Fishes are by far the largest group of vertebrate ani­

mals. T heir numbers may approach 28,000 extant spe­

cies (Nelson, 1984), well over half of all vertebrates,

and thousands of other fish species lived in past geo­

logic times. Over half the world's species of fishes live

in marine environments; these are most diverse in the

seas of the tropical Indo-Pacific region and in the Carib­

bean. Freshwater fishes are most diverse in South

America, which may harbor 5,000 species, and in

southeastern Asia. North America, including Mexico,

has a moderately diverse native freshwater fauna of

about 1,000 species.

However, it should be pointed out that "fishes" is a

collective term for actually three very different groups

of currently living vertebrates and thus is not a "natu­

ral" group, as, for instance, the mammals (class Mam­

malia) are presumed to be. Included in fishes are the

class (or superclass) Agnatha, the primitive jawless ver­

tebrates represented today by the lampreys and, under

older classifications, hagfishes; the class

Chondrichthyes, which includes the sharks, chimaeras,

skates, and rays, characterized by a cartilagenous rather

than bony skeleton; and the class Osteichthyes, the

bony fishes. Osteichthyes alone contains over 25,000 species, making it the largest vertebrate class. All tetra­

pod vertebrate groups are believed to have evolved from

the bony fishes, though the exact fish lineage (Jung­

fishes, lobefins, or other) from which they evolved has

been in dispute. To put this into perspective, "higher"

vertebrates, including humans, are more closely related

to bony fishes than these fishes are to some other "fish"

groups, such as sharks.

All fish groups are characterized by gills, which evo­

lutionarily gave rise to various structures, including the

jaws of chondrichthyans and bony fishes and the ear

bones of tetrapods. Chondrichthyans and bony fishes

share paired fins, which gave rise to the limbs of tetra­

pods. These adaptations and many others allow fishes to

respire, move, feed, excrete, reproduce, and otherwise

function in their watery environment.

Habitats and Lifestyles

Over half of the species of fishes, including many

whole families, are restricted to the marine environ-

About Fishes

ment. Some families are virtually restricted to fresh wa­

ter (e.g., percids) while others with many freshwater

representatives may show some tolerance for brackish

environments or have a few members occurring pri­

marily in marine habitats (e.g, cyprinodontiform fami­

lies). Still other, primarily marine, groups have a few

members which have invaded freshwater environments

(e.g., gobies, atheriniform fishes). T hese latter are best

represented in regions where strictly freshwater groups

are absent or less common, such as Australia, lower

Central America, and oceanic islands. Groups that are

able to move freely between waters of varied salinity

are termed euryhaline. Examples are the cyprinodon­

tiform fishes (topminnows, mollies, and related

groups). Many fishes are migratory, especially in re­

sponse to reproduction, and some are diadromous,

spending portions of their lives in both marine and

freshwater environments. Fishes that live most of their

lives in the ocean but spawn in rivers (e.g., salmon,

herrings) are termed anadromous; those that do the op­

posite (e.g., freshwater eels) are catadromous fishes.

Most fishes spend much of their life in one or two

basic habitat types, such as rivers or smaller streams

(lotic habitats), springs, lakes, and swamps (lentic), es­

tuararies, coral reef, oceanic, and so on. A few are so

generalized in habitat that they may be encountered in

several habitats within freshwater or marine realms.

For purposes of general reference, roughly based on

dimensions, stream habitats include very large rivers a

half kilometer or more in width, such as the Mississippi

or lower Tennessee, large rivers averaging 200-400 m

in width (e.g., lower Cumberland, upper Tennessee),

medium rivers 50-200 m width, (e.g., lower Duck or Clinch), small rivers under 50 m width (e.g., Little,

Buffalo), large and small creeks, and spring runs.

Creeks and small rivers may be montane (high gradient,

fast-flowing, predominately rocky substrate), upland

(moderate gradient and flow, substrate variable but gen­

erally v-.ith extensive rocky areas), or lowland (low gra­

dient, sluggish current, predominantly depositional

substrates such as sand and silt) in character; larger

streams may be upland or lowland in character. All of

these basic categories and subcategories of stream types

have characteristic fish faunas associated with them

which are further modified in composition by prevailing

habitat types associated with regional geology (see gen-

ABOUT FISHES 37

eral descriptions in previous section, Waters and Geol­ogy of Tennessee). Because fishes have evolved such close associations with these habitat types, many spe­cies show great fidelity to physiographic provinces. This fidelity can promote fragmentation and isolation of populations over time as the surface geology changes, and it probably has resulted in the evolution of several new species.

Natural len tic habitats consist of several kinds of lakes (lacustrine habitats), swamps, and backwaters of larger streams. Natural lakes are created in a variety of ways, including glacial modification of the land surface (as in much of northern North America), subsidence due to tectonic activity (e.g., Reelfoot Lake) or dissolu­tion of underlying strata (e.g., karst lakes of Florida), changing river courses (oxbow lakes), or natural dam­ming of streams by landslides or beavers. Humans have added a large contingent of unnatural lentic environ­ments through the impoundment of streams in which some len tic fish species have flourished while stream fishes largely disappeared.

Swamps or marshes are characterized by expanses of shallow, standing water which may fluctuate seasonally with rainfall. Permanent swamp habitats usually have submergent or emergent vegetation and may be forested (e.g., cypress, tupelo). Waters may be clear, stained brown by tannic-acid, or seasonally turbid (muddy). A number of fish species are strictly or often associated with swampy environments.

Caves constitute an additional unique habitat type. A few species of fish lead a strictly subterranean (hypo­

gean or trogloditic) existence. Other species (troglophilic) are often associated with caves but may lead an epigean (surface) existence part of the time. Hy­pogean fishes, which are typically blind and devoid of pigment, have evolved several specializations to cope with their lightless and low-nutrient environment, as discussed in the section herein on amblyopsid fishes .

Beyond general habitat preferences, fishes demon­strate a variety of basic behaviors and physical adapta­tions. Some fishes are rather sedentary, spending much of their time near a favored haunt or moving about a small area. Such fishes tend to be solitary or occur in

small, loose aggregations, and some may be constantly or seasonally territorial . Other fishes rove freely over large areas and are constantly on the move. These tend to be schooling fishes and may frequent the midwater (pelagic) realm or cruise very near the surface or bot­tom. Many midwater species are filter feeders (e.g., shad, paddlefish), straining plankton from the water with special gill rakers, while others actively feed on mid water organisms or dash to the surface to take fallen

38 The Fishes of Tennessee

prey (e.g., many minnows). Still others, such as striped bass, are roving predators feeding on other midwater fishes. Surface-oriented fishes (e.g., topminnows, some silversides) cruise just beneath the surface, feeding on fallen or emergent organisms; such fishes usually have an upturned mouth and flattened dorsal profile to facili­tate this life-style. Many fishes are bottom-oriented (benthic) in behavior, swimming just above the bottom (epibenthic; e.g., suckers), resting upon it (e.g., darters and scuJpins), or even burrowing into it. Some of these species have a downwardly oriented mouth to facilitate feeding. Those benthic species that frequent swifter wa­ters may have several hydrodynamic adaptations, in­cluding a depressed or streamlined body profile, modified fins, and reduction of the swimbladder to lessen buoyancy. Benthic and epibenthic species are generally very closely associated with a specific sub­strate type (gravel, sand, small or large rocks, vegeta­tion, detritus) and current regime. Though none occur in North America, a few fishes (e.g., mudskippers, climbing perches) have remarkably evolved adaptations permitting them to spend short periods of time on land!

Because they feed on a variety of organisms and, in tum, are preyed upon by a diversity of predators, in­cluding terrestrial ones, fishes are vital links in nature's food web, especially between the aquatic and terrestrial environments. Fishes of different kinds or life-stages feed on everything imaginable-including microscopic organisms, such as diatoms and bacteria; plankton; plants; macroinvertebrates (e.g., insects, crustaceans, mollusks); other fishes of all sizes; and other verte­brates, both aquatic and terrestrial. In tum, they are fed upon by everything from large aquatic insects to other fishes and reptiles and a variety of birds and mammals, including humans, for whom they have provided one of the most important protein sources for thousands of years.

Like nearly every living organism, fishes are host to a variety of parasites, both external and internal, includ­ing: fungi; protozoans; acanthocephalan, cestode, trem­atode, and nematode worms; and various arthropods, such as mites, copepods, and isopods (see Hoffman, 1967). Perhaps among the most obvious to collectors of freshwater species are the immature stages (metacer­cariae) of certain trematode worms which form con­

spicuous black cysts on the skin and may be extremely abundant on fishes in some populations. Also con­spicuous when present are such external parasites as leeches, "anchor worms" (females of certain copepods which attach to the skin or fins), and "fish lice" (copepods). Cope pods and large isopods may some­

times be found living in the gill cavity of fishes. When

dissecting a fish , it is not uncommon to find worms (trematodes , nematodes , cestodes) l iving in the gut or

freely in the body cavity, or to find immature stages en­cysted in the flesh . Perhaps one of the most interesting parasites is the larval stage (glochidia) of freshwater mussels (Unionidae) , which are discharged by the fe­male mussel on the gil ls or fins of passing fishes where they undergo early development; subsequently these drop to the stream bottom to begin their sedentary life­style after having had their dispersal facilitated by the fish host . Some mussels have actually evolved color

patterns to lure fishes in proximity to facilitate larval in­festation . Though sometimes unsightly, when in natural equ il ibrium, fish parasites seldom have an impact on the health of their hosts .

Anatomy and Function

In the following sections we attempt to acquaint readers with the basic physical features of fishes and how they function to facil itate their existence in the aquatic envi­ronment. Many terms will be introduced which will be of use to those who wish to identify fishes through the

keys and descriptions in this book . Much of the infor­mation presented here , plus much additional detailed in­forn1ation , is inferable from the general works of Lagler et al . (1977), Bond (1979) , and Moyle and Cech (1982, 1988) , and from special ized works such as those of Al­exander (1967) , Hoar and Randall (1969-1984), Harder (1975) , and Caillet et al . (1986).

Head, Body, Fins, and Locomotion. The body forms and fin configurations and positioning in fishes consti­tute an almost endless array; some of the more basic

forms are illustrated in Figure 10 . In lateral profile , ba­sic body shapes range from horizontally elongate , or extremely attenuate , to deep-bodied (vertically exagger­ated) ; in cross section , elongate fishes may be terete or fusiform (cylindrical) (e . g . , chub minnows , gar, mack­erel) , somewhat laterally compressed (parallel-sided) (e . g . , striped bass), or dorsally depressed (flattened from above; e . g . , sculpins) . Deep-bodied fishes are all laterally compressed to varied degrees; dorsally de­pressed fishes range from relatively elongate (e . g . , sculpins) to extremely broad (e . g . , skates and rays) in dorsal view.

Figure 10. Some basic body forms of fishes: top, terete (chub minnow); middle, laterally compressed (sunfish); bottom, dorsally

depressed (sculpin).

ABOUT FISHES 3 9

gill cover I / /

\opercle isthmus

gill juncture

a

spinous dor sal fin

\ TRUNK

dorsal spines \

lIy �

pelvic fin

anal spines \ myomeres

anal fin

REGION

/th orac ic positionl

adipose fin

c axillary process \ pelvic fin /abdominal position\

b

urogenital opening

Figure 11. Body regions and basic anatomical features of a fish: a) a spiny-rayed fish (perch) with anteriorly (thoracic) positioned

pelvic fins; b) a darter (small percid) showing details of urogenital area; c) a soft-rayed fish (trout) with abdominally positioned

pelvic fins and other fin features shown.

The body of a bony fish is divided into three main re­

gions: the head, trunk, and caudal (Fig. l l a). The head

includes that region from the tip of the snout to the pos­

terior extremity of the gill cover. Dorsally, it is gener­

ally delimited by the area where dorsal body muscu­

lature attaches to the rear of the skull (the occiput) and is generally discemable as a transverse line an­

terior to a hump in the dorsal profile. Ventrally, the

40 The Fishes of Tennessee

juncture of the gill flaps generally serves to demarcate

the rear of the head. The trunk, which contains the ab­

dominal cavity, lies between the head and caudal re­

gion. Dorsally, the region of the trunk lying between

the occiput and the dorsal fin is termed the nape. Ven­

trally, the trunk subregions are the narrow isthmus (in­

terposed between the gill flaps), the breast (anterior to

the pelvic fins), and belly. In most fishes the body is

distinctly constricted just behind the anal fin to form the caudal peduncle. In some fishes the caudal region , the beginning of which technically corresponds internally to the first vertebra with a ventral spine (haemal spine) rather than ribs (see Fig . 17) , s imply tapers from some­where near midbody without a distinct peduncle being formed ; this i s especially true in groups in which the anal fin is reduced or lacking , as in eel s .

In lampreys (Fig . 12a) , the head is considered t o ex­tend posterior to and include the body segment bearing the seventh (last) gill aperture . The trunk extends from

behind the head to the muscle segment (myomere) bear­ing the anus or urogenital opening , and the remainder is considered the c audal region .

The main external features of the head of bony fishes and bones of the interior of the mouth are illustrated in Figures 13a and b. Mouth shapes and positions vary greatly among fi shes . In many fishes the mouth is ter­

minal, located at the anterior extremity of the head , but in others it may be sl ightly overhung by the snout (sub­

terminal) , far beneath the snout (inferior), or upturned (superior) for surface feeding. In general , terminally lo­cated mouths are utilized for biting and seizing (e . g . , sunfish), or water intake for filter feeding (e . g . , shad) ,

while more inferior mouths may be suctorial or mod­ified for scraping algae and associated organisms from the bottom (e . g . , sturgeon , suckers) . The jaws may be protractile (extendable) or nonproctractile ; in the latter case the premaxilla is either broadly or narrowly con­nected to the snout by a fleshy median frenum (Fi g . 14) . In some fish groups (e . g . , gars , needlefishes) the jaw bones may be greatly modified into beaklike or other structures . In other groups (e . g . , catfishes , some min­nows) , barbels, which are usually well endowed with taste buds , may be present on the maxillary and mandi­bular areas (Fig . 15) .

The presence , locations , and types of teeth also vary greatly among fishes . They may be present or absent on any of the jaw or interior mouth bones (Fig . 13b) , and a few fi sh groups (e . g . , minnows) lack teeth in the mouth region altogether. Various tooth types are shown in Fig­ure 16 .

In lampreys , jaws are lacking; instead , the mouth consists of a buccal funnel, a suction device , which has variously developed toothlike structures (see terminol­ogy in Fig . 12b) . In parasitic species , the buccal funnel and tooth structures function in attaching and holding to slippery host fishes and to rasp a hole from which blood

trunk head

a

anterior teeth buccal

lateral teeth

marginal tooth row ----tc

transverse lingual lamina

b

caudal region

myome<es '\ diphycercal

' caudal fin

urogenital papilla & opening

supraoral lamina

ct======�circumoral teeth

infra oral lamina

Figure 12. Lamprey anatomy: a) body regions and external features; b) features of oral region.

ABOUT FISHES 41

hyomandibular

sphenotic

prefrontal

posttemporal

lachrymal epiotic

opercular

premaxillary pectoral girdle

subopercular

/��c��iJ--___ pre opercular

dentary

interopercular

maxillary anchiostegal

articular

a pte'rygoid sympletic

angular qua rate

meta pterygoid

endopterygoids

maxillaries vomer

b para sphenoid

premaxillaries

palatines

Figure 13, Bones of the head region of a fish: a) jaw bones and superficial skull bones (redrawn and modified from Lagler et a!.,

1977); b) bones of the roof of the mouth region (ventral view),

42 The Fishes of Tennessee

up per lip (p remaxillaries)

frenum

Figure 14. Dorsal view of head of fish with non-protractile upper jaw showing connection of frenum.

can be drawn . In nonparasitic lampreys , this organ fa­cilitates attachment to the substrate. It should be noted that , unl ike bony fishes , the nostril in lampreys is a sin­gle median opening before the eye s . On the sides of the head are seven circular openings which serve as the gill apertures . The lateral line pores of a lamprey's head are mainly microscopic and may not be visible as illus­trated .

The superficial facial bones and sensory canal system of a bony fish are shown in Figure 13 and 26. The size of the eye, or orbit, varies greatly among fishes and varies with growth (allometry) in an individual, with the eyes of larvae and juveniles usually being propor­tionately larger than those of adults . The infraorbital (or suborbital) bones may be poorly developed or partially absent in some fishes with corresponding alterations of the canal system borne by those bones . The sensory canal system (cephalic latera lis system; see Fig . 26) varies in development among fishes, and some canal portions may be elaborated, interrupted, or absent, or even cavernous, as in fishes of the drum family. The bones (opercle, preopercle, and others) associated with the gill flap region vary widely in shape and may or may not bear spines or other features .

The body of a fish is composed of sequential muscle segments termed myomeres (Fig . Ila) which, in small­scaled or scaleless species, may be visible through the skin or as creases on preserved specimens . In lampreys (Fig . 12a), they are usually quite evident, and their numbers are important in identification of species .

In many bony fish groups a lateral-line canal (see Sense Organs section , below) is evident along the side of the body near the midline (Fig . II a) extending from the cephalic system on the head near the top of the gill opening to the base of, or onto, the caudal fi n . It may

Figure 15. Barbels of fishes: left, a minnow (carp); center, a catfish; right, a sturgeon.

a b c

Figure 16. Some basic types of dentition in fishes: a) canifonn; b) villifonn or cardifonn (ventral view of upper jaw of catfish); c) anterior incisifonn and posterior molarifonn teeth of lower jaw of a marine porgy (Sparidae).

ABOUT FISHES 43

be borne in a row of scales, or in the dermis of scaleless species, and may be incomplete , interrupted, or absent in some groups and is even variable within species in some groups . In some fishes (e . g ., cavefishes) the lat­eral line has vertical elaboration s .

Also externally visible on the body are the urogenital

opening and anus (Fig . lib) . In most groups these openings are just before the anal fin, but in two families (pirateperch, cavefishes) these migrate forward during development to a jugular position just behind the is­thmus . The urogenital opening is posterior to the anus and serves both the kidneys and gonads; it is well sepa­rated from the anus or even produced into a tube called a genital papilla. The term genital papilla is used when the opening is on an elevated mound or tube .

The most prominent external features on the body of most fishes are the fins . Individual and collective termi­nology for fins and their support structures are shown in Figures lla,c, and 17. Fins vary greatly in shape and size among and within fishes, and some groups lack certain fins . Dorsal fins are present in most fishes; soft­rayed fishes (e . g ., trout, minnows) typically have a sin­gle dorsal fin (two or three in cods and relatives) while spiny-rayed fishes (e . g ., perch) may have two separate fins or a single continuous fin. Anal fins are single in most fishes. The dorsal and anal fins are supported prin­cipally by the bladelike pterygiophores, inserted medi­ally between the lateral body musculature, and the fin rays . Soft rays consist of bilaterally paired, segmented structures which, except for the anterior one or few, are

vertebral column

predorsal bones

pectoral girdle

pelvic girdle

usually branched one or more times tow ard the extremi­

ties . "True" spines, such as those of perciform fishes, are solid, unpaired, and unsegmented structures . The "spines" of catfishes, some minnows (e . g . , carp), and a few other " lower" bony fish groups, which are gener­ally single, differ in that they develop from hyperostosis (profuse bone tissue deposition) and fusion of seg­mented rays .

The caudal fin, or tail fin, is well developed in the

majority of fish groups but may be reduced or absent in some (e . g ., some eels). The three basic types of caudal fin are shown in Figures 17 and 18. Lampreys have a diphycercal caudal fin in which the rays radiate from and surround the tip of the notochord . Certain primitive groups, including the sturgeons, paddlefishes, gars, bowfin, and sharks, have a heterocercal caudal fin in which the upper lobe emanates from the upturned termi­nal vertebrae and the lower from the ventral aspects of those bones . Typical of most bony fishes is the homo­

cereal, usually relatively symetrical, configuration ema­nating entirely from bladelike modifications of the upturned terminal vertebra termed the hypural plate

complex (Fig . 17) . Homocercal caudal fins may have a variety of shapes, including rounded, truncate, forked, crescentic, and others .

Numerous families of lower teleosts (e . g ., smelt, trout, catfish) have an additional median fin, the ad­

ipose fin. It is a fleshy fin lacking rays or spines and is typically located between the dorsal and caudal fins .

pterygiophores

homocercal ca udal fin

hypural complex

haemal spines

ple ural ribs

Figure 17. Axial skeleton, fin support structures, and caudal fin type of a bony fish (centrarchid sunfish).

44 The Fishes of Tennessee

a

b

Figure 18. Primitive caudal fin types: a) diphycercal; b) heterocercal.

The paired fins (Fig. 1 1 a), the pectorals and the pel­vics, are bilateral appendages which vary considerably

in placement among fish groups. Pectoral fins are al­ways just behind the head but may be inserted relatively low (e.g., suckers) or high (e.g., silvers ides) on the sides of the body. The pelvic fins (also called ventrals) may be situated near midbody (abdominal; e.g., trout, minnows), which is thought to be the evolutionarily more primitive positioning, or they may be situated an­teriorly (thoracic or jugular) beneath or in advance of the pectoral fins (e.g., perch, sunfish), as is characteris­tic of "higher" fish groups. The pectoral and pelvic fins are supported internally (Fig. 17) by complexes of large bones (girdles) which are not directly associated with the vertebral column. In most groups only soft rays are

present in the pectoral fin, but in some (e.g., catfishes) the first rays may be fused into a spine. In many spiny rayed fishes the pelvic fin has a single spine. The axil­

lary processes shown in Figure 1 1 are characteristic of only a few fish groups, such as salmonids and herrings, and a small axillary process occurs in many cyprinids.

Fishes move through the water by using both body movements and the fins. Primarily, forward motion is attained by side-to-side flexure of the body, caused by

alternating contractions of the myomeres, culminating in thrust from the caudal fin. Fins are used primarily for short bursts, stabilization, and stopping or reversing. Median fins serve as stabilizers and, when curled to the side, as brakes. Paired fins are for short maneuvering,

ascending, and descending. Some species with very long-based dorsal fins, such as bowfins, achieve some propulsion by wavelike undulations of this fin.

Gills and Respiration. In bony fishes the gills consist of five arches, each composed of several bones, suspended posteriorly beneath the skull (Fig. 19). These pharyn­

geal arches and their associated structures are collec­tively sometimes called the branchial basket. Each arch bears numerous filaments posteriorly that are richly sup­plied with arterial blood. Anteriorly, the major bones (limbs) of the arch usually have inwardly directed gill

rakers of varied numbers and shapes which serve to strain food or other matter passing over the gills. In many fish, especially perciforms, teeth are present on various pharyngeal bones; these may be patches of small teeth for grasping or large molariform (rounded, molarlike) teeth for grinding prey, such as mollusks. In minnows and suckers, in place of the gill rakers, the fifth arch has developed large pharyngeal teeth (Fig. 20) which li� just ahead of the esophagus and serve to tear or grind food, and which vary in shape from sharp­pointed to hooked to molariform.

The heart of a bony fish is located just posterior to the gills in the ventral portion of the body cavity (see Fig. 23), and blood is pumped directly into the gill arches. Respiration is achieved by pumping water over the gills with alternating expansions of the mouth cavity

and opening of the gill flaps or by passage of water through the gill chamber while swimming. Gaseous ex­change (oxygen uptake, carbon dioxide release) occurs in the gill filaments, and oxygenated blood is pumped to the rest of the body. Gill filaments are also the site of chloride cells which serve to maintain salt balance in fishes. In many fishes, additional filamentous organs, the pseudobranchs, are located on the lining of the gill

chamber in the preopercle area (Fig. 19). These may provide oxygen to the eyes and possibly augment gas production for the swimbladder.

In lampreys, the gills consist simply of seven paired pouches on either side of the alimentary tract which open medially to this tract and have small external vents visible along the sides of the head (Fig. 12a). These pouches are lined with filaments through which gas ex­change takes place when water is pumped over them by contractions and expansions of the pouches. Gills of sharks, rays, and their relatives are more similar to those of bony fishes, consisting of arches, but the

chamber has multiple slits opening to the outside rather than a single gill flap, and a spiracle (reduced anterior gill slit) may be present dorsally for intake of water.

ABOUT FIS HES 45

rt. pharyngeal arches 1 - 4

pseudobranch (rt. side)

1 st left arch

gill ra kers

lower l imb

mod ified 5th arc h w ith tooth plates

Figure 1 9 . G ill chamber region of a f i sh showing gill structures and pseudobranch location.

Skin and Scales. The bodies of fishes are covered with a somewhat permeable skin , and most have bony scales or derivitive structures on much of the skin surface , but a few groups (e . g . , freshwater sculpins and several cat­fish families) lack scale s . The skin is endowed with mu­cous cells which produce the characteristic slime of fishes ; this secretion may function as a sealant against infections and to reduce friction in swimming. Also

present in the skin are the organs of color, the chroma­

tophores and iridocytes. The former are capable of con­traction and expansion to change coloration and impart true color through the possession of different pigments (e . g . , melanophores have black pigment , erythrophores

red , and so on) . Iridocytes reflect external l ight sources and result in silvery reflections and iridescent effects .

46 The Fishes of Tennessee

The basic scale types (Fig . 2 1a-d) are placoid

(sharks and relatives) , ganoid (gars , sturgeons), and cycloid, the more famil iar type found on most fishes . The first two types are thick and enamel-l ike and lack the circular bony ridges characteristic of cycloid scales . The parts of a cycloid scale are shown in Figure 2 1c .

The concentric growth rings and annual growth checks

(annuli) are important in age studies of fishes . In many

fish groups (primarily spiny-rayed) cycloid scales are modified (Fig . 2 I d) , having tiny spines on the posterior surface either as an integral part of the scale (few ma­rine groups) or attached at the base by platelets (ctenoid) , giving the fishes a rough texture (e . g . , perch , sunfish) . Some specialized derivatives of scales are the external armorl ike pl atings of seahorses and pipefishe s ,

c

Figure 20. Modified fifth pharyngeal arches of fishes: a) with partly hooked teeth for ripping and tearing (minnow); b) with comb-like teeth (sucker); c) with molarifonn teeth (redear sunfish, a snail-eater); d) heavily modified with molariform teeth for crushing mollusks (freshwater drum).

a

b

a n n u l u s

a n t e r i o r

f i e l d

c

r a d i i

d

.. . ' ...... . .. • .... . . . . ".- - - -.- - - . �. ,,, . . .. . ' -" . .

f o c u s

e x p o s e d

f i e l d

c i r c u l i

c t e n i i

Figure 2 1 . Basic fish scale types: a) placoid ; b) ganoid; c) cycloid; d) ctenoid (drawn, in part, by P. Yarrington).

ABOUT FISHES 47

Figure 22. Examples of fish breeding tubercles: a) head and body tubercles of a stoneroller minnow (genus Campostoma); b) tuberc les on pectoral f in rays of a cyprinid in uniserial (one per fin-ray segment) configuration.

belly scutes of herring , and the scalpel-like spine on the

caudal peduncle of the marine surgeonfishes.

Scales vary tremendously in size between and within

fish groups and may be firmly attached or easily lost

(deciduous). The coverage of scales (squamation) also

varies greatly among fishes . When present , scales are

usually distributed on much of the sides and back of the

body but may be present or absent on the belly, breast ,

and nape (dorsal surface from occiput to dorsal fin ori­

gin). On the head , they may be completely absent , or

present or absent on the opercles, cheek area , and top

of the head . Only a few groups have the head almost

completely scaled .

A variety of other features may be found on the skin

of fishes . Many species have external taste buds, usu­

ally visible only microscopically as tiny bumps , located

on the skin surface, particularly on the ventral portion

of the head , and some even on the body. Small fleshy

appendages , such as lappets or papillae, are present on

the skin of some fishes (e . g . , sculpins) . Perhaps some

of the most remarkable organs of the skin are the lumi-

48 The Fishes of Tennessee

nous photophores of deep-sea fishes which may func­

tion in species recognition and illumination of prey.

In several fish groups , breeding tubercles, or "pearl

organs ," develop on the head , body, or fins (Fig . 22) during the spawning season (see Wiley and Collette ,

1 970). T hese are horny protuberances which are se­

creted in response to seasonal hormonal changes , pri­

marily in males , and presumably function in enhanced

contact during spawning . T hese tubercles slough off af­

ter the spawning season .

Sense Organs and Senses. T he sensory systems of

fishes are necessarily quite different from those of ter­

restrial vertebrates . For instance , in bony fishes, the

cephalic lateralis system is instrumental in detection of

sound or vibrations . T he canals of the head and lateral

line are equipped with cells called neuromasts which

have hairlike cilia oriented in such a way as to detect di­

rectionality of vibrations . T his information is transmit­

ted directly to the brain via nerves which lie in proximity to the canals . T hus the "ears" of fishes are in

part very different from those of terrestrial vertebrates .

T he function of this system and its relationships to be­

havior were treated extensively by Disler ( 1 960) . T here is also an inner ear, more analagous to those of

other vertebrates, but which, instead of containing ear

bones or ossicles, has three calcareous "ear stones" or

otoliths. One of these (the lapillus) functions in main­

taining body orientation and the remaining two (as­

ter icus, sagitta) in sound reception . In several fish

groups , features have evolved to enhance transmission

of vibrations to the inner ear area . In minnows , suckers ,

catfishes , and relatives (ostariophysan or otophysan

fishes) , the anterior vertebrae are modified to form a

chain of bones , the Weberian ossicles, interlinking the

swimbladder, which is very sensitive to vibrations, to

the inner ear area . In a few other groups the swimblad-

der has forward extensions to the auditory region . Lampreys lack an elaborate inner ear system, having only a small auditory capsule . Thanks to the aid of sen­sitive hydrophonic equipment , many fishes , including even small minnows , are now known, to produce sounds (see Tavolga , 197 1 ). Sound communications may be much more important in fishes than was pre­viously thought.

Chemoreceptors are well developed in fishes . Taste buds , or gustatory organs , may occur externally on the skin of fish (e . g . , some catfishes) and on barbels about the mouth ; these organs are variously distributed on the

fins , lips , and in the mouth cavity and throat area as wel l . Intimately related with taste in the location of food is the sense of smell , which also is important in orientation , communication , and perhaps predator de­tection. The olfactory senses are extremely well devel­oped in fishes , and some species of fishes are believed capable of detecting substances at a few parts per tril­lion . For example , salmon homing to their natal streams to spawn after years at sea are believed to locate these

waters over hundreds of miles through smell which was imprinted soon after hatching. Odiferous chemicals , called pheromones , are emitted by some fishes (e.g . , the "fright" substance of minnows) for communication through the water to signal danger, locate one another, or perhaps facilitate courtship. The organs of smell , the nasal rosettes, are located in the capsule served by the incurrent and excurrent nares (Fig. 1 3a) through which water is funneled. The single nasal opening of lampreys (Fig . 12a) , through which water is alternately taken in and expelled , leads to a sac lined with olfactory tissue.

Vision in fishes is of varied utility among groups. Many fishes are sight feeders (e.g. , minnows , trout , sunfishes) , and many depend on well-developed vision for not only feeding but species recognition and locat­ing cover. Other species , which inhabit turbid or very deep waters , or subterranean waters , may have the eyes and visual capacity much reduced or even nonfunctional with concommitant increased development of other senses , such as sound detection (e . g . , cave fishes) or taste (e .g . , catfishes). In other fishes (e .g . , walleye , some marine groups) which are nocturnally active , the eyes have evolved a brilliant reflective layer (tapetum

lucidum) on the inner surface of the eyeball to maxi­mize light gathering and , in some of these groups , the eyes are very large. Several species of fishes are known to be capable of color recognition and , based on the bright color patterns , which probably facilitate recogni­tion in many species , this capability may be pervasive among at least species inhabiting well-lighted environ­ments .

The eye of a fish , like that of a terrestrial vertebrate , is a ball-shaped organ with a transparent cornea and cir­cular opaque iris ; however, unlike the eye of terrestrial vertebrates , the iris of the fish eye is scarcely dilative and the lens is essentially spherical rather than elliptical to compensate for the refractive properties of water. The eyeball bulges somewhat from the head , and the lens protrudes from within it , to give the fish virtually a circular field of vision; within the field , the lens focuses at some distance to the sides but at close range to the front , presumably to facilitate feeding . In some groups (e .g . , herrings) vertically oriented , fixed eyelids (adipose eyelids) have evolved (Fig. 33) .

As for other senses , fishes are capable of temperature perception , and many species remain closely associated with certain temperature regimes. The sense of touch is thought to be somewhat limited in fishes , but some spe­cies (e .g. , some catfishes) are known to be sensitive to tactile stimuli . Finally, one sensory system that may be very important in fishes is that of electroreception . The extent of development of these capabilities to sense ex­tremely weak currents is not well known for many fishes , but it is known to be developed to varied degrees in a number of families , reaching its highest state in such groups as the South American electricfishes (Gym­notiformes) which are also capable of emitting weak charges . (The ability to emit stunning voltage levels has developed in the notorious electric eel genus , Elec­

trophorous . ) Catfishes have microscopic pit organs in the skin for reception of minute electrical currents. This system may be very important in location of objects or prey as well as in communication , orientation , and nav­igation in some groups.

Skeleton and Internal Organs . We have already touched on various skeletal components associated with the head and fins and will only briefly further discuss the fish skeleton . In higher bony fishes (Osteichthyes) the body is supported internally by a more or less com­pletely ossified (bony) skeleton (Fig . 17) . However, in chondrostean fishes (sturgeons , paddlefishes) much of the skeleton is cartilagenous . In lampreys , the skeleton is even less developed , consisting of a cartilagenous cranial structure and gil l complex and a spinal cord around which vertebrae do not completely form; thus the embryonic notochord is retained through adulthood.

In bony fishes , skeletal features vary considerably among groups . The presence and number of predorsal

bones varies , as does the number of epipleural ribs , and epineurals , also known as "intramuscular" bones , which are common to such groups as herrings , pikes , trout , suckers , and minnows but not to "higher" groups ,

ABOUT FISHES 49

such as perches and sunfishes ; these are often responsi­ble for complaints about some of these fishes being "too bony to eat . "

When one dissects a fish , or guts one while cleaning , several internal organs are readily apparent (Fig . 23) . The organs are contained within the thin layer l ining of the body cavity, the peritoneum, which is usually s ilver or whitish but may be speckled with black pigment or virtually black , in which case it is usually visible as such through the body wal l . Dark peritonea are typical of some detritivorous and herbivorous species . The more variable organs to be discussed are the alimentary tract , gas bladder, and gonads .

The alimentary tracts of many fishes have a well­developed stomach (e . g . , trout , perch , sunfishes) while those of others are not well defined , appearing as part of the tubular intestine . In such species as detritivorous and herbivorous minnows , the gut may be extremely long with many loops or coils . In species with well­defined stomachs , pyloric caeca often append from the union between the stomach and gut; these vary in size and number among groups .

swi m b ladder

k idney

In many fishes the gas bladder, or "swimbladder, " is one of the largest internal organs , located dorsally in

the body cavity and having the appearance of a white

balloon . This organ functions in regulating buoyancy and is thus reduced or even absent in fishes that have more bottom-oriented life-styles . There are two basic types of gas bladder. In many "lower" bony fishes , such as minnows and trouts , a physostomous gas bladder ex­ists , which maintains a primitive connection to the ali­mentary tract , and which is presumably initially filled during early development by gUlping air; the bladder is regulated in later life by gas secretions from the blood . The tubular connection between the esophagus and swimbladder (pneumatic duct) allows some of these fishes (e . g . , gars , bowfins) to use the swimbladder as a lung and breathe by gulping air at the water surface . The primitive connection has been completely lost in higher fish groups (physoclistous condition). Gas blad­ders may be single chambered but are multichambered in many species . The gas bladder also serves as a reso­nating chamber for the reception and production of sound . Connections from this organ to the inner ear

u ri na ry b ladder

i ntest i ne

Figure 2 3 . Major internal organs of a fish (trout). Liver i s distended ventrally to reveal underlying organs; kidney i s generally

bound in tissue along ventral aspect of vertebral column.

50 The Fishes of Tennessee

area are discussed above under Sense Organs and Senses . For sound production , in some groups , spe­cialized muscles have evolved externally or internally about the swimbladder to generate vibrations (e .g . , in the drum family) .

In most fishes the gonads are tubular organs lying on each side of the body cavity and terminating at the urogenital opening , though they have different shapes in a few fish groups . In some primitive groups (e .g . , bowfin, paddlefish) and salmon ids and relatives , they do not connect directly to the oviducts , with the eggs first being shed into the body cavity and funneled into the ducts . The gonad of lampreys is a single tubular organ which is also remote from the urogenital opening . Near the spawning season , the gonads of fishes are ob­vious and , in the case of females , may fill a substantial portion of the body cavity; out of the spawning season the gonads may be flaccid and difficult to discern . Ova­ries are recognizable by their typically yellowish color and the granular appearance caused by the spherical egg bodies within them, while testes are usually white and contain thick fluid (milt) .

Reproduction and Early Development

Fishes exhibit diverse modes of reproduction; for a sys­tematic treatment of these , see Breder and Rosen ( 1966). The majority of fishes spawn by having the female extrude eggs , which are externally fertilized by sperm-bearing milt from the male (oviparous reproduc­tion) . These fishes may simply broadcast buoyant eggs in midwater (e .g . , shad) or sinking (demersal) eggs on the bottom (e .g . , walleye) , in which case the number extruded is generally extremely high . Other species uti­lize crevices in rocks or submerged logs while others construct gravel nests in which to deposit a lesser num­ber of eggs and abandon (e . g . , salmonids) or guard them (e . g . , catfish , some darters) ; this latter task is of­ten accomplished by the male. A few fishes (e .g . , am­blyopsid cavefishes) incubate eggs in the mouth or gill cavity until hatching . Some groups (e .g . , the "live­bearers ," Poeciliidae) have internal fertilization accom­plished by a male intromittant organ , the gonopodium ,

formed from a modified anal fin , with eggs incubated internally in the female and born fully developed (viv­

iparity) . This process is described more fully under the family Poeciliidae .

In a few species (e . g . , some poeciliids , cyprinids) re­

production by gynogenesis or hybridogenesis has evolved in which all-female clonal populations have re­sulted from certain hybrid crosses (Dawley and Bogart ,

1989; Allendorf and Ferguson , 1990) . In gynogenesis , females of the hybrid "species" develop diploid eggs such that , while sperm from the males of one of the original parental species stimulates initial development of the eggs , none of their genetic material is incorpo­rated into the zygote , thus resulting in exact clones of the females . In hybridogenesis , diploid females of hybrid origin produce haploid eggs containing the un­altered genome of only one of the original parents . These are fertilized by normal males to produce "hemi­clonal" progeny that are all female and will repeat the process to produce additional , different hemiclones .

Some fishes are hermaphroditic , possessing both male and female gonadal tissues , and functioning both as males and females in successive spawning acts (syn­chronous hermaphroditism); this condition is common in some marine groups and is known occasionally in such groups as topminnows (Fundulidae) and striped bass (Moronidae) . Other species (mostly marine) un­dergo complete sex reversal , functioning as one sex when younger and the other as an older adult (consecu­tive hermaphroditism).

With a few exceptions , most temperate fishes spawn sometime between late winter and midsummer when waters are warming . Before spawning , fishes generally engage in some form of courtship, which may range from simple following behavior to elaborate displays­especially by the males-which can consist of swim­ming patterns , physical contact , fin-flaring , and colora­tion changes . Before and after spawning , males of nesting species often vigorously defend territories about their nests against intruders of the same or different spe­cies . Some nest-building species (e .g . , sunfishes) en­gage in "cuckoldry," in which younger males posing as females enter the nests and fertilize some of the eggs .

During the actual spawning act , in mid water spawners , the spawning pair usually just swim upward in close proximity extruding eggs and sperm . In benthic spawners , it is usual for the male to press the female to the substrate in some fashion with the urogenital open­ings in close proximity. Fishes are generally observed to vibrate briefly at the time of releasing gonadal products . In species that bury eggs , the female or both sexes may vigorously vibrate the anal fin to entrench the eggs.

The eggs of mid water spawners may drift for several days and undergo early development while in transit. Demersal (heavier than water) eggs , which may be ad­hesive , generally develop while on the substrate . Newly hatched fishes (Fig . 24a ,b) are termed larvae or "fry." These have little resemblance to adult fishes and con­tinue to undergo development (ontogeny) after emer­gence from the egg . Earliest stages usually survive for

ABOUT FISHES 5 1

pectoral f in bud

b ra in lo bes oto l i t h s my omeres

y olk sac

a

oi l globule inte st ine

b

gas b l add er pelvic f in bud inc ip ient f in ray s

c

Figure 24. Early life h istory stages of a fish: a) yolk-sac larva.; b) pos tlarva; c) early j uvenile.

several days on nutrients gained from absorption of the

yolk sac . This stage is followed by a transition to an ac­

tively feeding stage (usually on microinvertebrates)

termed a postlarva . Eventually, postlarvae transform

into small fishes , more or less resembling adults ,

termed "juveniles . " The time of early development

from spawning through hatching and transition to the

postlarval stage ranges from a few days to several

weeks and , in general , is inversely related to water tem-

52 The Fishes of Tennessee

peratures . These early l ife"stages of a fish lead a very

tenuous existence , and a large percentage of fishes fall

victim to predation or other perils during this period and

thus never reach adulthood . References of particular in­

terest on the early development of fishes are Fish

( 1 932), Hogue et al . ( 1 976), S nyder ( 1 983) , Moser et

al . ( 1 984), and the series of volumes in preparation by

Wallus et al . (e . g . , 1 990) on Ohio River drainage larval

fishes .