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Fishes
Chapter 24
I. Diversity
 26,000 living species—more species than all
other vertebrate groups combined
 Adapted to live in a medium 800 x more dense
than air
 Can adjust to salt and water balance of
environment
 Gills extract oxygen from water that has 1/20 th
the oxygen of air
 Aquatic environment both shaped and
constrained their evolution
II. Ancestry and Evolution
 A. Ancestor
 Descended from freeswimming protochordate
ancestor
 B. Agnathans
 Earliest fish-like vertebrates
 Include extinct
ostracoderms, and living
lampreys and hagfishes
 C. Placoderms
 Fish with paired appendages
and jaws that went extinct in
Carboniferous with no living
descendants
D. & E.
 D. Cartilaginous Fishes
 Lost heavy armor and
adopted cartilage as
skeleton
 Flourished during some
periods, becoming nearly
extinct during others
 E. Acanthodians
 Resemble bony fish but have
heavy spines on all but
caudal fin; sister group to
bony fishes
 Went extinct in lower
Permian
F. Bony Fishes
 Dominant fishes today
 2 distinct lineages—ray finned and lobe finned
 Ray-finned radiated to form modern bony fishes
 Lobe-finned include lungfishes, the coelacanth,
and are sister group to tetrapods (amphibian
ancestors)
III. Superclass Agnatha: Jawless Fishes
 A. Characteristics
Lack jaws, internal ossification, scales, or
paired limbs
Pore-like gill openings and eel-like body
B. Class Myxini: Hagfishes
 43 species
 Entirely marine
 Scavengers and predators of
annelids, molluscs, dead or
dying fishes, etc
 Nearly blind but locates food by
acute sense of smell
 Rasps hole into prey then eats
from inside out with plate-like
tongue
 Glands secrete substance that
becomes slimy in contact with
seawater
C. Class Cephalaspidomorphi
 1. Diversity
 41 species; 22 in North
America and of these,
half are non-parasitic
brook variety
 Marine lampreys can
grow to length of 1 m.
 All lampreys reproduce
in freshwater streams,
dying soon after
2. Parasitic Lampreys
 Attach to fish by suckerlike mouth and sharp
teeth rasp away flesh
 Anticoagulant injected
into wound to stimulate
flow of blood
 Wound may be fatal to
host fish
 Non-parasitic lampreys do
not feed; digestive system
degenerates and fish die
after reproducing, within
2-3 years
3. Sea Lamprey Invasion
 No lampreys existed in
Great Lakes prior to 1829
when shipping canals
were built
 By the 1940’s, they
existed in all the lakes
 They decimated almost all
fish species until
populations finally
declined due to lack of
food and control
measures
IV. Class Chondrichthyes
 A. Overview
 850 species nearly all marine; 28 species live in freshwater
 Ancient lineage but have survived due to well-developed sense
organs and powerful jaws making them successful predators
 Largest living vertebrates, after whales, reaching up to 12 m in
length
Whale
shark
reaches
43’ in
length
B. Subclass Elasmobranchii
 1. Sharks
A. Orders
Carcharhiniformes—tiger and bull sharks
which are coastal sharks and the
hammerhead
Lamniformes-- white and mako sharks
which are large pelagic sharks
Squaliformes—some of these are deep sea
dwellers like dogfish sharks
Orectolobiformes—carpet sharks like
bamboo, nurse, and whale sharks
Sharks
More Sharks
b. Outer Physiology
 Streamlined fusiform body
shape
 Pointed nose with paired
nostrils in front of ventral
mouth; on hammerhead,
nostrils on ends of “hammer”
 Lateral eyes without lids
 Tail has longer upper lobe
(heterocercal)
 Paired pectoral and pelvic fins,
1-2 dorsal fins, 1 caudal fin,
and sometimes an anal fin
 Tough, leathery skin with
placoid scales that reduce water
turbulence
Body Structures of a Shark
c. Senses
 Olfactory organs can detect
chemicals diluted 1/10 billionth
their original concentration
 Lateral line senses low
frequency vibrations of prey
over large distances
 Excellent vision, even in dim
water, used at close range
 At close range, sharks are
guided to prey by electric fields
surrounding all animals
d. Inner Physiology
 Sharp triangular teeth in upper
and lower jaws; arranged in
rows that are constantly
replaced
 Mouth leads to pharynx with
openings to gill slits
 Osmoregulation accomplished
by rectal gland which secretes
sodium chloride; nitrogenous
compounds are also retained in
blood to increase solute
concentrations, making more
on par with seawater
e. Shark Attacks
 Only 32 species ( of 350) have been
documented to attack humans with
another 36 considered potentially
dangerous; these typically are the larger
size sharks; 80 % of sharks are harmless
 Great white, tiger, and bull sharks are the
more aggressive species
 50-75 attacks occur each year, with 8-12
fatalities; in contrast 30-100 million
sharks are killed every year
 Attacks usually occur by sandbars, steep
drop offs, or by river inlets and are
associated with mistaken
identity,territorial behavior, or feeding
behavior
2. Rays
 A. Order
Rajiformes—skates, sawfish rays, electric rays,
stingrays, manta rays and others
Make up half of all species of Elasmobranchii
b. Form and Function
 Specialized for benthic life
 Flattened dorsoventrally; enlarged pectoral fins
are used as swimming wings
 Water used in respiration enters large spiracles in
head
 Teeth adapted to act as rollers to crush
invertebrates and sometimes small fish
 Stingrays have whip-like tail with spines and
venom glands
 Electric rays have electric organs on sides of head
C. Subclass Holocephali: Chimeras
 31 species
 Ratfishes
 Diverged from earliest shark lineage
 Mouth has flat plates for crushing invertebrates;
also feeds on seaweed and small fish
D. Reproduction and Development
 Internal fertilization
 Oviparous sharks and rays lay an egg capsule
immediately after fertilization that attaches to kelp with
tendrils; may take up to 2 years before mini adult hatches
 Ovoviviparous sharks retain fertilized eggs in
reproductive system where they are nourished by yolk of
egg; “live” birth
 Viviparous sharks nourish embryos with maternal
bloodstream; “live” birth
 Live births make it more likely more of the young survive
but no other care is given after birth
Embryo Development
V. Superclass Osteichthyes
 A. Origin, Evolution, and Diversity
 Lineage developed in Silurian and now accounts for
96% of all fishes and all tetrapods
 Bone replaces cartilage as fish develops
 Lung or swim bladder evolved from an extension of
the gut; gas filled, it aids in buoyancy
 Bony operculum, a flap covering the gills that rotates
outward, draws water more efficiently over them
 Specialization of jaw musculature improves feeding;
also unique dental characters
B. Class Actinopterygii
 23,600 species comprise the ray-finned fishes
 Most familiar fish type
a. Palaeoniscids
 Earliest forms, existing from late Silurian to late
Paleozoic
 Small, large eyes, dorsal fin with bony rays,
heterocercal tail, and interlocking scales
 Survived as other fishes declined, suggesting
some adaptive advantage
 Gave rise to the chondrosteons and the
neopterygians
b. Chondrosteons
 Most primitive characteristics
 Heterocercal tail and ganoid scales
 Living species include sturgeons, paddlefishes,
and bichirs
c. Neopterygians
 One lineage gave rise to
modern bony fishes, the
teleosts
 Living species are bowfin
and gars which gulp air
and use vascularized swim
bladder to supplement the
gills
d. Teleosts
 96 % of all living fishes; half of all vertebrates
 10 mm to 17 m; up to 900 kg in weight
 Found at 5,200 m to 8,000 m below sea level
 Some can live in hot springs at 44 oC while others
can survive in Antarctic –2 oC.
 Some live in salt concentrations three times
seawater; others in swamps devoid of oxygen
2. Morphological Trends
Cycloid
Ctenoid
 Heavy armor replaced by light cycloid or
ctenoid scales which made fish more
mobile; some fish such as eels and catfish
have completely lost scales
 Fins changed to provide greater mobility
and serve a variety of functions: braking,
streamlining, and social communication
 Homocercal tail allowed greater speed and
buoyancy
 Swim bladder switched from primarily
respiratory to buoyancy in function
 Jaw changed to increase suctioning and
protrusion to secure food
C. Class Sarcopterygii
 1. Diversity
 Only 7 species alive today; 6
lungfishes and 1 coelacanth
 Early ones had lungs as well
as gills, heterocercal tail;
later tail became
symmetrical
 Skin covered in heavy scales
overlaid by an enamel
 Fleshy, paired lobes are
used to scuttle along bottom
 South American and African
lungfishes can survive out of
water or long periods of
time
2. Coelacanth
 Thought to have been
extinct for 70 million
years until one was
dredged up off of coast of
Africa in 1938
 More were caught off the
coast of the Comoro
Islands in 1998
VI. Structural and Function Adaptations
 A. Locomotion
 1. Mechanism
Trunk and tail muscles propel fish forward by
undulations
Large, rigid head minimizes yaw
Very rigid body creates less yaw and a fast fish
The largest fin is the tail or caudal fin for rapid
forward movement.
Dorsal fins on the top and anal fins underneath
assist with lateral stability.
Pectoral fins behind the gill covers (operculum)
assist with hovering and slow turning.
Pelvic fins are often small for open water swimmers
but larger on bottom dwellers which use them for
resting on.
2. Speed and Energy
 Larger fish swim faster
 Short bursts of speed are possible for a few
seconds
 Swimming is most economical means of motion
since water buoys the animal; swimming expends
0.30 Kcal, 1.45 Kcal for walking, and 5.43 Kcal
for flying
B. Swim Bladder
 Fish are slightly heavier than water
 A shark has a very fatty liver that makes it a little
buoyant; must also keep swimming to move it
forward and angle itself up
 Bottom dwelling fishes also lack swim bladder
 Fish can control depth by adjusting volume of
gas in swim bladder
 Gas gland removes or adds gases from blood to
remove or add gas to bladder
 Some fish gulp air to fill swim bladder
C. Respiration
 Gill filaments are folds of tissue inside the pharyngeal
cavity covered by the operculum
 Continuous water flow opposite blood flow through
capillaries maximizes gas exchange allowing some fish to
remove 85% of O2 from H2O
 Some fishes are dependent on ram ventilation as well, in
which forward movement pushes more water over gills;
such fish will die in an aquarium
 Lungfish use lungs; eels use skin; bowfin uses gills at low
temperatures and air bladder at higher temperatures;
electric eel has degenerate gills and must gulp air
D. Osmotic Regulation
 1. Freshwater Fishes
Freshwater has less salt than blood of fish so
water tends to enter fish’s cells and its salts
tend to leave
Hyperosmotic regulators: kidney pumps out
excess water and salt absorbing cells in skin
remove salts from water and add to blood
Euryhaline fishes live in estuary environments
where they are in contact with both fresh and
salt water
2. Marine Fishes
 Blood has lower salt content than surrounding
water so tend to lose water and gain salt
 Hypo-osmotic regulators: fish drinks water
bringing in more water but also salt; salt is
carried by blood to gills where it is secreted by
salt-secretory cells, some salt leaves in feces,
and others are excreted by kidneys
E. Feeding Behavior
 Most time devoted to searching for food and eating
 Most carnivores-feed on zooplankton, insect larvae, and
other aquatic animals
 Most don’t chew food since it would block flow of water
across gills; swallow food whole although a few have
teeth that crack prey or have some molars in throat
 Some herbivores--eat plants and algae
 Suspension feeders eat plankton, using gill rakers to
strain food; these fish swim in large schools
 Also have omnivores, scavengers, and parasites
 Stomach used for storage; intestines absorb and digest
nutrients
F. Migration
 1. Eels
 Catadromous—develop in freshwater but spawn in
seawater
 Adult eels spawn in Sargasso Sea at depths of 300 m.
 Larvae drift for 2 years before developing into elvers;
males remain in brackish water; females swim
hundreds of miles up rivers
 Females mature for 8-15 years before returning to the
sea ( 8 months to complete journey)
 American eels are separate species from European eels
2. Salmon
 Anadromous—living in sea but spawing in freshwater
 6 Pacific salmon species, and 1 Atlantic salmon species
that migrate
 Pacific species migrate downstream, live in Pacific for 4
years, and then return up the same stream it was
spawned in
 Young fish are imprinted with the odor of their stream
 Pacific salmon spawn and then die
 Endangered by stream degradation, logging, pollution,
and hydroelectric dams
G. Reproduction
 Most dioecious with external fertilization and
development
 Some are ovoviviparous where eggs develop in ovarian
cavity—sharks, guppies, mollies
 Oviparous marine fish lay large numbers of eggs,
upwards of several million
 Nearshore or bottom dwelling fish lay fewer, larger
nonbuoyant sticky eggs
 Some fish bury eggs, attach them to vegetation, incubate
them in their mouths
 Freshwater fish produce fewer, nonbuoyant eggs, and
more care is usually provided
 Many freshwater fish also have elaborate mating dances
before spawning
H. Growth
 Egg starts to take up water after it is laid, outer layer
hardens, and cell division begins
 Yolk is consumed during development
 Fish fry hatch carrying semitransparent yolk sac to supply
food until it can forage
 As fry change to adult, it may undergo dramatic changes
in body shape, fins, color patterns, etc
 Growth is temperature dependent; warmer fish grow
more rapidly
 Annual rings on scales reflect seasonal growth cycles
 Most fish continue to grow throughout life and do not
stop at maturity
Fish Development