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Fishes
CHAPTER 16
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Diversity
Overview
• “Fish” has many usages extending beyond what are
actually considered fishes today (e.g., starfish, etc.)
• A modern fish
– Aquatic vertebrate with gills, limbs (if present) in the form
of fins, and usually with a skin covered in scales of dermal
origin
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Diversity
• Gills are efficient at extracting oxygen from water
that has 1/20 the oxygen of air
• Lateral line system detects water currents and
vibrations, a sense of “distant touch”
• Evolution in an aquatic environment both shaped
and constrained its evolution
• “Fish” refers to one or more individuals of one
species
• “Fishes” refers to more than one species
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Ancestry and Relationships of Major Groups of Fishes
• Agnathans
– Include extinct ostracoderms and living hagfishes and
lampreys
• Hagfishes lack vertebrae
• Lampreys have rudimentary vertebrae
– Included in subphylum Vertebrata because they have a
cranium and other vertebrate homologies
– Remaining fish have paired appendages and join tetrapods
as a lineage of gnathostomes
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Ancestry and Relationships of Major Groups of Fishes
• Gnathostomes
• Cartilaginous Fishes
– Lost heavy dermal armor and adopted skeleton of cartilage
• Bony Fishes
– Dominant fishes today
– Two distinct lineages
• Ray-finned fishes
• Lobe-finned fishes
– Ray-finned fishes radiated to form modern bony fishes
– Lobe-finned fishes are a relict group with few species
today
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Living Jawless Fishes
Overview
• Living jawless fishes include hagfishes and lampreys
– About 43 species of hagfishes are known and about 41
species of lamprey are described
– Members of both groups lack jaws, internal ossification,
scales, or paired fins
– Both groups share porelike gill openings and an eel-like
body
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Living Jawless Fishes
Class Myxini: Hagfishes
• Entirely marine
• Scavengers and predators of annelids, molluscs, dead
or dying fishes, etc.
• Enters dead or dying animal through orifice or by
digging inside using keratinized plates on tongue
• Nearly blind
– Locate food by an acute sense of smell and touch
• To provide leverage
– Ties knot in tail and passes it forward to press against prey
• Special glands along body secrete fluid that becomes
slimy in contact with seawater
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Living Jawless Fishes
Class Petromyzontida: Lampreys
• Diversity
– All lampreys in Northern Hemisphere belong to the family
Petromyzontidae
– Marine lamprey Petromyzon marinus
• Occurs on both Atlantic coastlines
• Grows to a length of one meter
– 20 species of lampreys in North America
• Half belong to nonparasitic brook-dwelling species
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Living Jawless Fishes
• Parasitic Lampreys
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Marine, parasitic lampreys migrate to the sea
Other species remain in freshwater
Attach to a fish by a sucker-like mouth
Sharp teeth rasp through flesh as they suck fluids
Inject anticoagulant into a wound
When engorged, lamprey drops off but wound may be fatal
to fish
– Parasitic freshwater adults live 1–2 years before spawning
and dying
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Living Jawless Fishes
• Sea Lamprey Invasion of the Great Lakes Region
– No lampreys were in the U. S. Great Lakes west of Niagara
Falls until the Welland Ship Canal was deepened between
1913 and 1918
– Sea lampreys moved first through Lake Erie to Lakes Huron,
Michigan, and Superior
– Lampreys preferred lake trout
• Destroyed this commercial species along with overfishing
– Lamprey populations declined both from depletion
of food and control measures
• Chemical larvicides in spawning streams
• Release of sterile males
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Class Chondrichthyes: Cartilaginous Fishes
Overview
• Smaller and more ancient group
• Well-developed sense organs, powerful jaws, and
predaceous habits helped them survive
• True bone is completely absent throughout the class
• Phosphatized mineral tissues retained in teeth,
scales, and spines
• Nearly all are marine
– Only 28 species live primarily in freshwater
• After whales, sharks are the largest living vertebrates,
reaching 12 meters in length
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Class Chondrichthyes: Cartilaginous Fishes
Subclass Elasmobranchii: Sharks,
Skates and Rays
• Order Carcharhiniformes
– Contains the coastal tiger and bull sharks and the
hammerhead
• Order Lamniformes
– Contains large, pelagic sharks such as the white and mako
shark
• Order Squaliformes
– Contains dogfish sharks
• Order Rajiformes
– Contain skates
• Order Myliobatiformes
– Contains several groups of rays (stingrays, manta rays, etc.)
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Class Chondrichthyes: Cartilaginous Fishes
– In males, the medial part of the pelvic fin is modified to
form a clasper used in copulation
– Paired nostrils are anterior to mouth
– Lateral eyes are lidless
– Behind each eye is a spiracle
• Remnant of the first gill slit
– Tough, leathery skin with placoid scales
• Reduce water turbulence
– Detect prey at a distance by large olfactory organs
sensitive to one part per 10 billion
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Class Chondrichthyes: Cartilaginous Fishes
– Prey may also be located from long distances sensing low
frequency vibrations in the lateral line
– At close range, switch to vision
• Most have excellent vision even in dimly lit water
– Up close, sharks are guided by bioelectric fields that
surround all animals
– Electroreceptors, the ampullae of Lorenzini, are located on
the shark’s head
– Upper and lower jaws equipped with sharp, triangular teeth
that are constantly replaced
– Spiral valve in intestine slows passage of food and increases
absorptive area
– Rectal gland secretes NaCl
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Class Chondrichthyes: Cartilaginous Fishes
• Form and Function of Rays
– More than half of elasmobranchs are rays
– Most specialized for benthic life
– Dorsoventrally flattened body and enlarged pectoral fins
used to propel themselves
– Respiratory water enters through large spiracles on top of
the head
– Teeth adapted for crushing prey
• Molluscs, crustaceans, and sometimes small fish
– Stingrays
• Have whiplike tail with spines and venom glands
– Electric rays
• Have large electric organs on each side of head
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Class Chondrichthyes: Cartilaginous Fishes
Subclass Holocephali: Chimeras
• Small subclass
– Remnants of a line that diverged from the earliest shark
lineage
– 33 extant species
• Mouth lacks teeth
– Equipped with large flat plates for crushing food
– Upper jaw fused to the cranium
– Feed on seaweed, molluscs, echinoderms, crustaceans, and
fish
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Osteichthyes: Bony Fishes
Origin, Evolution and Diversity
• 3 features unite bony fishes and tetrapod
descendants
– Endochondral bone replaces cartilage during development
– Lung or swim bladder is present
• Evolved as an extension of gut
– Specialization of jaw musculature improved feeding
– Operculum increased respiratory efficiency
• Helped draw water across gills
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Class Actinopterygii: Ray-finned Fishes
• Ray-finned fishes, class Actinopterygii, radiated
to form modern bony fishes
• Lobe-finned fishes, class Sarcopterygii, include
lungfishes and the coelacanth
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Class Actinopterygii: Ray-finned Fishes
• Bichirs
– Have lungs & heavy ganoid scales
– 16 species of which all live in the freshwaters of Africa
• Chondrosteons
– 27 species of freshwater and anadromous sturgeons and
paddlefishes
– Dam construction, overfishing, and pollution have led to
their decline
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Class Actinopterygii: Ray-finned Fishes
• Neopterygians
• 2 surviving early neopterygians are the bowfin and gars
• Gars and bowfin gulp air and use the vascularized swim
bladder to supplement gills
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Class Actinopterygii: Ray-finned Fishes
• Teleosts
– Constitute 96% of all living fishes and half of all vertebrates
– Range from 10 millimeters to 17 meters long, and up to
900 kilograms in weight
– Survive from 5,200 meters altitude in Tibet to 8,000
meters below the ocean surface
– Some can live in hot springs at 44o C while others survive
under Antarctic ice at - 2o C
– Some live in salt concentrations three times seawater
– Others found swamps devoid of oxygen
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Class Actinopterygii: Ray-finned Fishes
Morphological Trends
• Heavy dermal armor replaced by light, thin, flexible
cycloid and ctenoid scales
– Some eels, catfishes, and others lost scales
– Increased mobility from shedding armor helps fish avoid
predators and improved feeding efficiency
• Fins changed to provide greater mobility and serve a
variety of functions
– Braking, streamlining, and social communication
• Jaw changed to increase suctioning and protrusion to
secure food
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Class Sarcopterygii: Lobe-finned
Fishes
• Lungfish
– Australia lungfishes, unlike close relatives, rely on gill
respiration and cannot survive long out of water
– South American and African lungfish can live out of water
for long periods of time
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Class Sarcopterygii: Lobe-finned
Fishes
• The Coelacanth
– Thought to be extinct 70 million years, a specimen was
dredged up in 1938
– More were caught off coast of the Comoro Islands and in
Indonesia
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Structural and Functional Adaptations of Fishes
Locomotion in Water
• Speed
– Most fishes swim maximally at ten body lengths per second
• Larger fish therefore swims faster
– Short bursts of speed are possible for a few seconds
• Mechanism
– Trunk and tail musculature propels a fish
– Muscles are arranged in zigzag bands called myomeres
• Have the shape of a W on the side of fish
• Internally the bands are folded and nested
• Each myomere pulls on several vertebrae
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Structural and Functional Adaptations of Fishes
• Economy
– Swimming is the most economical form of motion because
water buoys the animal
– Energy cost per kilogram of body weight for traveling one
kilometer is 0.39 Kcal for swimming, 1.45 Kcal for flying,
and 5.43 for walking
– Yet to be determined how aquatic animals can move
through water with little turbulence
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Structural and Functional Adaptations of Fishes
Neutral Buoyancy and the Swim Bladder
• Fish are slightly heavier than water
• To keep from sinking, a shark must continually move
forward
– Fins keep it “angled up”
• Shark liver has a special fatty hydrocarbon, or
squaline, that acts to keep the shark a little buoyant
• Swim bladder, as a gas-filled space, is the most
efficient flotation device
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Structural and Functional Adaptations of Fishes
Respiration
• Gills are inside the pharyngeal cavity and covered
with a movable flap, the operculum
• Operculum protects delicate gill filaments and
streamlines body
• Pumping action by operculum helps move water
through gills
• Although it appears pulsatile, water flow over gills is
continuous
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Structural and Functional Adaptations of Fishes
Osmotic Regulation
• Freshwater is hypotonic to fish blood
– Water enters body and salt is lost by diffusion
– Scaled and mucous-covered body is mostly impermeable,
but gills allow water and salt fluxes
– Opisthonephric kidney pumps excess water out
– Special salt-absorbing cells located in epithelium actively
move salt ions from water into fishes’ blood
– Systems are efficient
• Freshwater fish devote little energy to maintaining
osmotic balance
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Structural and Functional Adaptations of Fishes
• About 90% of bony fishes are restricted to either
freshwater or seawater habitats
• Euryhaline fishes live in estuaries where salinity
fluctuates throughout day
• Marine bony fishes
– Blood is hypotonic to seawater
– Tend to lose water and gain salt
• Risks “drying out”
– To compensate for water loss, a marine teleost drinks
seawater
• Brings in more unneeded salt
• Carried by blood to gills and secreted by special saltsecretory cells
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Structural and Functional Adaptations of Fishes
Feeding
• Intestine tends to be shorter in carnivores and long
and coiled in herbivores
• Stomach primarily stores food
• Intestine digests and absorbs nutrients
• Teleost fishes have pyloric ceca
– Apparently for fat absorption
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Structural and Functional Adaptations of Fishes
Migration
Eels
• Have presented a life history puzzle for centuries
• Catadromous, developing to maturity in freshwater
but migrating to sea to spawn
• In fall, adults swim downriver to the sea to spawn,
but none return
• In spring, many young eels or “elvers” appear in
coastal waters and swim upstream
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Structural and Functional Adaptations of Fishes
• Minute larvae journey back to the streams of Europe
and North America
• American eel larvae make trip back in 8 months since
the Sargasso Sea is much closer to American
coastline, whereas the European eel larvae take
three years
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Structural and Functional Adaptations of Fishes
Homing Salmon
• Salmon are anadromous
– Grow up in sea but return to freshwater to spawn
• 6 species of Pacific salmon and 1 Atlantic salmon
migrates
• Atlantic salmon makes repeated spawning runs
• Pacific species spawn once and die
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Structural and Functional Adaptations of Fishes
Reproduction and Growth
• Most fishes are dioecious with external fertilization
and external development
• Guppies and mollies represent ovoviviparous fish
that develop in ovarian cavity
• Some sharks are viviparous with some kind of
placental attachment to nourish young
• Most oviparous pelagic fish lay huge numbers of eggs
– Female cod may release 4–6 million eggs
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