Ray Finned Bony Fish
This diagram is typical of ray finned bony fish, the vast majority of
fish species, and almost all that appear in our fish markets. Ray finned
fish appear in an incredible array of sizes and shapes, including eels,
but are all made of the same components in the same general arrangement,
just exaggerated in different ways. This diagram does not cover sharks
and rays, which are not closely related. It also doesn't fit the bone
structure of lobe finned fishes (our ancestors) which have arm and leg
bones, but you are unlikely to encounter any of those.
Illustration by Marrabbio2 contributed to the Public Domain (liver
A: Dorsal fin
B: Fin rays
C: Lateral line (external)
E: Swim bladder
F: Weberian apparatus
G: Inner ear:
P: Gall bladder
R: Sex organs
(ovaries or testes)
S: Pelvic fins
U: Anal fin
V: Caudal fin
- Pectoral Fins are not shown. These fins are right behind the
gill covers (Opercula) on most fish, but they may be lower.
- The Swim Bladder does not appear in all fish.
- The Dorsal Fin is comprised of two regions. To the front are
spiny rays, often very sharp. At the rear end are the soft rays. The
spiny and soft ray regions may be divided into separate fins.
- Gill Rakers are a comb-like structure on the inner edge of
each gill to prevent food from escaping through the gills.
- The Adipose Fin appears on many fish. It is a small, fleshy,
rayless fin aft of the dorsal soft rays. Its purpose is still unknown.
- Barbels are long fleshy projections from the lower lip of
some fish, catfish for instance, which help locate food in sand
- Only Ostariophysian fish have the Webearian Apparatus
(Milkfish, Catfish, Piranhas, Carp). This device couples the swim
bladder to the inner ear, greatly enhancing hearing.
- The Skirt is our term for the fleshy wall enclosing the
body cavity. It is not official, but convenient for describing fish
Sharks & Rays
When the modern bony, ray fined fish (Teleostei) appeared, the older
types were under great pressure due to being eaten by the new fish. One
branch of the older fish responded by very rapidly evolving in two
directions - adopting some characteristics of its own predecessors, but
also evolving forward well beyond the Teleostei.
Advanced features include a bigger, more complex brain, allowing a
much more sophisticated social structure, internal fertilization and live
birth, and an array of very sensitive sensory organs for variations in
water pressure, sight, smell and minute electrical charges. Some even
developed a degree of warm bloodedness. So successful were these
adaptations, the following era is called "The Age of Sharks", and it was
the Teleostei who had to worry about getting eaten to extinction.
Illustration by Chris_huh, contributed to the Public Domain
- The shark Skeleton is not bony, but cartilaginous. This
relatively primitive structure is light weight, allowing sharks to
become quite large. They can also get along without a swim bladder,
enabling more vertical freedom in the water column. Actually, sharks
have negative buoyancy and their swimming is more like flying. If not
swimming or resting on the bottom, they will sink.
- Shark Scales are actually teeth, very hard and difficult to
penetrate. Many cultures use shark skin for sandpaper and graters.
- Shark Teeth are structured the same as their scales. They
are in several rows, and the front teeth are continuously replaced
over the life of the shark. Their teeth are shaped for ripping, not
- Shark Blood contains a large amount of urea so as to be in
osmotic balance with seawater, so most sharks can't survive in fresh
water, but some with modified kidneys can. When a shark dies, bacteria
degrade the urea into ammonia, so sharks must be well bled when caught,
and eaten or frozen in a short time. Do not buy shark or skate meat that
smells strongly of ammonia, it is not fresh.
- Shark Tails are quite asymmetrical, because the spine curves
up into the top lobe, allowing powerful propulsion and high swimming
- Rays and Skates are sharks who's pectoral fins have
become so large they are like wings. These sharks are mostly, but not
entirely, bottom dwellers. Rays give live birth, while Skates lay
eggs. The giant Manta Rays are deep water filter feeders. Once feared,
scuba divers have shown them to be harmless and even friendly.
- Temperature: Most sharks are cold blooded, but some of
the most active predators can keep their body temperature above ambient
for more vigorous activity than cold blooded fish can manage.
- Most sharks give Live Birth to fully developed young up to
2 feet long and able to fend for themselves. Some nourish their young
by placental attachment like mammals. Others use a different system
where some nourishment comes from the mother, but the first young to
hatch also eat unhatched eggs, and sometimes each other. This produces
few but relatively large well developed offspring. Rays and some
smaller sharks have returned to producing eggs, generally in leathery
- Shark Brains are similar in size compared to body mass to
birds and some mammals. Most tend to be quite social, and display
curiosity and play behavior similar to birds and mammals.
- The Spiracle is a special gill that feeds oxygen directly
to the eyes and brain. Teleostei fish do not have this feature, but
then they hardly have brains.
Lobe Finned Fish
In case you are curious as to what your remote ancestors looked like,
here is one of their relatives that is still alive today. Coelacanths
date from about 400 million years ago, and were thought long extinct
until science was shown living examples in 1938. Note that the pectoral
and pelvic fin pairs are on stems, which contain the jointed bones which
eventually developed into the arms and legs we enjoy today. These fin
stems are anchored to well developed internal bone structures. Some
species were quite lizard-like in shape, as is the related (and still
living) Queensland Lungfish. Living lobe finned fish are noted for
possessing either vestigial or functioning lungs. All in all, they are
more closely related to amphibians, reptiles and mammals than they are
to the ray finned bony fish.
Photo by Nkansah Rexford, distributed under license Creative
Attribution Share-Alike v3.0 Unported.