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Diversity
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“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
 Fishes
 In an evolutionary sense, can be defined as all vertebrates that are not tetrapods
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Diversity
 Common ancestor of fishes is also an ancestor of land vertebrates
 Approximately 24,600 living species
 Include more species than all other vertebrates combined
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Adapted to live in medium 800 times denser than air
Can adjust to the salt and water balance of their environment-osmotic balance

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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” and used in schooling
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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
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Earliest fish-like vertebrates: Group of agnathan fishes
Agnathans “jawless”
 Include extinct ostracoderms and living hagfishes and lampreys
 Hagfishes lack vertebrae
 Lampreys have rudimentary vertebrae
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Ancestry and Relationships of Major Groups of Fishes
 Gnathostomes “jawed”
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Appear in the Silurian fossil record with fully formed jaws
No intermediate forms between agnathans are known
The Devonian is called the Age of Fishes
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Ancestry and Relationships of Major Groups of Fishes
 Cartilaginous Fishes
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Lost heavy dermal armor and adopted skeleton of cartilage
Flourished during the Devonian and
Carboniferous
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Almost became extinct at end of the Paleozoic
Increased in numbers in the early Mesozoic, diversifying to form a modern shark assemblage
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Ancestry and Relationships of Major Groups of Fishes
 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
 Include the sister group of the tetrapods
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Living Jawless Fishes
 Living jawless fishes include hagfishes and lampreys
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Members of both groups lack jaws, internal ossification, scales, or paired fins
Both groups share porelike gill openings and an eel-like body
Hagfish
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Living Jawless Fishes
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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
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Living Jawless Fishes
 Reproduction of Hagfishes
 Copenhagen Academy of Science offered a prize over a century ago for information on hagfish breeding habits……… it remains unclaimed
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In some species, females outnumber males by
100 to 1
Females produce small numbers of large, yolky eggs 2-7 centimeters in diameter
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Living Jawless Fishes
 Diversity
 Marine lamprey
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 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
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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
Anadromous forms live 2 –3 years
Nonparasitic lampreys do not feed
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Digestive tract degenerates as an adult
They spawn and die
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Living Jawless Fishes
 Sea Lamprey Invasion of the Great Lakes
Region
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No lampreys were in the U. S. Great Lakes west of
Niagara Falls until the Welland Ship Canal was deepened between 1913 and 1918
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
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Class Chondrichthyes: Cartilaginous Fishes
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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
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 coastal tiger and bull sharks and the hammerhead large, pelagic sharks such as the white and mako shark
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 dogfish sharks skates
 several groups of rays (stingrays, manta rays, etc.)
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Class Chondrichthyes: Cartilaginous Fishes
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Form and Function of Sharks
Sharks are among the most gracefully streamlined of fishes
 In males, part of the pelvic fin is modified to form a clasper used in copulation
 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
 Nostrils to each side of the hammerhead shark may improve stereo-olfaction
 Prey may also be located from long distances sensing low frequency vibrations in the lateral line
 Consists of neuromasts in interconnected tubes and pores on side of body
 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
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Class Chondrichthyes: Cartilaginous Fishes
 Electroreceptors, the ampullae of Lorenzini, are located on the shark’s head
 Upper and lower jaws equipped with sharp, triangular teeth (modified placoid scales) that are constantly replaced
 Spiral valve in intestine slows passage of food and increases absorptive area
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Class Chondrichthyes: Cartilaginous Fishes
 Reproduction and Development
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Fertilization is internal
Maternal support of embryo is variable
 Includes oviparous, ovoviviparous, and viviparous species
“Mermaid’s purse”
 capsule encasing eggs laid by some oviparous species
 Sharks
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Class Chondrichthyes: Cartilaginous Fishes
 Rays & Skates
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Most specialized for benthic life flattened body and enlarged pectoral fins used to propel themselves
Respiratory water enters through large spiracles on top of the head to prevent clogging of gills.
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
 Stingrays
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Class Chondrichthyes: Cartilaginous Fishes
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Small subclass
 Remnants of a line that diverged from the earliest shark lineage
 33 extant species
Fossil chimaeras
 First appeared in the Carboniferous, reached a zenith in the Cretaceous and early Tertiary, and then declined
 Mouth lacks teeth
 Equipped with large flat plates for crushing food
 Upper jaw fused to the cranium
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 Feed on seaweed, molluscs, echinoderms, crustaceans, and fish

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Osteichthyes: Bony Fishes
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In early to middle Silurian, a lineage of fishes with bony endoskeletons gave rise to a clade that contains 96% of living fishes and all living tetrapods
3 features unite bony fishes and tetrapod descendants
 Endochondral bone replaces cartilage during development
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Lung or swim bladder is present
 Evolved as an extension of gut
Have several cranial and dental characters unique to clade
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Osteichthyes: Bony Fishes
 2 major lineages
 Ray-finned fishes,modern bony fishes
 Lobe-finned fishes, lungfishes and the coelacanth
 Operculum increased respiratory efficiency
 Helped draw water across gills
 Gas-filled structure off the esophagus
 Helped in buoyancy and also in hypoxic waters
 In fishes that use these pouches for respiration
 Pouches are called lungs
 In fishes that use these pouches for buoyancy
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 Pouches are called swim bladders
 Specialization of jaw musculature improved feeding

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Ray-finned Fishes
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Heavy dermal armor replaced by light, thin, flexible cycloid and ctenoid scales
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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
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Ancestor of tetrapods
 Group of extinct sarcopterygian fishes called rhipdistians
Early sarcopterygians
 Had lungs, gills
 Had powerful jaws, heavy, enameled scales with a dentine-like material called cosmine, and strong, fleshy, paired lobed fins
Today, clade is represented by 8 fish species
 6 species of lungfishes and 2 species of coelacanths
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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
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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
 Young coelacanths born fully formed after hatching from eggs up to 9 cm in diameter
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Structural and Functional Adaptations of Fishes
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Speed
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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
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Trunk and tail musculature propels a fish
Muscles are arranged in zigzag bands called myomeres
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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
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 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
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Fish are slightly heavier than water
To keep from sinking, a shark must continually move forward
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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
 Fish, like other vertebrates, detect sounds as vibrations in the inner ear.
 Weberian ossicles
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Structural and Functional Adaptations of Fishes
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Fish gills are filaments with thin epidermal membranes folded into plate-like lamellae
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
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Water flow is opposite to the blood flow
 Countercurrent exchange maximizes exchange of gases
Some bony fishes remove 85% of the oxygen from water passing over gills
Some active fishes use ram ventilation
 Forward movement is sufficient to force water across gills
 Such fishes are asphyxiated in a restrictive aquarium even if the water is saturated with oxygen
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Structural and Functional Adaptations of Fishes
 Fishes Out of Water
 Lungs of lungfishes allow them to respire in air
 Eels can wriggle over land during rainy weather
 Use skin as major respiratory surface
 A bowfin uses gills at cooler temperatures and lung-like swim bladder at higher temperatures
 Electric eel has degenerate gills and gulps air through vascular mouth cavity
 Indian climbing perch spends most of its time on land, breathing air in special chambers
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Structural and Functional Adaptations of Fishes
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Freshwater is hypotonic to fish blood
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Water enters body and salt is lost by diffusion
Scaled and mucous-covered body is mostly impermeable, but gills allow water and salt fluxes
Freshwater fishes are hyperosmotic regulators
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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
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About 90% of bony fishes are restricted to either freshwater or seawater habitats
Marine bony fishes are hypoosmotic regulators
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Blood is hypotonic to seawater
Tend to lose water and gain salt
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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 salt-secretory cells
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Structural and Functional Adaptations of Fishes
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Fish devote most of their time searching for food to eat and eating
With the evolution of jaws
 Fish left a passive filter-feeding life and entered a predator-prey battle
Most fish are carnivores
 Feed on zooplankton, insect larvae, and other aquatic animals
Most fish do not chew food
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Would block water flow across the gills
Blue fin tuna
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Structural and Functional Adaptations of Fishes
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A few can briefly crack prey items with teeth, or have molar-like teeth in throat
Most swallow food whole
 Easy with water pressure that sweeps food in when the mouth opens
Some are herbivores
 Eat plants and algae
 Crucial intermediates in food chain
Suspension feeders
 Crop abundant microorganisms of the sea
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Structural and Functional Adaptations of Fishes
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Many plankton feeders swim in large schools and using gill rakers to strain food
Omnivores feed on both plants and animals
Scavengers feed on organic debris
Detritovores consume fine particulate organic matter
Parasitic fishes suck body fluids of other fishes
Digestion follows the vertebrate plan
 A few lack stomachs
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Structural and Functional Adaptations of Fishes
 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
Homing Salmon
 Salmon are anadromous
 Grow up in sea but return to freshwater to spawn
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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
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Pacific species of sockeye salmon
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Migrates downstream
Roams the Pacific for 4 years
Returns to spawn in headwaters of parents’ stream
Young fish are imprinted on odor of their stream
May navigate to stream mouth by sensing magnetic field of the earth or angle of the sun, and then smelling their way home
Salmon are endangered by stream degradation from logging, pollution and hydroelectric dams
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Structural and Functional Adaptations of Fishes
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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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Structural and Functional Adaptations of Fishes
 Near-shore and bottom-dwelling species lay larger, typically yolky, nonbuoyant and adhesive eggs
 Some bury eggs
 Many attach eggs to vegetation
 Some incubate eggs in their mouth
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Structural and Functional Adaptations of Fishes
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Many benthic spawners guard eggs
 Male is usually the guard
Freshwater fishes produce nonbuoyant eggs
 The more care provided, the fewer the eggs produced
Freshwater fishes may have elaborate mating dances before spawning
Egg soon takes up water, the outer layer hardens and cleavage occurs
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Structural and Functional Adaptations of Fishes
 Blastoderm develops and yolk is consumed
 Fish hatches carrying a semitransparent yolk sac to supply food until it can forage
 Change from larva to adult may be dramatic in body shape, fins, color patterns, etc.
 Growth is temperature dependent
 Warmer fish grow more rapidly
 Annual rings on scales, otoliths, etc. reflect seasonal growth cycles
 Most fish continue to grow throughout life
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