characterized by greatly elongate pectoral and pelvic fin rays
Gas bladder
see: swim bladder
stage in embryonic development between blastula and embryonic axis
Germ ring
thickened margin of the blastodisc that advances over the yolk during epiboly
Germinal disc
the blastodisc
Gill arch(es)
see: branchial arches
Gill opening
measure distance between upper and lower ends of gill opening
Gill rakers
variously-shaped bony projections on anterior edge of the gill arches
Glass eel
stage of eel following leptocephali stage when they enter estuaries (after pigmentation they are called elvers)
a median bone of the tongue
Granular yolk
yolk consisting of discrete units of finely to coarsely granular material
Greatest body depth
greatest vertical distance of body excluding fins and/or finfolds
silvery or white pigment cells containing iridescent crystals of guanine
ventral region of the head anterior to the isthmus and below the lower jaw
Gular fold
transverse membrane across throat
Gular plate
median ventral bony plate or plates located behind the chin and between the sides of the lower jaw, as in Amia calva; large dermal bone on throat
Gular region
alimentary tube and associated organs
Gut length
distance from anterior margin of yolk sac to vent or from anterior margin of stomach (when distinguishable) to vent
Gut loop
loop, fold, or curve found along the axis of the gut



head depth
head length
head width
Haemal spine
a median spine on the ventral surface of a vertebral centrum, attached to the centrum by two bones that form an arch (haemal arch)
Head length (HL)
distance from the most anterior point on the snout to the posterior edge of the auditory vesicle, cleithrum or opercle as each develop; horizontal distance from the tip of the snout to the posterior margin of the cleithrum; or to posterodorsal point of gill opening, depending upon family
Head width (HW)
greatest dimension between opercles; transverse distance between the lateral margins of the head measured at the posterior margin of the orbit
caudal fin upper lobe much larger or longer than lower; tail in which the vertebral axis is flexed upward and extends nearly to tip of upper lobe of caudal fin; fin typically asymmetrical externally,
posterior region of the developing brain that includes the medulla
posterior part of the alimentary canal that includes the intestine and rectum
type of cleavage in which the entire egg, including the yolk, undergoes division
type of caudal fin in teleosts where the upper and lower lobes are symmetrical; internally, the major structural elements of the fin are an upturned urostyle (several fused vertebrae) that articulates with flattened bones (hypurals and parhypural) that support the principal fin rays
uniform in composition; opposite to segmented in referring to egg yolk
Horizontal myoseptum / septum
a sheet of connective tissue (in the horizontal plane) that separates the epaxial and hypaxial muscle masses; the lateral midline area in reference to pigmentation
bone or cartilage (usually elongate) in the cheek region that functions in jaw suspension
the portion of the body below the horizontal myoseptum; the part of the myomeres below the lateral midline (horizontal septum)
below the notochord; referring to the lower lobe of the caudal fin; a transitional rod of cells which develops under the notochord in the trunk region of some embryos
the expanded haemal spines of the posterior vertebrae which support most of the caudal fin



that part of the zooplankton consisting of the egg and larval stages of fishes
tentacle-like spine of the dorsal fin located on the snout of most lophiiform fishes, used as a lure to attract prey; illicium length measured from the point of insertion of the illicium on the head to the tip of the structure
becoming apparent
Incubation period
time from fertilization of egg to hatching
spatial anatomical term meaning lower in position; opposite to superior
Inferior mouth
snout projecting beyond the lower jaw; also referred to as subterminal mouth
space between eyes over top of head
Infraorbital pores
pores in infraorbital canal, including pores in lateral canal along upper jaw below eye, and postorbital pores (sometimes listed separately) in ascending canal behind eye (eels)
Insertion (of fin)
posterior (usually) point of attachment of a fin
an enveloping layer or membrane; coating or external skin
area between the nares on one side of the head or the other
lower bone of the gill cover lying below the preopercle
region on top of the head between the orbits of the eyes
Interorbital pores
pores in the supraorbital canal which are located between the eyes (eels)
Interorbital width
least distance between the orbits across dorsum of head
area between the fin rays
spaces between parr marks in salmonids
crystals of guanine having reflective and iridescent qualities
see: guanophores
symmetrical tail fin; vertebral column ending along the median line, caudal fin rays arising symmetrically from it; tail in which vertebral axis terminates in median line of fin, as in Gadiformes
fleshy space beneath the head and between the gill openings; ventral region of the head below the gills, often narrow, connecting the gular and cleithral regions; measure interbranchial distance as ventral distance between lower ends of gill opening



referring to the throat region; beneath the head; usually used to indicate the position of the pelvic fin in some fishes
phase of development from complete fin ray development and finfold absorption to sexual maturity; stage after transformation from the larval stage that is like the adult but not yet reproductively active; there is a full complement of fin rays, scales are formed (in species that have them), and the form is fundamentally similar to the adult



with a ridge or ridges
Kupffer's vesicle
a small, vesicular, ventro-caudal pocketing which forms as blastopore narrows



location of the liver
the anterior-most of the infraorbital (circumorbital) bones; enlarged and with spines in some species
slightly broad at the base and tapering to a point
Larva / larvae
phase of development from complete absorption of yolk to development of the full complement of adult fin rays and absorption of finfold; stage following hatching that is unlike the juvenile or adult in form and pigmentation, and must transform or metamorphose before assuming juvenile/adult characteristics
Last vertical blood vessel
the posterior-most large blood vessel extending from the dorsal aorta to the kidney (nephros); used as reference point and taxonomic character in eel leptocephali
Late embryo
stage prior to hatching in which the embryo has developed external characteristics of its hatching stage
Lateral line
a line along the lateral surface of the body (backward from head along sides) formed by a series of sensory pores that usually are associated with modified scales
Lateral line pores
pores in lateral-line canal (sometimes counted to anus, sometimes total count given) (eels)
Lateral line scales
pored or notched scales associated with the lateral line
Lateral midline
the region of the body between epaxial and hypaxial myomeres; the region of the horizontal septum
Lateral series scales
number of rows of scales crossing the midlateral surface or lateral line if complete
Lateral teeth
in petromyzontids, teeth of oral disc lateral to esophageal opening
replacements of actinotrichia; soft fin rays or spines; scale-like structures that form the segments of soft rays in bony fishes
Leptocephalus (leptocephali)
transparent, ribbon-like, often large larvae of elopiform fishes with an internal cavity filled with acellular mucinous material; leptocephali usually have a small head and prominent teeth
requiring gravel as a substrate for spawning
see: viviparous
Longitudinal septum
a sheet of connective tissue that separates the muscle masses on the right and left sides; the dorsal longitudinal septum separates the epaxial muscle masses and the ventral longitudinal septum separates the hypaxial masses
Low terminal mouth
mouth that is slightly oblique to horizontal with anterior end of upper lip at or below bottom-of-eye level and either even with or the most anterior margin of snout
Lower jaw
measure from tip of lower jaw to angle of mouth
crescent (moon) shaped



lower jaw, comprised of three bones: dentary, angular and articular
Mandibular pores
pores along lower jaw, part of preoperculomandibular canal series (eels)
Maxilla / maxillae
the posterior, lateral bones of the upper jaw; the longest paired bones of the upper jaw; located above the premaxillae
the dorsalmost of the two bones in the upper jaw
Meckel's cartilage
embryonic cartilaginous axis of the lower jaw in bony fishes
referring to the midline plane that divides a bilaterally symmetrical animal into right and left halves; usually synonymous with mesial
melanin-bearing pigment cell; ameboid black or brown pigment cells of various shapes and sizes derived from the neural crest region of the embryo; black chromatophores
pertaining to the chin
Meristic characters / meristics
countable structures occurring in series (e.g., myomeres, vertebrae, fin rays)
type of cleavage in which only the blastodisc undergoes division
midbrain; serves optic functions
phase of larval development (used by Snyder et. al.) characterized by presence of at least one dorsal, anal, or caudal-fin spine or ray but either lacking the adult complement of principal soft rays in at least one median (dorsal, anal, or caudal) fin or lacking pelvic-fin buds or pelvic fins (if present in adult)
zone from 200m to 1000m depth in the open ocean
middle of three dermal bones of the upper jaw
phase of larval development (used by Snyder et. al.) characterized by presence of adult complement of principal soft rays in all median fins and pelvic-fin buds or pelvic fins (if present in adult)
a marked change in form or structure at the end of the larval stage involving acquisition of adult characters and loss of larval characters; synonymous with transformation
portion of the brain immediately behind the mesencephalon
principle path of sperm entry through the chorion (vitelline membrane) of an egg
region of the developing brain that includes the optic and cerebellar lobes
the median plane or line of the body either on the dorsal or ventral surface; sometimes used as the middle of a body part
referring to a tooth with a flat grinding surface
stage in development of egg in which blastomeres from a mulberry-like cluster
Mucosal folds
folds of tissue lining the cavity of the intestine; pronounced in some fish larvae giving a striated or rugose appearance
Myomere formula
preanal myomeres + postanal myomeres = total myomeres
muscle segments of the body occurring in series; approximately equal to number of vertebrae in adults; the preanal myomeres are defined as all myomeres anterior to a vertical from the posterior margin of the anus and postanal myomeres are those myomeres posterior to a vertical from the posterior margin of the anus; in many fishes the number of preanal and postanal myomeres approximate the number of precaudal (abdominal) and caudal vertebrae, respectively
Myoseptum / myosepta
thin partition of connective tissue which joins myomeres



notochord length (see also standard length - used as, prior to and during notochord flexion)
dorsal region of the body immediately behind the head
nostrils, openings leading to the olfactory organs; the openings of the nasal organs or rosettes
pertaining to the nares
pertaining to region of the nostrils, or to the specific bone in that region
Nasal capsule
paired, more-or-less spherical structures on the snout that contain the olfactory organs
Natural hybrids
other species in the study area with which interbreeding has occurred naturally
motile, marine organisms living in open water (rather than the sea floor) and capable of swimming against currents; small to moderate-sized nektonic organisms (e.g., midwater fishes, juvenile fishes, some cephalopods) are referred to as micronekton
kidney; often used to refer to larval kidney
pelagic coastal zone extending seaward to the margin of the continental shelf
Neural crest
region of the neural ridge of the developing embryo that differentiates into many kinds of tissue and cells, including melanophores
Neural spine
a median spine on the dorsal surface of a vertebral centrum, attached to the centrum by two bones that form an arch (neural arch) through which the spinal cord passes
inhabiting the surface of the ocean; plankton living in this zone are referred to as neuston
longitudinal cartilaginous rod that supports the axis of the body; eventually replaced by the vertebral column in teleostean fishes
Notochord length (NL)
the distance from the tip of the snout to the posterior tip of the notochord; used prior to and during notochord flexion (see also: standard length)
referring to the region of the nape; immediately behind the head, dorsally



with a blunt or rounded end; an angle greater than 90 degrees
Occipital crest
bony ridge located posteriorly on top of the head
Occipital region
area on dorsal surface of head, beginning above or immediately behind eyes and extending backwards to end of head
see: occipital region
open sea zone seaward of the continental shelf or slope
Oil globule
discrete spheres of oil or fatty material within the yolk of some fish eggs
Olfactory buds
incipient olfactory organs
Ontogenetic characters
those characters associated with developmental stages
developmental history of an organism from zygote to maturity
upper posterior and usually largest bone of the gill cover of a fish; often used as synonym of operculum
bony plate of the gill cover
Ophioblennium stage
pelagic stage of some salariin blenniids characterized by enlarged pectoral fins and enlarged, hooked teeth anteriorly in the lower jaw or in both jaws
the embryonic or larval kidney
Optic vesicles
embryonic vesicular structures which give rise to the eyes
the bony socket of the eye
the relatively advanced period of embryonic development characterized by formation of the organ systems
Origin (of fin)
anterior (usually) point of attachment of a fin
process of bone formation involving calcification of cartilage or connective tissue
region of the head containing the auditory or hearing organs
small, calcareous, secreted bodies within the inner ear
Over yearling
fish having spent at least one winter in a stream; applies to trout and salmon
producing eggs that develop outside the maternal body



P or P1
pectoral fin
P2 or V
pelvic fin
preanal length
postanal length
preanal length
predorsal fin length
predorsal finfold length
referring to the phylogenetic retention of juvenile (or larval) characters in the adult stage
Palatine teeth
teeth on the paired palatine bones in the roof of the mouth of some fishes
paired bones on the roof of the mouth, often bearing teeth
a fleshy projection or protuberance
distribution of species or other taxa that meet in a very narrow zone of overlap
along the same plane as the spinal column
lowermost supporting bone of the principal caudal fin rays
paired bones of the roof of the skull
Parr marks
dark vertical oval bars along the sides, typical of salmonids
Pectoral (fin) bud
swelling at site of future pectoral fin; anlage of pectoral fin
Pectoral fin
paired lateral (sometimes ventrolateral) fins behind the head, articulating with the pectoral girdle
Pectoral fin base
supporting structure of the pectoral fin; in larvae it is peduncular and contains the muscles that operate the fin; its length is measured along the longitudinal axis from the point of insertion on the body to the point of attachment of the fin blade or rays; its depth is measured on the transverse axis at its widest (usually most distal) point
Pectoral fin length
distance from base to farthest tip of fin; prior to ray formation, pectoral fin length is measured from the base of the blade (finfold) to the greatest distal margin of the blade; after ray formation, pectoral fin length is measured from the point of insertion of the longest ray to the tip of the ray
a small, short stalk
fleshy end of the body between the anal and caudal fins; a narrow part or stalk that connects a structure to the body (e.g., caudal peduncle connecting caudal fin to body)
living in the open water habitat, as opposed to bottom living or inshore inhabitants; free-living in the sea away from the sea bottom, usually beyond the continental shelf; not necessarily near the surface
Pelvic bud
swelling at site of future pelvic (ventral) fins; anlage of pelvic fin
Pelvic fins
paired fins usually located ventrally on the body; various in position from beneath the head (jugular), to below the pectoral region (thoracic), to the gut region (abdominal); high on the sides in some stomioids; prior to ray formation, pelvic fin length is measured from the base of the blade (finfold) to the greatest distal margin of the blade; after ray formation pelvic fin length is measured from the point of insertion of the longest ray to the tip of the ray; articulating with the pelvic girdle; ventral fins
thin membrane lying below the embryo and surrounding the yolk in teleosts; the space surrounding the yolk is invaded by nuclei, forming a syncytial region of unknown function; observed as a thin border around blastula
cavity in which the heart lies
region of the body associated with the gut or the membrane of the peritoneum; often synonymous with perivisceral
membranous lining of the abdominal cavity; the membrane and associated connective tissue lining the gut cavity
Perivitelline space
fluid-filled space between the embryo and shell or chorion and yolk material of an egg. Width of perivitelline space is the distance between yolk and egg capsule expressed either as direct measurement or a ratio of the egg diameter
Pharyngeal teeth
bony tooth-like projections derived from the fifth (
Pharyngeal) gill arch; teeth on the pharyngeal bones of the branchial skeleton
luminous organs
referring to the evolutionary lineage of an organism
having no connection between the esophagus and the pneumatic duct; typical of perciform fishes
having the swim bladder connected to the esophagus by the pneumatic duct
small, free-living, weakly swimming or passively floating marine or fresh water organisms that drift with the currents
wrinkle-like folds found on the lips of some catostomids
Post yolk-sac larva(e)
see: larva
posterior to the anus
Postanal length (PosAL)
distance from the most posterior point of the anus to the most posterior point on the caudal fin or median finfold
Postanal myomeres
number of whole myomeres posterior to an imaginary vertical line at the most posterior point of the anus, including one urostylar element
towards the back or caudal region; opposite to anterior
posterior bone to which branchiostegal rays attach, formerly epihyal
Postflexion larva
phase following upward flexion of the tip of the notochord (more precisely considered to begin with formation of all principal caudal fin rays) [note: for purposes of these keys, we do not make a distinction between pre- and post-flexion stages]
behind the eye or eye socket
Postorbital length
distance from posterior margin of eye to posterior edge of opercular membrane
Postorbital pores
pores in ascending canal behind eye, part of infraorbital canal series (eels)
Posttemporal spine
a spine that emerges from the posttemporal bone located on the posterolateral upper region of the skull
located anterior to the anus
Preanal length (PAL or PreAL)
distance from the most anterior point on the snout to the most posterior point of the anus; synonymous with snout-anus distance
Preanal myomeres
number of myomeres from the nape to, and including any myomeres bisected by, an imaginary vertical line at the most posterior point of the anus
Preanal vertebrae
from the first vertebra to and including the vertebra intersected by a line drawn perpendicularly from the base of the first anal-fin ray (eels)
Prebranchial length
in petromyzontids, distance between the tip of the snout and the anterior margin of the first gill opening
Precaudal vertebrae
the anterior group of vertebrae that includes all centra anterior to the centrum with the first heamal spine
specialized early formation of a structure (e.g., fins or fin elements) compared to typical developmental sequences in most fishes; does not infer abnormality
Predorsal length
distance from the most anterior point on the snout to the anterior margin of the base of the first dorsal fin ray when formed
Predorsal myomeres
number of myomeres from nape to dorsal origin of median finfold
Predorsal scales
scales along dorsal ridge from occiput to origin of dorsal fin
Predorsal vertebrae
from the first vertebra to and including the vertebra intersected by a line drawn perpendicularly from the base of the first dorsal-fin ray (eels)
Preflexion larva
phase between hatching and upward flexing of the tip of the notochord (or appearance of first caudal fin rays] [note: for purposes of these keys, we do not make a distinction between pre- and post-flexion stages]
developmental stage immediately following acquisition of minimum fin ray complement of adult and before assumption of adult-like body form; used only where strikingly different from juvenile
Premaxilla / premaxillae
primary bone of the upper jaw in most fish, usually bearing teeth; paired bones of the upper jaw anterior to the maxillae and usually bearing teeth; often protrusile, and extending ventrad of most of the maxillae in advanced teleosts
the ventralmost of the two bones included in the upper jaw
upper anterior bone of the gill cover, often bearing serial spines; spines at the margin of the bone are referred to as posterior preopercular spines and those on a bony ridge forward of the margin are referred to as anterior preopercular spines
Preoperculomandibular pores
pores in preoperculomandibular canal, including mandibular pores along lower jaw and those in preopercular canal (both sometimes listed separately) (eels)
large bone anterior to the eye
rudimentary form of an anatomical structure; anlage
Principle anal and dorsal fin rays
in certain fishes (e.g., Cyprinidae and Catostomidae), the principal rays include the branched rays plus one unbranched ray [the anteriorly adjacent, usually longest, unbranched ray]; the last two bases [branched rays, both of which articulate with the most posterior pterygiophore] are counted as one ray.
Principle caudal rays
caudal-fin rays originating on the hypural and parhypural elements; the number of principal rays is generally defined as the number of branched rays plus two.
Procurrent caudal rays
small dorsal and ventral rays of the caudal fin located anterior to the principal rays and not supported by hypural/parhypural elements
Pronephric ducts
ducts of pronephric kidney of early developmental stages
Protocercal tail
form with body axis extending through the caudal finfold
phase of larval development (used by Snyder et. al.) characterized by absence of dorsal, anal, and caudal fin spines and rays
describing premaxillae which can be extended
near the point of attachment or origin; opposite to distal
requiring sand as a substrate for spawning
Pterotic spines
spines emerging from the pterotic bone located posterior to the upper region of the orbit in the temporal region of the skull
dermal bone of the upper jaw
bone of the anal internal skeleton supporting the dorsal and fins; cartilaginous or bony elements that form the fin base and support the fin rays of a fish
Punctate melanophore
round or dot-like melanophore





segmented fin supports that are usually bilaterally paired and often branched
an excavated area or nest into which trout spawn
net-like or web-like in appearance
angled backward; usually referring to direction of a spine
River system
a group of streams or lakes which lead into a drainage (e.g., Fox
River system)
a prolongation of the snout, sometimes ending in a spine (rostral spine); rostrum length measured from the anterior edge of the upper jaw to the tip of the rostrum or spine (holocentrids)
Rudimentary fin rays
in certain fishes (e.g., Cyprinidae and Catostomidae), size-graded series of shorter, unbranched soft rays anterior to the principal rays of the dorsal, anal, and caudal fins; also called secondary rays or, in the case of the caudal fin, procurrent rays; In traditional texts, formulas are represented by lower case
Roman numerals
, e.g., iv instead of 4 and separated from the principal ray count in Arabic, by commas. These are not able to be displayed appropriately in an electronic key.
having a wrinkled appearance; often used to describe bone with a highly textured surface; sometimes used to describe the appearance of the gut, caused by numerous mucosal folds



standard length
snout to anus
Saddle markings
pigment patterns which cover the dorsal and lateral aspects and give an overall appearance of a saddle
referring to an egg shell with ornamentation or surface features of various shapes and textures
a modified, thickened scale, often spiny or keeled
Secondary fin rays
see: rudimentary fin rays or procurrent rays
particulate or divided; opposite of homogeneous in referring to egg yolk
referring to eggs which do not float nor sink, but remain suspended in the water column
the membrane that encloses an egg; generally, equivalent to chorion
Sigmoid heart
S-shaped heart which develops from the primitive heart tube
forward part of the head anterior to the eye; snout length measured in the longitudinal axis from the anteriormost pigmented surface of the eye to the tip of the snout
Snout-anus (Sn-A) (or snout-to-vent) length
distance from the tip of the snout to the posterior margin of the anus, measured at the longitudinal axis; equivalent to preanal length (PAL)
Snout to rictus (upper jaw)
tip of snout to angle of mouth
Soft rays
bilaterally paired, usually segmented, fin supports
primitive, segmented, mesodermal tissue along each side of notochord
having a rounded apex and tapering to a base; flattened or spoon-shaped
Spawning habitat
description of the environment in which spawning has been documented
Spawning season
months during which spawning has been documented for the Great Lakes region, unless otherwise stated
Spawning substrate
type of material upon which spawning is known to occur
Spawning temperature
water temperatures at which spawning has been documented
requiring cavities as a substrate for spawning
bone at the upper part of the skull, often forming part of the orbit
supporting elements in the fins that are unsegmented, unpaired, unbranched, and usually (but not always) stiff, pungent and sharp (sometimes referred to as spinous rays); also, refers to pointed projections arising from various bones, usually on the head
Spinous rays
in certain otherwise soft-rayed fish, soft rays that during embryonic or larval development are thickened, fused, and hardened into spine-like structures, sometimes with moderate to strong serrations or barbs along their posterior margins. In formulas for fin-ray counts, fully spinous rays may be designated by Roman numerals like true spines.
Spinous scale
specialized larval scales with spines (not the serrated ctenoid scale of adult fishes)
covering of scales
Stalked eye
eye borne on a stalk or peduncle
Standard length (SL)
distance from the most anterior point on the snout to the most posterior point of the notochord or hypural complex
Stellate melanophore
referring to a melanophore which is expanded into a star-like shape; the stellate condition can be temporary and the melanophore can become punctate after contraction
primordial mouth; the anterior pitted portion of the embryonic gut
surface features in the form of lines or bands
a horizontal band of pigment on the lateral surface of the head or body; sometimes occurring in a series
occurring beneath the skin
beneath the lower jaw; along the edge of the lower jaw
posterior bone of the gill cover lying below the opercle
Subterminal mouth
underneath or set back from the tip of the snout, sometimes referred to as an inferior mouth
spatial anatomical term meaning upper in position; opposite to inferior
Superior mouth
condition when the lower jaw extends upward and the mouth opens dorsally
Supracleithral spine
a spine originating from the supracleithrum located near the upper posterior margin of the head
small dermal bone attached posterior and dorsal to the maxilla
Supraoccipital spine
spine or crest on the midline of the back of the head originating from the supraoccipital bone
above the mouth; referring to the teeth of the oral disc in lampreys which are anterior to the mouth opening
Supraoral tooth plate
in petromyzontids, tooth plate immediately anterior to esophageal opening
Supraorbital pores
pores on dorsal surface of snout and head, including those in ethmoid canal at tip of snout (sometimes listed separately) and those on dorsal surface of snout and head; the posteriormost pores situated between the eyes are sometimes referred to as interorbital pores (eels)
Supratemporal pores total number of pores in supratemporal canal (usually including one median pore) (eels)
Swim bladder
gas-filled sac lying beneath the spinal column in the abdominal region (between the kidneys and alimentary canal in teleosts); also referred to as air bladder or gas bladder
species inhabiting the same or overlapping geographic areas
a multinucleate mass of protoplasm



total length
portion of the body posterior to the anus; the postanal region
Tail-bud stage
stage of embryonic development characterized by a prominent caudal bulge and marked development of cephalic region
Tail-free stage
stage of embryonic development characterized by separation of the tail from the yolk
Tail length
anus to tail tip; in petromyzontids, distance from cloacal slit to tip of caudal fin
Taxon / Taxa
taxonomic category or unit, as a species, genus, family, etc.
bony fishes
Telescopic eye
type of elongate, cylindrical eye that protrudes forward or upward within an envelope of skin
Temporal canal
connects the lateral line with the postorbital portion of the infraorbital canal; rarely contains pores (eels)
Terminal mouth
condition when lower and upper jaws are equal in length and the mouth opens terminally; type of mouth that opens anteriorly; typical of most fish larvae
markings or colors arranged into squares
pelvic region; referring to the chest area; usually the region of the body of a fish below the pectoral fin base
Total length (TL)
distance from the most anterior point on the snout to the most posterior point on the caudal fin or fin fold
Total vertebrae
total number of vertebrae including the first reduced vertebra and the hypural (eels)
the process (synonymous with metamorphosis) at the end of the larval stage, characterized by a marked change in form or structure and involving acquisition of juvenile or adult characters and loss of larval characters; also refers to the stage where this process occurs; the term "transitional" is used sometimes for larvae that undergo a gradual transformation (e.g., in scombrids)
ending abruptly along a vertical line
portion of the body between the head and the anus; from posterodorsal point of gill opening to anus
Trunk length
in petromyzontids, distance between posterior margin of last gill opening and cloacal slit
Trunk myomeres
in petromyzontids, myomeres between the most posterior gill opening and the cloacal slit
small knobby protuberance



a median bone in the throat region to which the sternohyoid muscles attach; not part of the hyoid arch
final vertebral segment usually modified for caudal fin support; complex bony structure (usually upturned) at the terminus of the vertebral column formed from the fusion of several vertebrae



V [or P2]
central or pelvic fin
opening on the ventral surface of a fish where the alimentary and urinary canal open; essentially equivalent to the anus in larval fish
lower part of the body; opposite to dorsal
Ventral fins
pelvic fins; paired fins articulating with the pelvic girdle
lowermost part of the body; ventral body margin; opposite to dorsum
having worm-like markings
part of the body that once had a function but has become functionless in the course of evolution
Vertebral formula
expressed as the range of counts of predorsal, preanal, and total vertebrae (eels)
Vertical blood vessel
one or more of the vertically oriented blood vessels connecting the aorta with the gut or kidney in leptocephali
Vexillifer stage
larval stage of caraphids characterized by the tentacle-like dorsal fin ray
highly modified elongate anterior dorsal-fin ray in larval carapids
in the form of finger-like projections
Vitelline membrane
after water hardening, the membrane surrounding the egg proper (animal and vegetal material)
Vitelline vessels
arteries and veins of yolk region
type of reproduction where the embryos develop within the ovary and receive maternal nutrition; live-bearing
anterior, median bone of the roof of the mouth/palate (=prevomer)



Water hardening
process of membrane delamination and fluid formation which forms the perivitelline space bordered by the chorion and vitelline membrane; expansion and toughening of egg capsule due to absorption of water into the perivitelline space
Weberian vertebrae
first four vertebrae in cyprinids, catostomids, and ictalurids which are modified to connect the swim bladder to the inner ear



chromatophores bearing yellow pigment



yolk-sac depth
yolk-sac larva(e) or yolk-sac length
a fish in its second year
nutritive material of the egg or in a sac-like mass (yolk sac) below the abdominal region of a newly hatched larva; usually seen as a yellowish sphere diminishing in size as development proceeds
Yolk diameter
greatest diameter of yolk; more accurately measurable prior to embryo formation
Yolk plug
yolk within the blastopore
Yolk sac
a bag-like ventral extension of the primitive gut containing the yolk
Yolk-sac larvae (YSL)
phase of development from the moment of hatching to complete absorption of yolk; early larval stage with yolk present in a sac-like region of the gut
Yolk-sac length (YSL)
horizontal distance from most anterior to most posterior portion of yolk-sac
Yolk-sac depth
vertical distance from dorsum to venter of yolk sac