Fins & Swimming
Most sharks are designed for efficient motion through the water. They have three types of median fins (dorsal, anal, and caudal) and two sets of paired fins (pelvic and pectoral). Swimming is achieved by side-to-side undulations of the caudal, or tail fin, and often part of the trunk; these motions propel the shark forward.
Unlike those of bony fish, shark fins generally have broad bases, and are fleshy and relatively inflexible. The tail fin is the driving force, dorsal and anal fins provide stability, and pectoral fins (along with the broad snout) provide lift and diving control, somewhat like airplane wings or the planes of a submarine.
As with any other aspect of shark biology, there are differences in fins among different species. For example, the hammerhead shark’s expanded head provides some lift, so they have smaller pectoral fins. Blue sharks and oceanic whitetips have long pectoral fins for increased lift in the pelagic environment.
Pectoral fins have other uses as well. They can be angled downward to turn or descend quickly. They can be used for display of social signals. But they cannot be used to make a shark swim backwards, like they can in many bony fishes. The small pelvic fins provide some lift, too.
Dorsal fins stabilize the shark, keeping it from rolling to the side and helping it swim in a straight line. The trailing edge of the first dorsal fin may create a low pressure area extending to the tail, increasing the efficiency of the tail’s forward thrust and helping to conserve energy.
Skin covering also plays a role in swimming. Whereas bony fish usually have flat, round, overlapping scales, sharks’ scales (denticles) have a structure similar to teeth. On fast swimming sharks, denticles have sharp peaks and small grooves running from front to back, helping water flow over the body more efficiently. The denticles of some sharks even have cross-ridges and pits (like a golf ball) to further increase swimming efficiency. Like teeth, denticles are continuously shed and replaced throughout a shark’s life.
Gills & Respiration
Gills allow fish to take in oxygen from the surrounding water and eliminate carbon dioxide from the blood. Sharks can have up to seven external gill openings, but most species have five.
Gill arches are considered part of the skeleton; they hold the gills in place. The arches support one or two rows of gill filaments. The filaments are designed so that water flows in one direction alongside them, while inside the filaments blood flows in the opposite direction. This countercurrent system is the most efficient method of exchanging oxygen and carbon dioxide between the water and blood.
Bony fish generally have four gill arches on each side, covered and protected by a single external bony plate. Sharks do not have a protective bony covering over their gill slits, which leaves gills more vulnerable to injury.
Many sharks, especially bottom-dwelling species, have paired openings called spiracles located between the eye and the gill slits. Spiracles are used to take in water and ventilate the gills, even while the shark may be feeding or at rest on the bottom.
Some sharks must maintain a certain swimming speed in order to ventilate their gills with water taken in through the mouth. But others have specialized muscles in their pharynx (throat) which they use to pump water over their gills even when they’re not moving.
Skeleton & Buoyancy
A shark’s body is supported by a skeleton very similar to that of other fishes, except it is made of cartilage rather than bone. Some parts of the skeleton, including the vertebrae and skull, are strengthened by increased calcification.
All sharks are slightly negatively buoyant, which means they sink. Unlike many bony fishes, sharks do not have a swim bladder to provide buoyancy. To help compensate for their tendency to sink, their livers contain large amounts of oil that is less dense than seawater. Pelagic (open water) sharks generally have larger livers, with more and lighter oil, than sharks which live in shallower water or near the ocean bottom.
The combination of a cartilaginous skeleton, which is lighter than bone, and an oil-filled liver work together to increase swimming efficiency and buoyancy.
Teeth & Jaws
Shark teeth are formed from specialized skin tissue on the jaw cartilage. Teeth are arranged in rows and attached to the jaws by connective tissue. Usually only the front one or two rows are functional. The other teeth, in some cases up to 13 rows, are folded back against the inside of the jaw, where they develop.
As the connective tissue moves slowly forward, new teeth continuously replace older ones as they are lost or just fall out. Lost teeth can sometimes be replaced in as little as 24 hours. Some sharks may shed 50,000 teeth in a lifetime. Not all sharks shed their teeth one or two at a time. The cookiecutter shark sheds its entire lower plate of teeth at once, often swallowing the teeth with its meal.
Each species of shark has its own distinctively shaped teeth, although the teeth of some species are very similar. Comparing some local species, the tiger shark has notched, serrated teeth on both jaws; the mako shark has dagger-like teeth and the white shark has serrated triangular teeth.
Tooth shape, size, and structure vary with a shark’s primary food source. Teeth may be designed to grasp, tear, or cut. In some sharks teeth are modified to grind or crush mollusks or crustaceans.
Nearly all sharks have what is known as a subterminal mouth, located on the ventral surface (underside) of the head behind the snout. The upper jaw is suspended below the skull, attached by ligaments, muscle, and connective tissue. The lower jaw is connected to the upper jaw at the corners of the mouth and attached to massive muscles used for biting.
Each jaw is somewhat flexible, consisting of right and left halves joined in the center at what is termed the symphysis. During a bite, many sharks can extend the entire jaw structure forward, thrusting it out from the skull. This helps some sharks bite off parts of prey that are too big to swallow whole.
The pressure exerted by shark jaws has been measured at around 42,000 pounds per square inch. This is actually about the same order of magnitude as the pressure humans can exert with their molars. But because shark teeth are so sharp, their biting force is distributed over a much smaller surface area, and as a result can do considerably more damage than human teeth.
The Shark Inside
The inside of the shark is designed for maximum efficiency, just like its outside.
Sharks have large J-shaped stomachs that can expand considerably. When prey is captured, it is usually swallowed whole or in large pieces. The stomach produces an acid that is strong enough to dissolve metal. Large bones and other indigestible objects are prevented from going past the stomach due to the small size of the opening to the intestine, but can be regurgitated through the mouth.
To rid their stomachs of indigestible material, some sharks can force their stomachs inside-out through their mouths, wash it in sea water, then pull it back to its normal location.
Shark intestines are short and compact. The surface area of the intestine is increased by internal valves, or coils, that can take any of several forms. The increased surface area slows food passage through the intestine, and speeds up the rate at which it can be digested and absorbed into the blood.
Shark livers are huge, consisting of two large lobes surrounding the digestive tract. In some sharks the liver comprises up to 30% of their body weight. The liver stores carbohydrates and fats, releases sugars for energy when needed, and contains oils that assist with buoyancy.
In marine fishes, the salt concentration of body tissue fluids is less than half that of the surrounding water, so there would normally be a tendency for water to leave the shark’s body across the gills by osmosis. But sharks compensate for this by retaining urea, a waste product of food produced by the liver. By maintaining elevated blood levels of urea, along with sodium and chloride ions and trimethylamine oxide (TMAO), sharks avoid dehydration in a saltwater environment.
Like other fish, sharks often have two types of muscle tissue, red and white (somewhat like dark and light meat on a turkey). Red muscle contains a high concentration of myoglobin, and can store a lot of oxygen. It is used for sustained swimming over long distances.
In some sharks, such as whites and makos (and in some bony fishes), the heat generated by red muscle is returned to it by networks of blood vessels. This raises the internal body temperature, and increases the efficiency of red muscle tissue. White muscle makes up the majority of muscle mass in sharks, but it doesn’t store much oxygen and can’t sustain swimming for long periods of time.