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Fish Biology
Fish are cold-blooded aquatic animals with backbones, gills, and fins. Most fishes are torpedoshaped (fusiform) for efficient travel through water, but much variation occurs, from flattened and
rounded, as in flounders, to vertical and angular, as in sea horses. Fishes range in size from the pygmy
goby, Pandaka pygmaea, of the Philippines, which reaches only 12 mm (0.5 in) long and about 1.5 g (0.05
oz) in weight and is sexually mature at 6 mm (0.25 in), to the whale shark, Rhincodon typus, which grows
to 18 m (60 ft) long and over 20 tons in weight.
Fish Distribution
Fish are found throughout the world, from altitudes of more than 5,000 m, as in Lake Titicaca,
located 3,800 m above sea level in the Andes, to depths of about 10 km in the Pacific Ocean. Some, like
certain killifishes, Cyprinodon, inhabit hot springs, where the water temperature may reach 45 deg C;
others, like the icefishes, Chaenocephalus, are found in Antarctic seas, where water temperature may
fall below 0 deg C. About 107 species, including the swordfish, Xiphias, are distributed worldwide in
tropical and subtropical waters, but many species have very limited ranges, among the smallest being
that of the killifish Cyprinodon diabolis, which is confined to a single spring in Nevada.
About 70% of the Earth's surface is covered by oceans and seas, and about 3.5% of the land
surface (1% of the Earth's total surface) is covered by fresh water. Inhabiting these waters are an
estimated 20,000 or more fish species, equal to or exceeding the number of all other vertebrate
species combined. Bird species number approximately 8,600; reptiles, 6,000; mammals, 4,500; and
amphibians, 2,500. About 60% of the fish species live in marine waters; the remaining 40% are found in
fresh water.
Most of the world's fishes are continental in orientation, living either as part of the freshwater
systems on land or as sea-dwellers staying near and influenced by the coastal environment. High
densities of marine fish populations occur near coasts, because the waters there are extremely rich in
nutrients. Coastal benefits include chemical and organic enrichment discharged by rivers, upwellings
from the ocean depths that recycle previously deposited nitrates and phosphates, aeration caused by
surf and tide, and the penetration of sunlight.
Fish Anatomy
The living species of fish are usually divided into three classes: the Agnatha, the jawless fishes,
comprising the hagfishes and lampreys; the Chondrichthyes, the cartilaginous-skeleton fishes, such as
sharks and rays; and the Osteichthyes, the bony-skeleton fishes, comprising all other living fishes. The
skeletons of these three groups vary in fundamental ways. In the hagfishes and lampreys the backbone
is basically a notochord, a rodlike structure composed of unique notochordal tissue. In sharks and rays
the notochord is surrounded and constricted by spaced rings of cartilage, the vertebrae, to form a
backbone. The remainder of the skeleton is also cartilaginous, not bony, but in many forms the cartilage
is partly calcified, and thereby hardened, by the addition of calcareous salts. In primitive bony fishes,
such as the sturgeon, the vertebrae spaced along the notochord are still largely cartilaginous, but in
most advanced bony fishes the vertebrae are bony and are united to form the backbone, and the
notochord is no longer present.
The body appendages of fish are fins. Fins are either median or paired. Median fins are
situated along the centerline of the body, at the top, the bottom, and the end. The top, or dorsal, fin
may consist of one to several fins, one behind the other, and may include a fleshy fin, called the adipose
fin, near the tail. The bottom, or anal, fin is located on the belly behind the vent, or anus. The end fin is
called the tail, or caudal fin.
The paired fins correspond to the arms and legs of land vertebrates. The pectoral fins are
situated at the front of the body behind the gill openings and generally function to provide
maneuverability, but may be highly modified to fulfill other functions. The pelvic fins, also called the
ventral fins, are located along the bottom of the body but vary considerably in their placement. They
may be located in the middle of the belly, as in salmon; below the pectorals, as in the largemouth bass;
or in front of the pectorals, as in cods. Pelvic fins also serve as maneuvering structures and also may be
modified to serve other uses.
The scales of fish are colorless; a fish's coloring arises from structures beneath or closely
associated with the scales. Not all species of fishes have scales, or the scales may be so small as to
make the fish appear scaleless. Scales also may be present only on small areas of the body. The
arrangement of scales may be imbricate (overlapping like the shingles on a roof) or mosaic (fitting
closely together or just minutely separated).
Four basic scale types can be distinguished on the basis of structure. Placoid scales, also called
dermal denticles, are found on sharks and rays and are toothlike in structure. Indeed, modified and
enlarged placoid scales have become the teeth of sharks. Placoid scales do not increase in size as do the
scales of bony fishes, and new scales must be added as a shark grows.
Cosmoid scales are found on the primitive coelacanth. They also occur on lungfishes, but in a
highly modified, single-layered form. The cosmoid scale of the coelacanth is a four-layered bony scale.
Ganoid scales, as found on gars, are typically squarish (rhombic) in shape and consist of a single
bony layer, a layer of cosmine, and a covering of a very hard enamellike substance called ganoin.
Leptoid scales are believed to have been derived from ganoid scales by the loss of the ganoin
layer; they consist of a single layer of bone. Leptoid scales are found on the higher bony fishes and
occur in two forms: cycloid (circular) and ctenoid (toothed), the latter bearing tiny comblike projections.
Fish Circulation
The blood of the fish serves, as does the blood of other vertebrates, to transport oxygen,
nutrients, and wastes. The typical fish's circulation is a single circuit: heart-gills-body-heart. In
contrast, mammals have two circuits: heart-lungs-heart and heart-body-heart. The fish heart proper is
two-chambered, consisting of an upper atrium and a lower ventricle. Amphibians, basically, have a threechambered heart, two atria and one ventricle; reptiles have a three- or four-chambered heart; and
mammals and birds have a four-chambered heart consisting of two atria and two ventricles.
Fish Respiration
In order to live, fish must extract oxygen from the water and transfer it to their bloodstream.
This is done by gills, lungs, specialized chambers, or skin, any of which must be richly supplied with blood
vessels in order to act as a respiratory organ. Extracting oxygen from water is more difficult and
requires a greater expenditure of energy than does extracting oxygen from air. Water is a thousand
times more dense (heavier per unit volume) than air; it has 50 times more viscosity (resistance to flow)
than air, and contains only 3% as much oxygen as an equal volume of air. Fishes, therefore, have
necessarily evolved very efficient systems for extracting oxygen from water; some fishes are able to
extract as much as 80% of the oxygen contained in the water passing over the gills, whereas humans can
extract only about 25% of the oxygen from the air taken into the lungs.
Gills are made efficient in a number of ways. (1) A large surface area for gaseous exchange means
that more oxygen can enter the bloodstream over a given period of time. The surface area of the gills is
commonly 10 to 60 times more than that of the whole body surface. (2) A short diffusion, or travel,
distance for the oxygen increases the rate of oxygen entry into the blood. The blood traveling in the
folds of the gill filaments is very close to the oxygen-containing water, being separated from it by a very
thin membrane. (3) By using countercurrent circulation in the gill, the blood in the filament folds travels
forward, in the opposite direction to the water flow, so that a constant imbalance is maintained between
the lower amount of oxygen in the blood and the higher amount in the water, ensuring passage of oxygen
to the blood. (4) Water flows continuously in only one direction over the gills, as contrasted with the
interrupted, two-way flow of air in and out of lungs of mammals.
Fish Body Temperature
Fish are described as cold-blooded, meaning that their body temperature varies with the
external temperature. Fish do, however, produce metabolic heat (that is, heat derived from the
oxidation, or "burning," of food and from other processes), but much of this heat is lost to the outside
at the gills. Blood passing through the gills loses heat to the water quite rapidly, so that a fish's body
temperature is usually within a degree or so of the water temperature. Tunas and mackerel sharks,
however, are warm-bodied fishes. They have evolved countercurrent circulatory networks that consist
basically of paired ingoing and outgoing blood vessels. In this way the heat of the warm blood going to
the gills is transferred to the cooled blood coming from the gills, and the heat is kept within the fish's
body. By using these networks, yellowfin and skipjack tuna are able to keep their body temperature from
about 5 deg to almost 12 deg C above the water temperature.
One of the advantages of warm-bodiedness is an increase in muscle power. Muscles contract
more rapidly when warm without loss of force. If with a 10 C degree rise in body temperature a muscle
can contract three times as fast, then three times the power is available from that muscle. More muscle
power means more speed in pursuing prey, escaping enemies, and shortening the time required for longdistance migration.
Fish Water Balance
The blood of freshwater fishes is typically more salty than the water in which they live.
Osmotic pressure, the force that tends to equalize differences in salt concentrations, causes water to
diffuse, or enter, into the fish's body, primarily through the gills, mouth membranes, and intestine. To
eliminate this excess water, freshwater fishes produce a large amount of very dilute urine. Lampreys,
for example, may daily produce an amount of urine equal to as much as 36% of their total body weight;
bony fishes commonly produce amounts of urine equaling from 5 to 12% of their body weight per day. As
these fishes are gaining water, they are losing salts. Salts contained in their foods are insufficient to
maintain the proper salt balance. Freshwater fishes have therefore developed the capacity to absorb
salts from water by means of their gills.
Marine bony fishes, in contrast, have blood that is less salty than sea water, and consequently
they lose water and absorb salts. To offset this loss of fluid, marine fishes drink seawater and produce
very little urine. The drinking of seawater, however, adds to the concentration of salts. These salts are
eliminated in several ways. Calcium, magnesium, and sulfates are passed out through the anus along with
wastes. Sodium, potassium, chloride, and nitrogenous compounds, such as urea, are excreted through the
gills.
The hagfishes and the sharks have approached the problem of fluid balance in other ways.
Hagfish blood has a total salt concentration approximately equal to that of seawater. Sharks' gills do
not excrete the nitrogenous waste product urea, retaining it instead in the blood. The presence of urea
and another waste product, trimethylamine oxide, as well as various salts, keeps the shark's blood at a
slightly higher solute concentration than that of seawater.
Fish Gas Bladder
Because a weightless, or buoyant, body requires a minimum of energy to keep it at a given depth,
and because a weightless body requires less energy than a weighted body to move at a given speed, many
fishes have evolved means of reducing their body weight, or density, relative to the density of water. A
fish whose total body density equaled that of water would be effectively weightless, neither rising nor
sinking. Because fat is less dense than water, one method of reducing body density would be to increase
the proportion of fat within the body. Theoretically, about one-third of a fish's body weight would have
to be made up of fat in order to make the fish weightless in seawater. This condition is approached in
some species of deep-sea sharks having very large livers that contain a great amount of squalene, a
fatty substance that is significantly less dense than seawater.
Another method of reducing density is to include gases within the body. Many fishes have a gasfilled bladder that serves this function. The gases within the bladder are similar to those in air but are
present in different and widely varying proportions. The degree of body volume that must be taken up
by gas in order to achieve weightlessness depends mainly upon whether the fish is freshwater or marine.
Fresh water is less dense than seawater and consequently provides less buoyancy. Freshwater fishes,
therefore, require a larger gas bladder than do marine fishes to keep them from sinking. In actual
measurements, freshwater fishes' gas bladders have been found to range from 7 to 11% of body volume,
while those of marine fishes have ranged from 4 to 6% of body volume.
If the gas bladder contained an unchanging quantity of gas, the fish possessing it would be
weightless at only one depth. The quantity of gas within a fish's bladder must therefore be adjustable.
If, as in the carp, the gas bladder is connected by a duct to the gullet, gas may be expelled through the
mouth and gill cavities as the fish rises, and, in a similar manner, gas may be added to the bladder by
swallowing air at the water surface.
For most fishes, however, coming to the surface to gulp air prior to going deeper is impractical,
and in many fishes the gas bladder has no connection to the outside. In these fishes there must be
another means of adjusting the quantity of gas within the bladder. This is done by transferring gases
from the gas bladder to adjoining blood vessels and back again.
Fish Lateral Line System
The lateral line system, found in many fishes and in some aquatic amphibians, is sensitive to
differences in water pressure. These differences may be due to changes in depth or to the currentlike
waves caused by approaching objects. The basic sensory unit of the lateral line system is the neuromast,
which is a bundle of sensory and supporting cells whose projecting hairs are encased in a gelatinous cap.
The nueromasts continuously send out trains of nerve impulses. When pressure waves cause the
gelatinous caps of the neuromasts to move, bending the enclosed hairs, the frequency of the nerve
impulses is either increased or decreased, depending on the direction of bending.
Neuromasts may occur singly, in small groups called pit organs, or in rows within grooves or canals,
when they are referred to as the lateral line system. The lateral line system runs along the sides of the
body onto the head, where it divides into three branches, two to the snout and one to the lower jaw.
A swimming fish sets up a pressure wave in the water that is detectable by the lateral line
systems of other fishes. It also sets up a bow wave in front of itself, the pressure of which is higher
than that of the wave flow along its sides. These near-field differences are registered by its own
lateral line system. As the fish approaches an object, such as a rock or the glass wall of an aquarium, the
pressure waves around its body are distorted, and these changes are quickly detected by the lateral line
system, enabling the fish to swerve or to take other suitable action. Because sound waves are waves of
pressure, the lateral line system is also able to detect very low-frequency sounds of 100Hz or less.
Fish Reproduction
Most fishes are egg-layers, but many bear living young. Live-bearing fishes may be ovoviviparous,
in which the eggs essentially simply hatch within the female, or viviparous, in which the unborn young are
supplied nourishment through the mother's tissues.
In live-bearing fishes and in some egg-layers, fertilization occurs internally, and methods have
been evolved for introducing the sperm into the female's body. In sharks the pelvic fins of the male are
modified into intromittent organs called myxoptergia, and in the male topminnows the anal fin is
modified into a similar-functioning intromittent organ called the gonopodium.
At least three modes of reproduction--heterosexual, hermaphroditic, and parthenogenetic--are
found in fishes. In the most common form, heterosexual reproduction, there are separate male and
female parents, but even here there is considerable variation. In some live-bearing fishes, the female is
able to store sperm for up to 8 or even 10 months, and this sperm is used to fertilize new batches of
eggs as they develop. In some cases, a female may carry sperm from several males at once.
In hermaphroditic reproduction, a single fish is both male and female, produces both eggs and
sperm (either at the same time or at different times), and mates with other similar hermaphroditic
fishes. External self-fertilization occurs in one hermaphroditic fish, which sheds egg and sperm
simultaneously. In another, internal self-fertilization may occur. In certain fishes there is a time
sequence of hermaphroditism, young fishes reversing their sex as they grow older.
In parthenogenetic reproduction, unfertilized eggs develop into embryos. This is known to exist
in at least one fish species, Poecilia formosa, of the Amazon River; however, even though development
proceeds without fertilization in some of these females, mating with a male is still required to stimulate
egg development.
Parental care also shows great diversity. Some fishes, like the Atlantic herring, form huge
schools of males and females and freely shed their eggs and sperm (milt), and then abandon the eggs.
Other fishes build nests and care for both the eggs and newly hatched young. Others have evolved
methods of carrying the eggs with them, commonly in their mouths, but also in gill cavities or in special
pouches on the body.
Taken from: http://www.lookd.com/fish/index.html