Fishes
Chapter 24
Diversity
 “Fish” has many usages extending beyond
what are actually considered fishes today (e.g.,
starfish, etc.).
 Fishes do not form a monophyletic group.
 In an evolutionary sense, can be defined as all
vertebrates that are not tetrapods.
 Common ancestor of fishes is also an ancestor of
land vertebrates.
 Therefore in pure cladistics, would make land vertebrates
“fish.
 Approximately 24,600 living species.
 Adapted to live in medium 800 times denser than air.
 Can adjust to the salt and water balance of their
environment.
Diversity
 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.
Ancestry of Fishes
 Fishes have descended from an unknown freeswimming protochordate ancestor.
 Agnathans including ostracoderms.
 Gnathostomes derived from one group of
ostracoderms.
 Four groups of gnathostomes flourished during the
Devonian, two survive today.
Fossils of Early Vertebrates
 Armored, jawless
vertebrates called
ostracoderms
had defensive
plates of bone on
their skin.
 One group of
ostracoderms led
to the
gnathostomes.
Fossils of Early Vertebrates
 Placoderms, one group of early jawed fishes,
died out during the Carboniferous.
 Left no descendents.
Fossils of Early Vertebrates
 Another group, the acanthodians, were
common during the Devonian, but became
extinct during the Permian.
 They were distinguished by having heavy spines on
all fins except the caudal (tail) fin.
 Possible sister group of the bony fishes.
Fossils of Early Vertebrates
 A third group of gnathostomes, the
cartilaginous fishes (Class Chondrichthyes)
lost the dermal armor and uses cartilage rather
than bone for the skeleton.
 Sharks, skates, rays, chimaeras.
Fossils of Early Vertebrates
 The last group, the bony fishes, are the
dominant fishes today.
 Ray-finned fishes include most modern bony fishes.
 Lobe-finned fishes contain few living species.
 Includes sister group of tetrapods.
 Lung fishes & coelacanths.
Origins of Bone and Teeth
 Mineralization appears to have originated with
vertebrate mouthparts.
 The vertebrate endoskeleton became fully
mineralized much later.
Agnathans
 The least derived vertebrate lineages that still
survives are class Myxini, the hagfishes and
class Petromyzontida, the lampreys.
 They lack: jaws, internal ossification, scales, and
paired fins.
 Pore-like gill openings along the side of the body.
Class Myxini - Hagfish
 Entirely marine.
 Feeds on annelids, molluscs, crustaceans, &
dead or dying fishes.
 Predators or scavengers.
Class Myxini - Hagfish
 Hagfishes are jawless marine vertebrates that have a
cartilaginous skull and axial rod of cartilage derived
from the notochord.
 They lack vertebrae.
Class Myxini - Hagfish
 A hagfish can tie
itself in knots to
increase leverage
when burrowing into
a dead fish.
 Produces large
amounts of slime.
Class Petromyzontida - Lampreys
 Lampreys (Class Petromyzontida) are found
in fresh and saltwater.
 Lampreys have cartilaginous segments
surrounding the notochord and arching partly
over the nerve cord.
Class Petromyzontida - Lampreys
 All ascend
freshwater streams
to breed.
 Marine forms are
anadromous.
 Freshwater forms
move between
lakes & streams.
Class Petromyzontida - Lampreys
 Lamprey larvae are called ammocoetes.
 Larvae look much like amphioxus.
 Possess basic chordate characteristics in simplified
form.
 Suspension feeders.
Class Petromyzontida - Lampreys
 Many are parasitic
as adults.
 Those that are not,
do not feed as
adults.
Derived Characters of
Gnathostomes
 Gnathostomes have jaws that
evolved from skeletal supports
of the pharyngeal slits.
Derived Characters of
Gnathostomes
 Other characters common to gnathostomes
include:
 Enhanced sensory systems, including the lateral line
system.
 An extensively mineralized endoskeleton.
 Paired appendages.
Fossil Gnathostomes
 The earliest gnathostomes in the fossil record
are an extinct lineage of armored vertebrates
called placoderms.
Fossil Gnathostomes
 Another group of jawed vertebrates called
acanthodians radiated during the Devonian
period.
 Closely related to the ancestors of
osteichthyans (bony fishes).
Class Chondrichthyes
 Members of class Chondrichthyes have a
skeleton that is composed primarily of cartilage.
 The cartilaginous skeleton evolved
secondarily from an ancestral mineralized
skeleton.
Subclass Elasmobranchii
 The largest and most diverse subclass of
Chondrichthyes, Elasmobranchii, includes the
sharks and rays.
Subclass Elasmobranchii
 Most sharks have a streamlined body and are swift
swimmers.
 Heterocercal tail – the upper lobe of the tail is longer
than the lower.
 Placoid scales.
 The upper & lower jaws have a front, functional row of
teeth and several developing rows growing behind as
replacements.
Subclass Elasmobranchii
 Spiral valve in intestine slows passage of food
and increases absorptive area.
 Large fatty liver aids in buoyancy.
Subclass Elasmobranchii– Acute
Senses
 Prey is initially detected using large olfactory organs.
 Mechanorecptors in the lateral line system sense lowfrequency vibrations from far away.
 Vision is important at close range.
 Bioelectric fields surrounding their prey can be detected
using electroreceptors in the ampullae of Lorenzini on the
shark’s head.
Subclass Elasmobranchii
 All chondrichthyans have internal
fertilization.
 Oviparous species lay large yolky eggs
soon after fertilization.
 Some lay eggs in a capsule called a
“mermaid’s purse” that often have tendrils
to attach it to a some object.
Subclass Elasmobranchii
 Ovoviviparous species retain developing
young in the uterus while they are being
nourished by the yolk.
Subclass Elasmobranchii
 In viviparous species, young receive nourishment
from the maternal bloodstream through a placenta,
or from nutritional secretions produced by the
mother.
 Some receive additional nutrition by eating eggs &
siblings.
 Parental care ends as soon as eggs are laid or
young are born.
Subclass Elasmobranchii
 Skates and rays are specialized for bottom
dwelling with a flattened body and enlarged
pectoral fins.
 Gill openings on ventral surface.
 Water enters through spiracles on dorsal
surface.
Subclass Elasmobranchii
 Stingrays have a slender
whip-like tail with one or
more saw-edged spines
with venom glands at the
base.
 Electric rays have large
electric organs that can
discharge high-amperage,
low voltage current into the
surrounding water.
Subclass Holocephali
 A second subclass is composed of a few dozen
species of chimaeras, or ratfishes.
 Flat plates instead of teeth.
 Upper jaw fused to cranium.
Osteichthyes
 Osteichthyes are the bony fishes.
 Bone replaces the cartilage during development.
 A swim bladder is present for controlling buoyancy
and respiration in some.
 Not a monophyletic group.
Osteichthyes
 Fishes breathe by drawing water over four or
five pairs of gills located in chambers covered
by a protective bony flap called the operculum.
Class Actinopterygii
 Ray-finned fishes
(class
Actinopterygii)
contain all the
familiar bony fishes
– more than 23,600
species.
Class Actinopterygii
 The fins, supported mainly by long, flexible
rays are modified for maneuvering, defense,
and other functions.
Class Actinopterygii
 Two main groups of
ray-finned fishes.
 Chondrosteans
(e.g. sturgeons)
have heterocercal
tails and ganoid
scales.
Class Actinopterygii
 Neopterygians –
one lineage of early
neopterygians led
to the modern bony
fishes (teleosts).
 Early type
neopterygians
include the bowfin
and gars.
Class Actinopterygii
 The major lineage of neopterygians are teleosts,
the modern bony fishes.
 Changes in fins increased maneuverability and
speed.
 Symmetrical, homocercal, tail allows increased
speed.
Teleosts
 Thinner, lighter cycloid and ctenoid scales
replace the heavy dermal armor of primitive
ray-finned fishes. Some (e.g. eels) lack scales.
Teleosts
 Fins diversified for a
variety of functions:
camouflage,
communication,
complex
movements,
streamlining, etc.
Teleosts
 The swim bladder shifted purpose from
primarily respiratory to buoyancy.
 Gill arches in many diversified into pharyngeal
jaws for chewing, grinding, and crushing.
Class Sarcopterygii
 Lobed-finned fishes (class Sarcopterygii)
include 2 species of coelacanths and 6 species
of lungfishes.
 This group was much more abundant during the
Devonian.
 Rhipidistians are an extinct group of
sarcopterygians that led to tetrapods.
Class Sarcopterygii
 All early sarcopterygians had lungs as well as gills and
a heterocercal tail.
 Later sarcopterygians have a continuous flexible fin
around the tail.
 They have fleshy, paired lobed fins that may have been
used like legs to scuttle along the bottom.
Class Sarcopterygii
 Some lungfishes can live out of the water for long
periods of time.
 During long dry seasons, the African lungfish can
burrow down into the mud and secrete lots of slime
forming a hard cocoon where they will estivate until
the rains return.
Class Sarcopterygii
 Coelocanths arose
during the Devonian and
peaked (max. species)
in the Mesozoic.
 One genus, two species
currently.
 Believed to be extinct
for 70 million years,
rediscovered in 1938.
 The second species was
discovered in 1998.
Locomotion in Water
 Fishes use trunk and tail musculature to propel
them through the water.
 Musculature is composed of zigzag bands
called myomeres.
Locomotion in Water
 Flexible fishes like eels
use a serpentine
movement.
 Not very efficient for
high speed.
 Fast swimmers are
less flexible.
 Body undulations
limited to caudal
region.
Locomotion in Water
 Many fast swimmers are streamlined with
grooves so their fins can lie flat.
Buoyancy
 Sharks must move constantly to avoid sinking.
 The heterocercal tail provides lift as it moves from
side to side.
 Broad head and angled, stiff fins add lift.
 Their large livers with fatty hydrocarbons aid in
buoyancy as well.
 Liver is like a large sack of buoyant oil.
Buoyancy
 Bony fishes use a gas-filled space to regulate
buoyancy – the swim bladder.
 Derived from a pair of lungs.
 Swim bladders are absent in tunas, abyssal fishes,
many bottom dwellers.
 Bony fishes will sink without the swim bladder
because they are denser than water.
Buoyancy
 Fishes must be able to regulate gas inside the swim
bladder.
 At depth, the gas will compress and the fish will sink.
 As it rises to the surface, the gas will expand and the
fish will rise faster.
 Gas may be removed in two ways.
Buoyancy
 Physostomous fishes (more primitive, e.g. trout) have
a pneumatic duct that connects the swim bladder and
the esophagus.
 Air can be expelled through the duct.
 Gas must be secreted into the swim bladder from the
blood, although some species can gulp air to fill the
swim bladder.
Buoyancy
 Physoclistous fishes
(more derived, e.g.
advanced teleosts) the
pneumatic duct has been
lost. Gas must be
absorbed by blood from
the highly vascularized
ovale.
 Gas is secreted into the
swim bladder from the
blood at the gas gland.
Hearing
 The bodies of fishes are
nearly the same density as
water.
 Makes hearing difficult.
 Weberian ossicles, found
in minnows, suckers, &
catfish, improves hearing.
 Sound detection starts in
swim bladder (sound
vibrates easily in air) and
is transmitted to the inner
ear by Weberian ossicles.
Respiration
 Fish gills are composed of
thin filaments covered with an
epidermal membrane that is
folded into lamellae.
 Richly supplied with blood
vessels.
 Located inside the
pharyngeal cavity.
 Covered with an operculum
in bony fishes.
 Elasmobranchs have gill slits.
Respiration
 Water must be continuously pumped over the
gills.
 A countercurrent system is found where the
flow of water is opposite to the flow of blood.
 Deoxygenated blood encounters the freshest
water with the highest oxygen content.
Osmotic Regulation
 Freshwater fishes (hyperosmotic regulators)
must have a way to get rid of water that enters
their bodies by diffusion through the gills.
 Water enters the body, salts are lost by diffusion.
 Water is pumped out by the opisthonephric kidney
which can form very dilute urine.
 Salt absorbing cells in the gill actively move salt
from the water into the blood.
Osmotic Regulation
 Saltwater fishes (hypoosmotic regulators)
have a lower blood salt concentration than the
seawater.
 Tend to lose water and gain salts.
 Marine teleosts drink seawater.
 Salts are carried by the blood to the gills where they
are secreted out by salt-secretory cells.
 Other salts are voided with feces or excreted by the
kidney.
Feeding Behavior
 Most fishes are carnivores and
prey on everything from
zooplankton to large vertebrates.
 Some deep-sea fishes can eat
victims twice their size – an
adaptation to scarce food.
 Most fishes can’t chew with their
jaws (this would block water flow
over the gills), many have
pharyngeal teeth in their throats.
 Large-mouthed predators can
suck prey in by suddenly opening
their mouths.
Feeding Behavior
 Herbivorous fishes
eat plants and microalgae.
 Most common on
coral reefs –
parrotfishes,
damselfishes,
surgeonfishes.
 And tropical
freshwater habitats –
minnows, characins,
catfishes.
Feeding Behavior
 Suspension feeders filter microorganisms from the
water using gill rakers.
 Herring-like fishes are common – menhaden,
herring, anchovies etc.
 Many larval fishes.
 Basking sharks.
 Most are pelagic fishes that travel in large schools.
Feeding Behavior
 Other groups are scavengers that eat dead
and dying animals,
 Detritivores that consume fine particulate
organic matter,
 Parasites that consume parts of other live
fishes.
Migration
 Freshwater eels are catadromous, they enter
the ocean as adults, migrate to a spawning
area where they spawn & then die.
 Larvae make their way back to the streams –
only females enter the streams.
Migration
 Anadromous salmon
spend their lives at
sea, returning to
freshwater to spawn.
 Die after spawning.
 Strong homing instinct
brings them to their
parent stream.
 Guided by odor of
parent stream.
Reproduction
 Most fishes are
dioecious with external
fertilization and
external development
– oviparity.
 Ovoviviparous species
(guppies, mollies,
surfperches) bear live
young after
development in the
ovarian cavity of the
female.
Reproduction
 Fertilized eggs may be
pelagic and hatch into
pelagic larvae.
 Large yolky benthic eggs
are often attached to
vegetation or deposited in
nests, buried, or even
carried in the mouth.
 Many benthic spawners
guard their eggs.
 Usually the male.
Reproduction
 In some species, males defend nest sites and
perform courtship rituals to entice females to
lay their eggs in his nest. Sometimes, several
females will lay eggs in a nest.
 The male will guard the eggs from predators and
will also fan them with his fins to aerate them.
Growth
 Larvae may depend on the yolk sac until their
mouths and digestive systems are fully
developed.
 Larvae then forage for their own food.
Growth
 Larvae metamorphose into juveniles with body
shape & color patterns usually similar to the
adults.
 Some species have different color patterns in
juveniles.
French Angelfishes (Pomacanthus paru) juvenile (left) and adult (right).
Growth
 Growth is temperature dependent.
 Fish grow faster in summer when the temperature is
warm and food is plentiful.
 Growth may nearly cease during the winter.
 Annual rings in scales, otoliths, and other bony
parts reflect seasonal growth.
 Fish continue to grow throughout life.
 Larger fishes produce more gametes.