The bony fish (Osteon
= "bone"; "icthys" = "fish") are the most
diverse and numerous of all vertebrates. Bony fish are first seen in fossils
from the Devonian era. Bony fish are the
largest and most diverse group of fishes, over 95% of all living fishes are in
this class and more than 24000 living species have been described. The bony
fishes represent more than half of all vertebrate species on earth.
Key
features
Members
of this class have several common features
· The skeleton is usually ossified, with
many vertebrae
· The tail is homocercal (tail lobes are
symmetrical with dorsal lobe approximately equal in size to the ventral lobe).
· Vertebral column ends at the base of
the tail (it does not extend into the dorsal lobe.
· Both median and paired fins are
present; they may be supported by skeletal elements called fin rays of
cartilage or bones.
· Gill slits open into a common chamber
covered by operculum, spiracle is absent.
· Well developed bony scales cover the
bodies of most members of this class, the type and number of scales is related
to life cycles.
· Their jaws are well developed,
articulated with the skull, and armed with teeth. They also have three pairs of
tooth-bearing dermal bones lining the jaws (dentary, premaxillary and
maxillary).
· Air filled chamber functions in either
respiration (as lungs) or in buoyancy (as swim bladder)
· Lungs and swim bladder are homologous
organs that appeared early in bony fish evolution the ancestral organ probably
functioned as a lung similar to those of modern air breathing fish
· The swim bladder maintains neutral
buoyancy at various depths; it is filled primarily with oxygen
Locomotion
Bony
fish moves by flexing the body from side to side. Two kinds of movement in bony
fish are;
· Anguilliform swimming- is an eel like movement where by the
fish flexes the entire body. This motion is similar to that found in most
sharks
· Carangiform swimming- the fish flexes less and receives a
propulsive boost from the caudal fin, drag is minimized. The fastest swimming
fish employs this e.g. Tunas etc.
Energy
cost of movement is minimized by neutral buoyancy which is usually achieved
through the presence of gas filled swim bladder. The volume of gas in swim
bladder regulates depth.
There
are two types of swim bladder found in teleosts
· Physostomous swim bladders are connected by a
pneumatic duct to the gut. It is inflated by gulping air at the water surface
and It is deflated by releasing air bubbles
· Physoclistous swim bladders are not connected with
the gut. They are filled and emptied by an adaptation of the circulatory system
called the rete mirabile. The rete is a counter current exchange system for the
transfer of blood gases. O2 and Co2 are secreted to
inflate the swim bladder and then gases diffuse back into the blood streams to
empty the swim bladder
Nutrition
Teleosts
have a variety of feeding modes, most are carnivorous but most are herbivorous,
detrivorous or omnivorous.
The
shape, size and orientation of the mouth is related to the feeding habits.
Bottom feeders have a subterminal, down oriented mouth while the surface
feeders have a mouth that faces upwards
Herbivorous
and detrivorous fish usually have long digestive tracts with adaptations for
increased surface area while carnivorous tend to have shorter digestive tracts.
Food is digested by enzymes and acid secretions in the stomach and intestines,
nutrients are absorbed in the intestines.
Reproduction
Modes
of reproduction and types of breeding behaviour in bony fish are diverse and
complex. Most bony fish are dioecious and employs external fertilization. Some
species are hermaphroditic and are capable of changing sex during their life
cycles; they may begin life as males or females and later change sex.
Spawning
is influenced by a number of factors such as presence of male, food, optimum
temperature & dissolved oxygen, tranquillity etc. some fish species takes
care of their eggs or juveniles by brooding eggs in mouth, guarding their eggs
etc but some of them abandons their eggs (no parental care).
Circulation
and Gas Exchange
In
bony fish blood is pumped from the heart, through the gills, and into arteries
that supply the tissues, blood returns through the vein to the heart. Blood of
bony fish contains red and white blood cells.
A unidirectional flow of water passes
over the gills (water enters the mouth, passes the gills and exits the
operculum) then gaseous exchange takes place.
The gills are characterized by a large
surface area with highly efficient gas exchange surface;
· The bony or cartilaginous arches
stiffens the gill filaments
· Each filament is coved with flattened
plates of epidermal tissues called lamellae which are the primary sites for
gaseous exchange
· Flow of water is opposite to that of
blood flow, providing a counter current exchange system. Fish can remove up to
85% of the oxygen from water passing over gills
Excretion
and Osmoregulation
Bony
fishes excretes most nitrogenous wastes as ammonia which is removed primarily
via gills
· Marine teleosts excrete excess
monovalent ions especially Na+ and Cl- via specialized
cells in gills. These cells use an active pumping mechanism to transport ions
against the concentration gradient back to the sea water. Excess divalent ions
are removed by the kidney
· Fresh water teleosts produce a large
volume of dilute urine to remove excess water that is constantly diffusing
across the gills. Monovalent ions also moving with the concentration gradient
are constantly being lost across the gills. Gill cells use active pumping
mechanism to regain these ions by transporting them against their conc.
gradient.
Nervous
system and sense organs
Brain
volume to body weight varies in bony fishes. Most bony fish are capable of
complex behaviours. Sensory capabilities of bony fish are similar to those of
cartilaginous fish.
Subclass
Acanthodii
These
are the oldest known gnathostomes that were mostly found in the Devonian era.
They were first marine then invaded freshwater.
In the past,
these fishes were classed sometimes as pre-cartilaginous fishes, sometimes as
placoderms and often as bony fishes. As better fossils have been found,
we now realize that they are probably most closely related to bony fishes and
may have given rise to them. These are
the earliest forms of jawed fishes found in the fossil record (Late Ordovician
≈ 450 mybp), living at the same time as the cephalaspid ostracoderms
(osteostraci). They survived until the Permian period.
Key
features
·
Their skeleton is cartilaginous but the skull is partially
ossified
·
Small fish with a fusiform body. Head and eyes are large
·
They
also possess minute, growing scales which have a special onion-like structure,
i.e. the crown consists of overlying layers of dentine or mesodentine.
·
Jaws were attached by their own process to the skull
(autodiastylic). They also had true teeth.
·
strong swimmers: up in water column (pelagic); not benthic like
ostracoderms and most placoderms
·
Lateral
fins consist of a series of pairs often as many as seven in all down the side
of the body.
·
The
fins were supported by the large spines from which the group derives its name.
·
They have two or one dorsal fin and an asymmetrical Caudal fin
Subclass Actinopterygii (ray finned fish)
Ray finned fishes originated in
Devonian - 325 million years ago and were rare until about 200 million years
ago. The fish are the dominant aquatic vertebrates today, making up about half
of all vertebrate species known. They are found in every aquatic habitat from
the abyssal depths of the ocean to freshwater streams and ponds; a few can even
crawl on land for short periods of time. They constitute a major source of food
for millions of people.
Key features
· Most Actinopterygians have complex
skeletons of true bone , sturgeons and paddlefishes are
exceptions
· Early and primitive forms have ganoid
scales (thick, bony, non-overlapping, relatively
inflexible scales). Scales can also be cycloid or may not be present at
all
· Dorsal lung or swim bladder (usually
with a dorsal connection) is present
· Internal nostrils absent, have external
nostrils
· Unlike the Chondrichthyes, the fins of the Actinopterygii are
webs of skin supported by bony or horny spines.
· Fins controlled by muscles in body wall
· Caudal fin primitively heterocercal
with no epaxial lobe
· Hyostylic jaw support
Infraclass chondrostei
The
chondrosteans are the most primitive ray finned fishes; living representatives
include sturgeons, bichirs and paddle fish.
Chondrosteans first appeared as fossils in the Middle Devonian Period.
They were most diverse in the Paleozoic and Mesozoic Era, after which the rise
of the teleosts caused their decline.
There are 52
Chondrosteans species are still alive today, divided into two orders:
Acipenseriformes (sturgeons and paddlefish) and Polypteriformes (bichirs and
reedfishes) along with many fossil species.
Key features
Chondrosteans usually have a
· Have a spiracle
· The caudal fin is heterocercal
· No premaxilla, upper jaw united with
skull
· Spiral valve intestine
· Reduction in bone. They are mostly
cartilaginous with some sign of ossification
(vertebral centra not ossified )
· Heavy, ganoid scales (thick enamel
cover over dentine and bone)
· Have many rows of radials & lots of
fin rays
· Have a small air bladder
· simple, scissor-like jaws
Order
Acipenseriformes
This order has two families which are
very different
i. Acipenseridae – sturgeons
Twenty six species are known all found in
northern hemisphere
o
They
are anadromous and with some entirely freshwater
o
Scales reduced to 5 rows of bony scutes on body
o
Have
four barbels,
o
Inferior
and protrusible mouth, no teeth in adults
o
Includes
largest freshwater fishes - beluga sturgeon, over 1 ton
o
feed
on invertebrates and smaller fish
o
They
are sluggish swimmers
o
require
clear fast flowing water over gravel for spawning
ii.
Polyodontidae -
paddlefishes
Two species are known, one in
Mississippi drainage - 2m long with a non-protrusible mouth and another in Yangtze
drainage - 3m long with a protrusible mouth
o
Both
have paddle like snout that is an electrical sense organ
o
have
ampullary organs, and with minute barbels
o Scales reduced to a few at
base of tail
o
Gill
covers is very long
o
American
species is a plankton feeder and the Asian species is a piscivore
Order Polypteriformes - bichirs and reedfishes
Eleven species of these fish have been
found only in Africa. These fish are fresh water and are very common in the
aquarium trade.
· They are relatively common, appeared
relatively late in fossil record but have many primitive characteristics
· Possession 5 to 18 dorsal finlets -
each with a single spine, and rays are attached to the spine
· Lungs are only used in low oxygen
situations and are paired
· Some species require access to air at
the surface
· Caudal fin is symmetrical
NOTE; these fishes are Similar to Sarcopterygians
because,
· They have lobed pectoral fins but
Lobes in these fish do not have a skeleton similar to Sarcopterygians and
tetrapods
· Have ventrally connected lungs,
· Their Larvae have external gills (like
lungfishes)
Infraclass Holostei
Radiation
began in Permian era and during the Triassic era holosteans fish almost
completely replaced their ancestors (palaeoniscids) and they flourished greatly
in the Jurassic era.
· Possess true teeth which are more
ossified. Some have large teeth which are adapted for crushing
· Air bladder present but still small
· Heterocercal tail became more
symmetrical possibly due to change in swimming habit made possible by
development of a swim bladder which is used as a hydrostatic organ
· Maxilla is forward free from the
pre-opercular bone and able to swing forward. As a result the lower jaw can now
be protruded in front of the upper jaw making it possible to draw the prey into
the mouth. This feature is mainly found in teleosts.
· Radials & fins rays are less bulky;
this enhances greater mobility of fins thus enabling the fish to perform more
maneuverable swimming
· some became deep and short bodied and
developed a small mouth with flat crushing teeth or beak
E.g.
Gar pike
Infraclass Teleostei
The
fishes are the predominant modern fish and are the largest and diverse group.
They are found in aquatic habitats and are specialized for every possible
niche. Began radiation from late Triassic & replaced holosteans by
Cretaceous
Key features
· The tail is usually homocercal
· Scales are reduced to thin bone with
enamel glaze, they may be either cycloid or ctenoid, both types are rounded
light weight protective structures, ctenoid scales have tiny comb like
projections along the posterior edge
· The jaw is flexible and protrusible
with minimal attachment to the skull bones
· The endoskeleton is almost completely
ossified, the notochord replaced by an ossified vertebral column
· Small number of radials & fin rays.
This makes the fins to be extremely flexible hence resulting to great
maneuverable swimming.
· Have large air bladder
Superorder Protacanthopterygii
· Lack spines have soft rays only
· Pectoral fins low and widely separated
from pelvic fins
· Abdominal pelvic fins and broad tail
E.g.
Salmon, trout etc
Superorder Acanthopterygii - spiny-rayed fish
This Superorder contains the most
successful fish. It includes 15,000 species in 300 families in 13 orders; the
most diverse of which is the Order Perciformes that have 150 families and 7000
species of perch-like fish
· Stiff spines at front of dorsal and
anal fins
· Premaxilla forms the tooth bearing
margin of the jaw, the toothless maxilla acts as a lever to push the jaw
forward to suck in food.
· Body is shortened and the pelvic fins
are thoracic
· Swim bladder is ductless
E.g.
Mackerels, jacks, tuna, perch, cichlids etc
Superorder Ostariophysi
Most
are fresh water species with a swim bladder divided into anterior and posterior
chambers.
Fish
in this group posses two unique features;
· Swim bladder connected to the inner ear
by a series of bones known as the “Weberian ossicles”. The apparatus enhance
hearing in fish and as a result they are very sensitive to sound and can hear a
wide range of sound than other fish.
· Presence of special epidermal cells
which produce an alarm substance. When the skin is
damaged, pheromones are released into the water and these stimulate a fright
reaction in other members of the species and other ostariophysians. In
response, they may quickly seek cover or school together
E.g. Catfish,
carps etc.
Subclass Sarcopterygii
Sarcopterygii (from Greek
meaning flesh fin) contains the so called lobe-finned fishes. Importantly, the
limb like fins of sarcopterygiians are so similar to the expected ancestral
form of tetrapod limbs that the scientific consensus has emerged they are the
ancestors of all tetrapods. Sarcopterygians are placed in the Osteichthyes
group (bony fishes), because their skeleton is bone rather than cartilage, and
are therefore most closely related to them
Key features
· Possess a fleshy leg like
structure of the pectoral and pelvic fins which enabled these fish to move
about on land. These fins evolved into legs of the first tetrapod land vertebrates, amphibians.
· the ancestral lobe finned
fishes had both lungs and gills
· In addition, early
Sarcopterygians were covered with a dentine-like material called cosmine.
· Had/have paired external nostrils
· Today the Sarcopterygians
are a very small group that includes only six species of lungfishes (Dipnoi)
and two species of coelacanths (Actinistia).
Order Crossopterygii
Fishes of this
order are chiefly Palaeozoic except Latimeria sp. they resemble early amphibians. Their Skeleton
corresponds closely to proximal skeletal elements of early tetrapod limbs and
their skull is similar to that of early amphibians. These fish had swim
bladders that may have been used as lungs
This order is subdivided into two suborders,
i.
Suborder Rhipidistia
The Rhipidistia were lobe-finned fishes that are the ancestors of
the tetrapods. These fish became extinct in early Permian
era but they were important because they gave rise to the tetrapods as well as
to the coelacanths.
Key features
· Skull was well ossified and
made of roof bones homologous with skull of tetrapods
· Paired fins have a
characteristic scaly lobed form from which the group derives its name and the
skeleton of the pectoral fin contained a basal element attached to the girdles
and a branching arrangement at the tip. This plan is distinctively different from
that found in ray finned fish and it could also easily have lead to the
evolution of a tetrapod limb because bones making these paired fins are
homologous with bones making limbs of tetrapods.
· Labyrinthodont teeth
typically present (characterized
by folded broad surface/enamel) and they were continually replaced.
· A strong bony skeleton.
· Had one pair of external and internal
nostrils. These fish may have breathed air because they had internal
nostrils. They also certainly possessed
gills covered with an operculum
o the Orders
Osteolepidoidea (including Eusthenopteron sp.) and
o Panderichthyida - the tetrapods.
ii.
Suborder coelacanthin
This group was thought to have gone extinct in the Cretaceous
until a living coelacanth (Latimeria chalumnae) from
the depths of the Indian Ocean off east coast of Africa was recognized in 1939.
A second species has been found near
Sulewesi in Indonesia. About 200 specimens have since been collected. These are
the only Sarcopterygians that still live in the ocean
Features
·
The fins of coelacanth are also different
instead of 1 dorsal fins they have 2.
·
They have an extra tail, the epicaudal.
·
Denticles are present which aids in
protection.
·
The skeleton were made mostly of cartilage
·
The head region of the coelacanth is also
a mystery. It is occupied by 6 small holes, two near each eye and two towards
the tip of the snout.
· The
eye features of the coelacanth are normally large which is attached to the
optic nerves. The fish were also nocturnal
· The
coelacanth in its natural habitat are distinguish from one another by having
white marks present on their body
Order
Dipnoi (lung
fish)
There are six species of lungfishes: one South American, one
Australian and four African species. Air breathing
capability allow survival in dry conditions or in stagnant deoxygenated water
· As their name suggests,
these fish, as all Sarcopterygians do, possess alveolar lungs and can breathe
air
· Extant Dipnoi have lost the
articulating toothed premaxillary and maxillary bones of the other Osteichthyes.
· They have crushing dental plates with
fan-shaped ridges and teeth scattered over the palate. In addition,
strong muscles attach the lower jaw to the cranium. Lungfishes are thus
specialized to feed on hard foods such as crustaceans and molluscs.
· The dorsal, caudal and anal
fins have fused into a single continuous fin that extends around the entire
rear third of the body.
· They were initially considered to be
salamanders when first described.
NB; The Australian lungfish
can gulp air and survive being in oxygen poor water, but cannot live out of
water. In contrast, the South American and African species can survive
out of water for long periods of time. African species normally lives in
seasonal steams and ponds that dry out, but the lungfish survives by burrowing
into the mud and forming a cocoon in which it survives until the water returns.
