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.
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.
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
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.ႜ