An intimidating picture of the Caribbean reef shark, carcharhinus perezi, inspired me to research this terrifying creature. Of all the marine organisms potentially dangerous to man, none are so universally feared and respected, as are sharks. This fear is drawn by common societies lack of knowledge concerning this predators history, biological makeup, and feeding habits.
It has taken 400 million years for the jawless fish to evolve into the flesh-shredding sharks known of today. Until the late 1880ís, there was little knowledge about the evolution of sharks: when an amateur archeologist found fossilized remnants in an ancient mud bed south of Cleveland, Ohio. Since then, sharks have been separated into 19 family classifications, and 368 species.
Body proportions, fin size, location, and other features are somewhat inconsistent in the categorization of sharks. Many of the body features such as dimension and coloration change with the shark's growth. Coloration is the LEAST consistent for identification in most cases as coloration depends on environmental and dietary habits. Although, there are three factors for identification in each species of shark; fin size, weight, and liver oil proportions. Liver oil proportions help in identification because they permit a shark to control the depth at which it swims.
The Caribbean reef shark can be identified from other sharks by three factors; its relatively large eyes, distance between pelvis and anal fins being less than 3/4 length of the anal fin base, and length of second dorsal being greater than its height. It can grow from a birth length of 38 inches to 8 feet in its life span of approximately 25 years. Depending on its location of habitat the Caribbean reef shark can appear to be an olive-gray or yellow gray. They inhabit reefs found in the Bahamas, West Indies, and the SE Florida coast (Pope 5-7). These sharks are usually seen actively swimming in the open ocean, but when in the caves 20m down, they lay on the bottom motionless (Harrigan "Shark Diving Bahamas Style").
Due to the fact that sharks have a skeleton composed of flexible cartilage, reinforced with some mineral deposits, they are able to swim gracefully through the oceansí waters. Cartilage is beneficial to sharks instead of bone because it is less dense. A skin similar to that of mans covers the sharkís cartilage. The skin is similar to that of manís because of its outer epidermis, which can regenerate itself and the second layer called the dermis, providing pigmentation of the hide. The epidermis is covered with placid scales called dermal denticles, which are like thousands of minute teeth firmly embedded in the skin. These tiny scales of a sandpaper texture possess a root canal, blood vessels and nerves. In some spaces the denticles are rounded, smooth and without points. Denticle sizes can be found in a wide array of serried ranks or spread more dispersely.
The eyes of a shark are covered by an upper and lower eyelid, in order to protect itself from struggling pray. The Caribbean Reef Shark is moderately far sighted, seeing up to 50 feet in front of them. Viewing their range of eyesight and movement to different depths, maximum dilation and constriction take about a minute, which is much faster than humans. This also helps with quickly attacking prey. They are colorblind since seawater absorbs most colors rapidly. Vision therefore, is not as important to them in detecting prey as their enhanced senses of smell and hearing.
The sense of smell is ideal for use in a sharkís hunt for itís next meal. Their keen sense of smell allows them to smell the blood of wounded prey from outrageous distances. Hearing is also primary for sharks to locate their prey: allowing them to hear low tone sounds 1000ís of yards from their location. They have an internal ear on each side of their head along with a lateral-line system that extends along the sides of their head and body. The lateral line consists of fluid-filled sensory canals that contain receptors sensitive to vibrations, pressure changes, waves and movements in the water.
The breathing process of a shark begins by swimming with its mouth open and allowing water to enter the body. With the throat blocked by muscular tissue, the water exits through the external five-gill slits, or clefts, on each side of its head. At the same time that the water is rushing through the gills oxygen is extracted and absorbed into the blood. Simultaneously, carbon dioxide is released from the blood into the water.
The tail plays a major role in providing the power and direction for swimming by sweeping from side to side to produce a sculling motion. Unlike most fish, sharks do not have a swim bladder. This makes them denser than ocean water and for this reason, sharks must constantly be swimming to avoid sinking. The sharkís liver also counteracts the tendency to sink. Occupying 90% of the body cavity, the liver consists of oils that are lighter than water, which decrease the density of its body. Constantly moving provides a steady stream of oxygenated water to the gills. Since oxygen is lighter than water, this also allows the shark to maintain its depth without sinking. The constant movement allots for the 35,000 miles that some sharks swim yearly.
During the breeding season, many female sharks are seen with torn and scarred fins, from male teeth marks. The reproduction of sharks is through the female being internally fertilized. This process of internal fertilization is through insertion of the maleís claspers into the cloaca, anal opening, of the female. The time elapsed from conception to birth is 12 months.
The teeth become important tools in the ultimate identification of any sharkís species, genera or family due to being different shapes and sizes. There are two types of teeth; incisors are for cutting and molars are for crushing. The transition from incisors to molars is very gradual and the upper jaw generally differs from the lower. Sharks, like humans, have teeth that are not implanted in sockets but are attached at their bases to a connective tissue, and anchored by a root. Frequently loosing and breaking teeth, sharks have what are called functional teeth. Functional teeth are several rows, commonly 5-7, under the gum tissue that are able to replace at a momentís notice. When a tooth is broken or lost, the next tooth behind it begins to move up and forward filing in the space. Replacement of teeth in the lower jaw takes 8 days and upper jaws 7 days. The teeth are replaced so quickly that an individual shark may shed as many as 30,000 teeth during its lifetime.
Nothing is more unpredictable than a sharkís behavior when faced with food. Sharks are extremely efficient when it comes to finding food because they can hear things that are thousands of yards away and detect pray by sight, smell, sound, touch, taste, water vibrations, and electric fields. Recent heavy rains attract sharks due to the washing down of dead animals. However, their appetite is quickly satisfied because it is only once or twice a month that they are able to eat an ill or dead dolphin. Sharks consume food equal to 3 to 14 percent of their body weight within one week.
The stomach of a shark is in the shape of a U and is therefore not coiled like that of humans. Having the intestines relatively short helps to digest food quickly but creates a problem with the rate of absorption, which the stomach mussels help with.
Sharks can sense one part mammal blood to 100 million parts water. However, individuals have a 1 in 5 million chance of being attacked by a shark. There is a better chance of being struck by lightening. Several sharks considered dangerous, but have few recorded attacks and far fewer fatalities, are the gray reef shark, blacktip reef shark, and reef shark. They are unagressive in situations where they meet divers, but will retaliate vigorously if they are disturbed, speared, or provoked. A common thought is that the youngest sharks, the smallest, are the most brazen. Even a very small shark, two feet in length can inflict dangerous wounds.
Sharks are extremely sensitive to odors of blood and are accustomed to attacking anything that floats. For divers the moments of entering and leaving the water are particularly dangerous since only a third of shark attacks occur in water deeper than 5 feet. However, sharks will never attack a diver below the surface immediately. They will circle around the individual, go away and then cautiously return. This gives the diver time to decide whether to remain or return to the surface. In clear water, and daylight a diver is in little danger if they encounter a shark, but it is dangerous to dive at night. Not only is it dangerous to dive at night, but to show fear of a shark, because he knows this by instinct, and can profit from it. The best protection lies in ease of movement in diving, swimming slowly and softly, and avoiding abrupt change of position. If a shark should swim toward you, try not to swim away but face them calmly
There still exists no effective means of keeping sharks away from the area in which you are diving, either by chemical products, sound waves or fields of electricity. "All things considered, diving in tropical waters is actually much less dangerous than riding a motorcycle" (Cousteau 240). Precautions to take are to always swim with a partner. If wearing scuba gear, its best to remain submerged in the water until you have reached the boat because sharks make the connection of fishing boats and the presence of fish at the end of the lines. Do not deliberately grab, injure, or provoke even a small seemingly harmless shark. Amongst the causes of premature death, sharks are ranked far below cars, motor cycles, alcoholism, excessive smoking, and narcotics.
Budker, Paul. The Life of Sharks. New York: Columbia University Press, 1971. 41-101
Cousteau, Jacques-Yves, and Philippe Cousteau. The Shark "Splendid Savage of the
Sea". Farden City, New York: Doubleday & Company, Inc.,1970. 229-240.
Faughnan, Victor R. The National Shark-O-Pedia. Honolulu, Hawaii: Undersea
Resoueces, Ltd,. 1980. 1-9.
Gold, Joy P., and Victor G. Springer. Sharks in Question. Washington, D.C.:
Smithsonian Institution Press, 1989. 51-94,
Harrigan, Bill. "Shark Diving Bahamas Style." Bahamas Diver 1998/1999: 10-12.
Pope, Patricia E. A Dictionary of Sharks copyright 1973 Great Outdoors Publishing
Company St. Petersburg, Florida. 5-7, 78-86.
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