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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 1/20th the oxygen of air
Aquatic environment both shaped and constrained their evolution
II. Ancestry and Evolution A. Ancestor
Descended from free-swimming 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. CharacteristicsLack jaws, internal ossification, scales,
or paired limbsPore-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
sucker-like 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. OrdersCarcharhiniformes—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
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
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. OrderRajiformes—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
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 developsLung 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
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 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
Cycloid
Ctenoid
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 yawVery rigid body creates less yaw and a fast fishThe 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 FishesFreshwater 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. EelsCatadromous—develop in freshwater but spawn
in seawaterAdult 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