I wrote this a long, LONG time ago, and my writing style's become considerably less stiff since then, but sharks are still super-cool, and I wanted to share some of this coolness with you! You can skip to your favorite section if you want (weird kinds of sharks, shark anatomy, shark lifestyle, etc etc)--just scroll down looking for the big words.
INTRO
All sharks are in the kingdom animalia, phylum chordata,
and class chondrichthyes, along with chimaeras, rays, and skates.
They are classified in subclass Elasmobranchii with skates and rays.
The most famous shark is undoubtedly the Great White, popularized by
horror movies, but there are over 370 different species of shark of
all shapes and sizes, ranging from a small member of the genus
Squaliolus (males—6” approx., females—8” approx.) to the
massive whale shark (Rhincodon typus)
measuring 40 ft. or more. The number of shark species is relatively
small, though, compared to the 2,800 different kinds of bony fish.
Nevertheless, the shark is a very diverse creature. We would like to
give you a brief overview on a few species before going into the
general characteristics of all sharks.i
ii
NEAT KINDS OF SHARKS
The small smooth dogfish shark (Mustelus
canis) averages 3 to 4 ft. in length and is
one of the most abundant sharks on the east coast, found in great
numbers in the Delaware Bay—more are caught here than all other
sharks combined. They leave the area in mid-October, although some
are still here by early November and don’t return north until the
water rises above 50 degrees Fahrenheit. These common sharks have
low, crushing, flat teeth—and a lot of ‘em—and feed mostly on
crustaceans like lobsters, crabs, and clams, as well as small fish. iii
One very interesting shark is the Megamouth shark,
(Megachasma pelagios,
meaning big cave of the sea) considered by some scientists to be one
of the oldest and most primitive species within the order
Lamniformes. A whole new genus had to be created when it was found.
It looks more like a small whale than a shark, with a flabby body,
long pectoral fins, and very short dorsal fins, and is thought to
have poor mobility and to be less active than whale sharks or basking
sharks. Very little is known about this shark; it has only been
sighted 27 times. It habits waters all over the world, although the
most sightings were seen in the Pacific, and seems to spend daytime
in deep waters and night in mid water depths. Its body tapers
posteriorly and its head looks big. It’s pretty much white on the
bottom and brown on the dorsal surface, darker towards the head, with
a whitish band on the snout. It has fifty rows of teeth, females
seeming to use fewer rows than males. The largest was 17 ft—males
mature by 13 ft and females by 16 ft. It filter-feeds mainly on
krill, shrimps, and other such organisms and is fed upon mostly by
sperm whales. iv
Another unusual shark is the cookie-cutter shark
(Isistius braselienus),
which is a very small fish that preys upon slow larger fish. It is
very aggressive and has been known to take bites out of submarines,
taking them for large whales. The cookie-cutter sharks live around
larger creatures and take bites out of them as they go along. They
frequent deeper waters during the day and mid-waters during the
night, making the Megamouth shark easy prey for them.iv
The Megatooth shark is also of interest, and it is the
largest fossil shark known to man, estimated to have reached at least
40 feet and 20 tons, like a very carnivorous whale shark.
Carcharodon megalodon
is thought to have fed on large fishes and mammals and seems to be
the closest “relative” to the Great White shark. Unfortunately,
sharks, lacking bones, don’t preserve well and this fossil is known
only by its teeth and jaws—but these are REALLY huge, kind of like
a REALLY GREAT WHITE. i
iv
Unfortunately we are not writing a book but only a
report, and cannot describe the basking shark, the Caribbean shark,
the Galapagos shark, the Tiger shark, the Lemon shark, the Nurse
shark, the Sand shark, the Sandbar shark, the black tip shark and
many others. We hope you have a very slight appreciation for the
diversity of subclass Elasmobranchii. Before we move on, though, we
HAVE to mention the Great White.
To put it bluntly, Cacharodon
cacharias is dumb and antisocial. It does
not tell its prey by shape, size or color but by taste, and will
attack pretty much anything that splashes on the surface. It feeds
primarily on aquatic mammals and occasionally a marine reptile, as
well as large fish. It never eats sea otters or aquatic
birds—perhaps, like people, they taste awful. It appears to prefer
high-energy, fatty foods, kind of like how Americans prefer high-carb
McDonald’s meals. For example, the Great White will strip the
blubber off of a whale, but not the muscle underneath. The White’s
skin is less rough and sand-papery than most other sharks, and is
rather smooth. The Great White roams waters all over the world, is
most viewable in the wintertime, and hates other Great Whites. If
two whites come upon each other from opposite direction, they will
both turn and leave. Often when one White wants another to clear
out, it will slap the water with its caudal fin, roll over in the
water, or leap up and smack itself against the water. These splashings are then detected through the "lateral lines" (specialized organs) of the other shark.
The Great White attacks prey in many different ways, sometimes
charging it horizontally across the surface, sometimes attacking from
below, and sometimes charging it directly vertically with such a
force that it leaps out of the water like a dolphin.v
iv
COOL SHARK ANATOMY
Despite all this variety, all sharks follow a uniform
definition, having one general body pattern and many similarities in
lifestyles. We will begin with the shark’s body plan and then move
to discussing lifestyles before talking a little on the other members
of the class chondrichthyes.
The shark’s skeleton is similar to other fish, but
made of cartilage instead of bone. This cartilage is usually
hardened by mineral deposits, especially around the vertebrae and the
skull, but it does not do well outside of water. The cartilage is
lighter than bone, which increases buoyancy. The skeleton is made up
of two main areas—the axial skeleton and the appendicular skeleton.
The axial skeleton is the vertebrae (backbone), cranium (skull), and
rib cage. There can be two regions of the skull, or
chondrocranium—the neurocranium and the splanchnocranium. The
neurocranium is the dorsal, or top part of the skull, that has to do
with protecting the brain and sensory organs. The splanchnocranium
is the ventral, or underneath part of the skull, concerned with
forming the jaw and the gill arches. The powerful jaws, of course,
contain the numerous teeth, which are attached by ligaments to the
jaw, and upon falling out, are replaced by teeth that were developing
all along behind layers of skin. The neurocranium has lots of little
foramina, or perforations in it, for such things as a primitive
“third eye”, protection for the lateral lines, openings from
sensory organs into the cranium, and other stuff. It also has the
rostrum, or snout, of the shark. The splanchnocranium has the
mandibular arch, which is the jaws, as well as other support for the
sides of the shark’s comparatively massive head. The axial
skeleton also includes the long vertebrae down to the shark’s tail,
which contains the notochord, the spinal cord, the caudal artery, and
the caudal vein. The appendicular skeleton contains the assorted
fins—dorsal, caudal, pelvic, and pectoral—as well as the pectoral
and pelvic girdles. Attached to the pectoral girdle, which is like a
big belt around the middle of the shark, are the two wing-like
pectoral fins, and attached to the pelvic girdle, a smaller belt
nearer the tail of the shark, are the pelvic fins. Each set of fins
is attached to its girdle and supported by a notochord. The shark’s
skeleton is complicated, but simpler than that of say, a human. vi
i
Sharks’ fins differ according to species, and are very
different from bony fish fins. Sharks that don’t dwell exclusively
on the bottom have much stiffer pectoral fins than those that do.
Shark fins are generally inflexible, unlike the fins of bony fishes.
The shark uses its tail as the main method of movement, with the fins
providing lift and buoyancy, kind of like airplane wings. The
hammerhead shark doesn’t have very large pectoral fins, for
example, because its weird head provides some lift for it. Dorsal
fins aren’t just for decoration, either. They aid in steering
efficiency and may help block water flow to the tail, increasing the
result of the thrust from the tail and preserving energy. Bottom
dwelling sharks have smaller dorsal fins that top dwelling sharks,
because they don’t need to move as quickly. The downward sweep of
the asymmetrical caudal fin, with its large dorsal lobe and small
ventral lobe, also helps with lift.
Despite the light skeleton, a shark is negatively
buoyant—it sinks. The shark also lacks a swim bladder, which other
fish have. Although a shark must be often in motion, it isn’t true
that it must be continuously in motion to stay afloat. The shark
contains a liver with very light oil to help it stay afloat.
Pelagic, or open-ocean sharks, which need to stay afloat more than
bottom dwelling sharks, contain larger livers with lighter oils.
This makes an effective buoyancy system.
The shark breathes by exchanging gases through its
gills. The water taken in through its mouth passes over the gill
filaments, where gas exchange takes place. The carbon dioxide is
released from the blood at the gill lamellae and exits via the gill
slits with the water. Sharks have between five and seven gill arches
just in front of the pectoral fins. They slits are never entirely
below the pectoral fins. Some sharks must swim continually to ensure
that they get the oxygen they need past the filaments, but others
have a special pharynx in their throats that pumps the water past the
gill filaments. There is no protective covering over the gill slits,
unlike with bony fish, so they are more vulnerable to injury. Many
sharks, especially bottom dwelling species, also have spiracles on
the dorsal side of their skulls that are kind of like extra gills, so
they can ventilate while feeding slowly on the bottom. These
spiracles have a special valve to keep things flowing.
Sharks’ guts are as interesting as any—if you’re
into lots of greasy yellow stuff. There’s a really cool fat
esophagus, which is really large considering the size of the shark,
since the food is normally swallowed in big, whole pieces. The
stomach can expand considerably, and follows the esophagus
immediately in a caudal directed J-shape. Following the stomach,
which is lined with white lobey things on the inside called rugae, is
a duodenum, connected to the gall bladder and the intestines, where
the bile breaks down the shark’s food. Following the duodenum is
the valvular intestine, or the rest of the small intestine, which
contains spiral valves to increase the surface area and absorption of
the intestines. The colon is the continuation of this intestine.
The rectal gland leads into the colon by means of a duct. It
excretes salt to regulate the shark’s body fluids. Normally, the
concentration of salt in the shark is lower than that in the sea, so
it is in danger of dehydrating through osmosis. The salt excreted by
the rectal gland increases the concentration of salt in the shark, in
a sense decreasing the concentration of water, so that the
concentration inside the shark is the same as outside, preventing any
osmosis from occurring. This is called an osmoregulator. The shark
has one final end to the digestive tract—the cloaca, meaning sewer.
Higher organisms such as humans have different organs for the
rectum, the reproductive system, and the urinary bladder, but the
shark just uses the cloaca for all three.
The shark’s liver is also really cool—and REALLY
HUGE!!! (In some it takes up 30% of the shark’s body weight, and
it takes up practically 50% of the inside room of some sharks) The
shark needs a lot of liver to store all the oils for all the energy
it needs, besides the fact that the liver aids with buoyancy, as
aforementioned.
Sometimes sharks have two types of muscle tissues, red
and white, kind of like light and dark poultry meat. The red
contains a high concentration of myoglobin that stores oxygen. The
red is used frequently on long trips, and the energy produced while
burning the oxygen is used for heat. White muscle makes up the
majority of the muscle mass, but doesn’t store much oxygen.
The shark has a complex circulatory system, like a
human’s, except with only a three-chambered heart rather than a
four chambered heart, with one big artery entering the heart from the
top. The three-chambered heart appears much less complex than the
four-chambered from the outside. There are veins leading away from
the heart and arteries leading to the heart, passing by the essential
gills on the way.
The senses of the shark are very highly specialized and
perfectly sharp. The neurological system is very advanced. The
shark’s hearing is especially amazing--sometimes sharks can hear
things from a mile away. They are especially sensitive to sounds in
the range of 20-300Hz, particularly at or below 40 Hz, which is about
the frequency of a struggling fish. The hole-thing above the eye,
though, is not the ear---that’s the aforementioned spiracle. The
outer ears are very small pore-like objects between the spiracles,
and while the inner ear is very extensive, hidden entirely inside the
otic capsule of the chondrocranium, the outer ear is small and almost
impossible to see without a magnifying glass or hand lens. The inner
ear also contains three semi-circular canals, which, as in humans,
provide the primary sense of balance, besides that provided by the
eye, by way of fluids and salt-like grains flowing past the canals in
a huge hole called the sacculus. The nose is also a wonderful
creation—blacktip sharks have been known to detect fish diluted to
one part per 10 billion parts of seawater. The shark can also smell
from many yards away. Its olfactory organ—or, “nose”—has two
external nares, each with two openings, one lateral, an incurrent
aperture, and one medial, and excurrent aperture. (Feel free to
refer to the attached sketches at any time) A part of the brain
called the olfactory bulbs touches the olfactory sacs, internal
cavities leading out to the nares. The shark actually takes in
water, passes it over some epithelium, or special sensors lining the
sac-insides, and then shoots it out again for more. This water flow
is regulated by a flap of skin between the lateral and medial
apertures. The eye of the shark is probably not as wonderful, and of
course picture quality depends on the quality of the water, but it is
relatively the same as that of the human. Any of you who happen to
be humans focus by changing the shape of the lens—the shark focuses
like a camera, moving the lens further or closer to the retina of the
eye. Just like you humans, the shark has a pupil to regulate the
amount of light entering the eye. The shark has both cone cells and
rod cells, so it can probably detect some color, and sees highly
contrasted objects well, although it is not known how well it can see
subtle details. Many sharks have a tapetum, or reflective layer,
behind the retina that allows them to make the most of the little
light they have. Many also have a nictitating membrane on each eye
that aids to protect the eye during encounters with prey. The shark
also has well-developed touch and taste—developed enough to reject
things it finds distasteful. The tongue is very short, so the shark
can only taste things far in its mouth. The shark also has a very
interesting lateral line system, which only fish and amphibian larvae
have, as far as we know. The connected lateral line canals form a
long line on either side of the shark’s body, just under the skin,
and are exposed to the outside water by little pores. The lateral
line system is made of neuromasts, which are the ciliated sensory
cells in the canals, and detects water current, so the shark can tell
if something is moving the water nearby and in what direction.
Another interesting feature of sharks is the ability to detect
electric fields produced by the bodies of some organisms. The fields
are detected by the pores of ampullae of lorenzini, which lead to the
ampullae of lorenzini, which store a jelly-like solution and are
attached to a sensory nerve. This aids the shark in knowing where
the prey is when it’s very close and just about to bite. They can
use this also to detect buried prey as the sharks go over the
ocean-bottom. The pores are on the underside of the snout and are
huge and clearly visible.
SHARK LIFE
Sharks have many different modes of reproduction.
Sharks do not stay together for life, and only come together during
mating season. The male has two claspers, which are modified pelvic
fins for transmission of sperm, on its underside, near the pelvic
fins, and the female has nothing but the cloaca and the pelvic fins.
Sharks utilize internal fertilization. Mating gets very violent as
the males will hold the females with their teeth to put them in the
mating position, which is why females have skin twice as thick. Some
sharks breed year round, and others migrate to a mating ground every
two years or so. Sharks tend to give birth in the summer, or spring,
or when the water is a tolerable temperature, not too hot and not too
cold. It is thought that female sharks give off pheromones, special
smells, when they are ready to mate, to let the males know they’re
available. This is when it ceases to be all the same for every
species. Some sharks are oviparous, and lay the fertilized eggs to
hatch on their own. Some sharks are live bearing. Some live bearing
sharks use placental vivipary, where the embryo is attached to the
mother and receives nutrients from her. Others have the embryo
attached to a yolk-sack within the uterus. If the yolk ever runs
out, the embryo will eat other embryos or eat the unfertilized eggs
in the oviduct that leads to the uterus tubes. Once the shark hatches
or is born, it looks just like its parent and is on its own.
Sometimes the mother will eat her babies. vii
Sharks have varying lifestyles. Many migrate
vertically, and many migrate from south to north for breeding and
temperature changes. Sharks also tolerate a wide range of pressures.
Centroscymnus coelolepis
has been found at depths of 8000 ft, while some sharks are restricted
to 600 ft. Most sharks are carnivorous and feed on fairly large
prey. All sharks have wonderful sensory reception and low
intelligence, although the temperament varies from species to
species. The sand shark (Carcharhinus
plumbeus) is sluggish and “lazy” while
the tiger shark has been known to actually consume people for no
reason at all. Some sharks live on the bottom, some in the middle,
and some are pelagic, on the top of the ocean. A shark’s lifestyle
depends on the shark. viii
THE SHARK AND YOU
It is hard to write about the shark without mentioning
what it has to do with you. Despite the ideas the Jaws series
inspired, sharks do not eat people, and the actions of one or two
deranged individuals should not dishonor the species. There are a
few more dangerous types of sharks—the tiger shark, the lemon
shark, large hammerheads, and the great white. These are the ones
most likely to attack without provocation. Often it seems they
simply wonder what you are and taste you, like a baby does. It just
so happens that you taste awful, or something, because they don’t
eat you, but rather take a bite and leave. This bite, though,
unfortunately results in the loss of limbs or even in fatalities.
Swimmers are more vulnerable than surfers, presumably because
swimmers look more like food. Actually, though, more people die from
wasp stings than white shark attacks. Off of Virginia there has only
been one fatal attack since 1670, and 3 non-fatal attacks. And while
you think about that, why not “tasting” a shark? Your taste
would be much more fatal to a shark than his would be to you. Shark
meat is wonderfully boneless, the best meat being firm and flaky.
The Atlantic Mako is an especially popular meat shark, because it
makes lovely leaps and runs when hooked. Conservation is important,
but there are many shark species that are not endangered and taste
good, too! For shark fishing, make sure you have some kind of rope,
hook, and pole, as well as something on your 35 to 40 ft. boat to
lift the shark out of the water with. When you have gutted your
shark, make sure to get all the blood out, or the meat will turn
brown and taste very dry. You can do it just like you would to any
other large fish, although instead of scraping off scales, you skin
it—kind of like hunting and fishing together. You can soak your
filets in an acidic solution like lemon juice to further remove any
yuckiness. When you preserve it, double-wrap your shark and use
un-iodized salt, like kosher salt—otherwise the shark will spoil
and turn black. Glaze the inside of any crock you use, preferably
plastic, after soaking the shark in a salt brine to make absolutely
sure you got all the blood out. After washing and draining, you can
put the shark in the crock layered with salt, covering the last layer
of shark with about an inch of salt. Shark meat can make anything
from fin soup to teriyaki! Just make sure you use the whole shark
and don’t waste these magnificent creatures! ix
THE DOGFISH SHARK
Our shark is a spiny dogfish, Squalus
acanthias, from the family squalidae in the
suborder squaloidei in the order sqauliformes. This order contains 7
families and 113 species like dogfish, cookie cutter sharks,
spurdogs, rough sharks, etc. The sharks in this order range in size
from less than 8 in. to over 20 ft. They all reproduce live bearing
without placenta, as far as we know.
The spiny dogfish shark has a gray/brown dorsal surface
dotted laterally with white spots that fade as the dogfish gets
older. It has no anal fin, almost perfectly equilateral pectoral
fins, a rear dorsal fin behind the pelvic fins, and a caudal fin
without a ridge or chip out of it. The dogfish has large, beautiful
eyes and a kind of flat head. Like the smooth dogfish, it has two
rows of low, grinding teeth, but also a row of small, sharp teeth.
It also has a spine in front of its dorsal fins which it can us in
defense by doubling up like a bow and striking the mildly venomous
spine into its attacker.
The spiny dogfish is migratory, found primarily north of
Cape Cod in the summer and south in the winter. It lives at almost
any depth and prefers highly saturated sea-water to brackish
freshwater. The deepest known depth of a spiny dogfish was 2950 ft.
The dogfish is a social shark and travels in groups
segregated by size and sex. Medium sized males travel with medium
sized males, and big females with big females, and so on. Only the
immature dogfish travel in mixed sex groups, although still sticking
with sharks their own sizes. The dogfish got its English name from
the way these sharks travel in packs and attack the small fish in
their way.
The spiny dogfish eats crustaceans like the smooth
dogfish, but feeds primarily on small fish such as herring. It often
tears fishing nets to get what’s inside and is a pest to commercial
fishing.
The spiny dogfish’s age has been found by growth zones
on its spine, and dogfish can live between 25 to 30 or even more
years. The dogfish has the longest gestation period of any known
vertebrae—almost two years! The spiny dogfish also grows up very
slowly. The Peter Pan of sharks, the spiny dogfish female isn’t
mature until at least 12 years of age! The dogfish’s size upon
reaching maturity varies from climate to climate. It grows between 1
½ to 3 feet in length, the largest around 4 ft.
The spiny dogfish is an aggressive little shark, the
young pups attacking fish two or three times their sizes. It is no
real harm to man, although the spines can prove dangerous upon
handling of this shark, and no dogfish shark that we know of has ever
killed a human being.
Although the spiny dogfish is considered a bore to
sportsfishermen and a pest to commercial fishermen, its flesh is
flaky and firm like haddock and well-loved in Europe. If you eat
fish and chips, the fish is very likely the spiny dogfish. x
And that’s it! We hope you enjoyed our whirlwind tour
into the life of a shark. The typist has especially come to love the
dogfish shark, and hopes that you did, too, or at least came to enjoy
the magnificent sharks the Lord has placed on this planet! Googling
“shark” will give you lots of results—try it some time, dissect
a shark, write a report, and tell us if you don’t discover
something amazing!
--Phillip Hines, Joy Lee, Megan Poe, and Jennifer
Veldhuyzen
Bibliography
i
Hawaii sharks.com—internet on sharks
ii
Encyclopedia Americana
iii
BeachNet.com—Sharks in the Delaware-Chesapeake area
iv
Ichthyology at the Florida Museum of Natural
History—website
iv
Shark diving of coast of South
Africa—diving—website hosted by TopHosting
ivi
Photo Manual and Dissection Guide of the Shark by Fred
Bohensky, Avery Publishing, copyright 1981
vii
all information from this paragraph and the 8
paragraphs proceeding are from the Hawaii Shark.com thing, the Photo
Manual, or personal experience…hee hee
viii
all info in this paragraph from sources stated in
footnotes i through vi
ix
numbers from source iv—fishing and other information
from source i㶷䱩
vx
All information on the spiny dogfish came from
iii and iv, as well as the Montery Bay Aquarium website and
SeaPics.com.ႜ
v禽
i솧
