1. The Earth Life Web, The Fish's Skeleton http://www.earthlife.net/fish/skeleton.html
The Fish's Skeleton
Because the term fish includes such a diverse array of animals it is difficult to talk in a general way about an
average fish. Most fish are active swimmers and the shape of their skeleton reflects the shape of their body,
which, in most cases is designed to allow them to move easily through the water they live in. Thus the skeleton
of an average bony fish looks like an arrow where the skull represents the arrows head, the backbone or spine
the arrows shaft and the tail represents the feathers. If we add a few barbs to our arrow to represent the spines
that run along the spine we have a fairly good idea of the basic fish skeleton.
The skeleton of a modern bony fish as shown above, and even the arrow, represent the end point of a long
period of evolution. The skeleton of the first fish was probably very like the skeleton of a modern Hagfish
which is little more than an amalgamation of pieces of cartilage. The skull of a Lamprey is a single
cartilaginous trough with a few lobes and spines while the spine is a simple sheath of cartilage surrounding the
notochord. There is also a simple cage of cartilage to support the front-most parts of the viscera. The skeleton
of a Hagfish is even simpler. Sharks and Rays have a more complex skeletal system, which, for the most part it
is still only cartilage and not bone. The skull however is more complex than that of the Lamprey and is called a
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2. The Earth Life Web, The Fish's Skeleton http://www.earthlife.net/fish/skeleton.html
chondocranium which surrounds the brain and supports the sense organs. Attached to the skull are the jaw
cartilages, called the palatoquadrate cartilage (upper) and Meckel's cartilage (lower). There are also
branchial cartilages supporting the gills. As fishes evolved the number of bones involved in the head-capsule
increased and their arrangement became more and more complex as Nature experimented with different
solutions to the problems of life in the sea.
As can be seen in the image to the right, in the
primitive bony fishes the individual bones are larger
and the amount of open space is restricted. Also, many
of the bones are in different places, or have different
shapes. Look particularly at the Maxilla and the
Premaxilla in this picture and in the one below. In the
primitive ray-finned fish the premaxillar is small, not
moveable and carries only a few front teeth, most of
the teeth are on maxilla itself. In comparison in the
more highly evolved (sometimes called 'derived')
rayfinned fish the premaxilla has taken over job of
being the upper jaw bone and it is now larger, much
more moveable and carries most, if not all, the teeth.
The maxilla has taken on a supportive role. You should note that all these structural changes accompany, or
allow a functional change in the way the jaw works.
The jaws of the primitive fish work simply, much the way lizard jaws do, whereas the derived fish's jaws are a
much more complex system of levers allowing the mouth to move backwards and forwards as it opens and
closes. However within the 28,000 species of fish there is room for a great deal of variety. For instance Eels of
the family Anguilidae (Anguilla rostrata, Anguilla anguilla) have no maxilla or premaxilla
As can be seen from the image at
the top of the page, and more fully
from the image to the right, the
skull of a bony fish is a puzzle of
extreme complexity with many
moving parts. In this collection of
small interlocked bones the fishes
strike an ecological balance
between strength and lightness. The
light-weight requirement is real
because bone, being denser than
water requires muscle to move, the
heavier a fish's skeleton is the more
slowly it moves, both when
escaping a larger enemy, and when
catching smaller prey.
The earliest fishes went in for
heavy armour, which made them slow moving and restricted them to living only on the bottom of the sea. The
picture below shows an artist's impression of what a cambrian fish of the genus Hemicyclaspis may have
looked like. Whatever the colours of the fish were in real-life isn't really important, what we can easily see
here, and what is known from the fossil record is that Hemicyclaspis had its entire head protected by a heavy,
bony shield. Evolution has created the modern fish, which in most instances, has a very low ratio of bone to
muscle, it is fast and highly maneuverable and it claims the whole of the sea as its domain.
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3. The Earth Life Web, The Fish's Skeleton http://www.earthlife.net/fish/skeleton.html
The vertebral column, or spine, of a fish is the main supporting structure for the muscles that the fish uses to
swim. In its evolutionary journey it has gone from being the simple cartilaginous tube of the Hagfishes through
the more complex cartilaginous tube of other early jawless fish to partial ossification (boniness) as in the
Ratfish to the fully bony tube of modern sardines. The individual bones of the spine meet at their round
centres, called 'centra' and there is usually one vertebrae per body segment. Two flattened rods of bone arise
from the upper side of the centrum, they are separated where they arise, but meet a little way above the
centrum, the space thus formed is called the 'Neural Arch'. collectively the neural arches of all the vertebrae
form a tube that encloses and protects the spinal cord. In most species of fish the combined bony rods rise up
as a Neural Spine above the Neural Arch. In some species of fish such as the Salmon a second pair of rods
extend downwards creating a 'Hemal Arch' that serves to protect various blood vessels.
Unlike those of mammals and reptiles the vertebrae of fish are not linked together, they are simply held in
place by a series of tendons. In fish both sides of the centrum are concave, the space between is filled with a
ball of cartilaginous substance that holds them a little apart allowing them to flex a bit. (There is an exception
to this rule however, Garfish (Lepisostidae) have interlocking vertebrae much like those of reptiles. In other
words the centra of the vertebrae are convex on the anterior or front face and concave on the posterior or
behind face allowing them to fit into each other). The vertebrae that connect the skull to the spine are called
the Atlas and the Axis, as in all vertebrates.
In the picture above we can see representative vertebrae from three fish and two sharks, a Sturgeon, a Cod
and a Salmon a White Shark and an Angel Shark. The Sturgeon, the Salmon and the sharks are seen front on
(transverse view) and the Cod is seen side on (lateral view) to show that the processes, and hence the spines
they make up, are not necessarily vertical.
Australian Fish Species Some of the best known fishes are found in the waters of Australia. www.TravelNT.com/Fish-Species
Read Google eBooks Flash and Bones On The Web, Android, iPhone & iPad. books.google.com.au/eBooks
Travel to Peru Find all you need to know before travelling to Peru. Visit us now! Peru.travel
The sturgeon is an ancient fish and represents the ancestral state, in fact the vertebrae of Sturgeon are not
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4. The Earth Life Web, The Fish's Skeleton http://www.earthlife.net/fish/skeleton.html
ossified (made into bone) and all the parts you see are in fact cartilage. The arrow represents the direction of
evolution. The notochord is broken into sections, some of which become the bones of the centrum and the rest
becomes the cartilaginous balls that keep the bones from damaging each other. The dorsal and ventral
cartilages become ossified and are then called the dorsal and ventral processes. In most fish the ventral
processes of the caudal vertebrae (tail bones) move closer together to form a hemal arch, and in some species
this hemal arch forms below the other vertebrae as well, as in the salmon. In sharks and rays you can see
transitional states whereby only part of each vertebrae is calcified, these hardened, or bony, parts, which are
normally in the form of rings or struts, are embedded in a matrix of ground material that is still cartilage.
The vertebrae of the trunk (the main part of the body) and the skull support a number of additional sets of
bones, all of which may be present in varying degrees in different fish species.
Extending sideways are the ribs which protect the visceral cavity (the space where
the guts are). Reaching up are a series of dorsal spines which maybe, but usually are
not, in actual contact with the vertebrae. The pelvic and pectoral fins are supported
by simple pelvic and pectoral girdles which are attached to the skull. The dorsal fin
or fins and the anal fin are supported by spines that may, or may not be connected to
the vertebrae. The tail is supported by the caudal vertebrae (the Hypurals, Epurals
and the Urostyle). The Urostyle is the calcified unsegmented final portion of the old
notochord. In those sharks which have highly asymmetric caudal fins cartilaginous
end of the vertebral column often extends into, and supports the larger upper lobe.
The bones mentioned above extend only a small way, or in the case
of most dorsal and anal fins not at all into the fins themselves are.
Instead spines or bristles of toughened cartilaginous material called
finrays reach out into the fins from the bones that remain encased by
the flesh of the body. Movement of both the dorsal and anal fins,
either sideways, or merely to raise and lower the fin is made possible
by simple hinges at the point of attachment of the finrays to the bones
that support them. For more information on the function of fins see
the page on the external anatomy of a fish.
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5. The Earth Life Web, The Fish's Skeleton http://www.earthlife.net/fish/skeleton.html
Bibliography
The Fish Anatomy Menu
Anatomy Fins Blood erves Magnetism Swim-bladder
Skeleton Sight Scales Hearing Electricity Osmoregulation
Digestion Gills Smell Muscles Lateral Line Thermoregulation
Have You Seen The Other Earthlife Web Chapters
This page was designed and written by Mr Gordon Ramel
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