Sharks (Chondrichthyes/Elasmobranchia): The Bahamas

This topic submitted by Kelly Haas ( haaskn@miamioh.edu) at 11:57 PM on 6/5/06.

Jamie and his research team work on the "Moon Lab" in San Salvador, Bahamas. See other phenomena from the Bahamas.

Tropical Field Courses -Western Program-Miami University



Kelly Haas
Tropical Marine Ecology
June 5, 2006

Sharks (Chondrichthyes/Elasmobranchia): The Bahamas

There are an estimated 25,000 species of fish living in the Earth’s water today. Of those known, 368 species are sharks, belonging to the class Chondrichthyes (also called Elasmobranchia). The top of the food chain the ocean over comprises only one percent of all fish species, managing to be the most effective predator in every habitat and niche. Unfortunately, due to their relative scarcity, shyness, and their tendency to migrate and roam frequently, scientists know very little about these amazing creatures (Tricas, 14).

Sharks have roamed Earth’s waters since at least 350 million years ago (some estimates which include shark’s ancestors say 400 million years), making sharks older than dinosaurs, which evolved 230 million years ago. Modern sharks have not evolved much in the past 150 million years. So what has made them so successful? Sharks are the fiercest carnivores (Great White) and the largest fish (Whale and Basking) in the sea. Their teeth, along with their innately mysterious environment, have long made them frightening and intimidating creatures to land-dwelling people (“Sharks;” Tricas, 15).

The average shark lives about 25 years, while some can grow to be 100 or more (especially Whale sharks). This old age is balanced by the low birth rates, and long gestation periods. Some sharks remain pregnant for up to 2 years, while birthing only one or two pups per pregnancy. Reaching maturity after many years (much slower than most of the rest of the animal world). At adulthood, the smallest shark (Pigmy) grows to an average of 7 inches, while the largest can easily break 50 feet (the Whale and sometimes Basking sharks)(“Sharks”).

Unlike most fish, sharks cannot swim backwards. They also lack a swim bladder, but make up for it with an oily liver that helps them rise or sink like a submarine. Their very light and flexible cartilaginous skeletons also help keep these fish from sinking uncontrollably or from wasting energy hauling around a heavy, calcified bone skeleton. Most sharks have between three and five rows of teeth – all finely serrated and triangular for strong, piercing bites – with the first row being the largest and most important. Other, bottom-dwelling sharks that feed on shelled creatures such as snails and clams have jaw surfaces covered by flat teeth, perfect for crushing hard exoskeletons. Shark teeth are typically replaced about every eight days. There may be up to 3,000 teeth in a shark’s mouth at any time. In a lifetime, a shark may lose as many as 30,000 teeth in a lifetime. They are the only animals that have a mobile upper – as well as lower – jaw, which dislocates from the skull, remaining attached to the lower jaw by a small piece of cartilage, preceding a bite (see “Jaw Attachment” at left). This allows them to open their mouths wider/higher for bigger bites and helps to create a sawing effect on prey when the shark thrashes back and forth. Sharks differ from the bony fishes in that all sharks’ gills are unprotected (between 5 and 7 gill slits, plus a spiracle). In bony fishes, the gills are covered by gill flaps. An extra gill has evolved into what is known as a spiracle. The spiracle is a small hole near the eye, where water is taken in for breathing. It successfully cleans the water, not allowing debris into the gills, as the outer gill slits might. But, the most amazing tool which the shark uses to seek out prey is by far the Ampullae of Lorenzini. These little detectors, located on the snout, can sense very low electrical fields from up to a foot of distance, even through barriers of sand or rock where many prey are likely to hide). Like most great predators, the Chondrichthyes have become the scariest and most dangerous predators of the sea, by skillfully taking advantage of a weakened or dead creature’s vulnerability: scavenging. Most sharks actively scavenge, because it is easier to do than to pursue a healthy, terrified individual (“Sharks”).

While we are afraid of sharks, they are the ones who should really be afraid. Many sharks are translocating due to overfishing, while other species are simply becoming endangered because of it. The Mako shark’s nasal cartilage was once used in ancient burial rituals by the Chumash people in what is now Santa Barbara, California. Shark teeth used to be popular in weapons, generally banded together in rows to make blades of knives and swords. Some cultures in Melanesia, Micronesia, and Polynesia used to make drums from sharkskin. Shark fin soup has been considered a delicacy in China for more than two millennia. And even in the United States, shark meat has been gaining popularity. But, it is often misleadingly labeled (as “grayfish”), so that consumers do not realize or know that it is shark. And, until the 1950s, when vitamin A could finally be synthesized, the shark’s oily liver used to be prized for its concentration of the vitamin. But recently, shark liver has become popular again, but for a new reason: squalene oil. It is used across machinery, cosmetics, and pharmaceuticals. The market value of sharks is increasing again due to the viability of corneal transplants. Even today there are shark-fishing tournaments around the world that mean a bloodbath for sharks. There is one tournament which actually aims to help the sharks. It is put on by Mote Marine Laboratory, Florida. At the end of the contest, the sharks are tagged and released in order to gain more knowledge about such things as migration/movement of sharks and their sensitivity to capture and release (Tricas, 61).

Habitat helps to identify and classify sharks. There are two general categories: pelagic and benthic, along with more specific latitudinal classifications: epipelagic, mesopelagic, and bathypelagic (see diagram below). Pelagic sharks live in the open ocean, are more likely to travel great distances daily or by migrations, and tend to be more dangerous than the lazy, bottom-dwelling benthic-zone sharks. Knowing the area in which a shark was found can significantly simplify the process of finding out the species.

Along with region of the ocean, there are many obvious physical features that can help at once to identify a shark (see diagram below). These include number of gill slits (there are few sharks with six or seven), presence/absence of anal fins, dorsal spines, spiracle, or nictating eyelid (which usually flashes before an attack); mouth-position, size of shark, and teeth shape and number (Tricas, 92).

There are only 25 recorded species that have attacked humans. Although there is some dispute between rankings four and below, the three sharks that are undisputedly the most dangerous to humans are (in order) the Great White (Carcharodon carcharias), the Tiger shark (Galeocerdo cuvier), and the Bull shark (Carcharhinus leucas). A person bitten by a great white is most likely to die, but a person is much likelier to be bitten by a Bull shark. The Oceanic Whitetip shark (which is said to attack more humans than all other shark species combined), Gray shark, Blue shark, Hammerhead shark, Mako shark, Nurse shark, Lemon shark, Blacktip reef shark, Wobbegong, Sand Tiger, Spitting shark, and Porbeagle have also been known to attack people on rare occasions. From 1749 to 2005, there were only 24 shark attacks including 1 fatality (in 1968) in the Bahamas. Most attacks take place near or between sandbars, where many sharks naturally feed (“1749-2005 Map”).

There are three classifications of shark attacks: hit and run, bump and bite, and sneak. The hit and run is the most common and usually occurs in surf waters, where sharks presumably mistake humans for prey and therefore fail to return for a second bite; the damage is usually inflicted below the knee and is nonfatal. (Surfers often look like sea turtles when their legs hang over the surf board). The bump and bite is the second most common, occurring generally in deeper waters with divers or swimmers. It is characterized by the shark first circling, then bumping, and finally biting the victim. This type causes more damage and many more fatalities than the hit and run. This is a more aggressive, perhaps territorial attack. Sneak attacks are not very common, but they are the most dangerous with repeated bitings characteristic of a sneak attack. This type causes the most fatalities per attack and is thought to be provoked by touching, feeding, or antagonistic behavior (Rodriguez). (The diagram to the left displays the differences between peaceful and aggressive behavior.)

There are measures a person can take to decrease the chance of shark encounter: always staying with a group (sharks usually attack persons who are alone in the water), avoiding being in the water at night, dusk, or dawn (when sharks usually feed), staying out of the water when bleeding or menstruating (sharks can blood in water at such low concentrations as 1 part per billion), not wearing shiny things (i.e. jewelry – it looks like fish scales to sharks); excessively splashing (it draws attention and makes the person appear wounded and vulnerable: like prey). Once a shark is actually spotted the diver should remain calm, back away slowly (if at all), and maintain eye contact. Many sharks will inspect divers out of pure curiosity and will not do any harm. If the shark does come too close, however, a hard punch on the snout will usually send it away (Tricas, 97).

The three most commonly spotted sharks of the Bahamas are the Tiger, the Bull, and Caribbean Reef – all three of these are considered to be very dangerous to humans (Tricas, 240-241).

 The Tiger shark (Galeocerdo cuvier), aptly named for its unique skin coloration, also has similar behavior to its terrestrial counterpart. The Tiger shark will initially attack and flee, especially in the case of its favorite meal: sea turtle, who snaps inside his shell. Although the turtle will eventually come back out of its shell, there is a predator below just waiting to pounce. Once the turtle does feel safe enough to continue on his way and slowly reveals his arms and head, the Tiger shark is there in a second, biting whichever appendage he chooses. This process continues until the turtle dies or the shark gives up (upon failing multiple attempts). Along with sea turtles, the Tiger shark eats rays, lobsters, and birds and carcasses from the surface. The Tiger sharks found in the Atlantic, however, seem to be far less aggressive than those found in the Pacific (Benchley).

 The Bull shark (Carcharhinus leucas) is especially frightening. It is a stout-looking, girthy shark. Small eyes, general body build, and a blunt round snout are the identifying features for this shark. It is usually a dark gray with a whitish underbelly. The Bull is usually found by divers in deeper waters, not by snorkelers in surface reefs. It has even been found in and has attacked people in brackish and fresh water – in rivers that connect to the sea – whereas most sharks cannot tolerate anything less saline than ocean-salt water. It can grow between 7 and 11.5 feet long, will eat anything (including people), and is therefore is considered aggressive towards humans (Abernethy; “Sharks”).

 Caribbean reef sharks (Carcharhinus perezi) eat a variety of bony fishes, is very common in the Bahamas, and ranges in length from 4 to 9 feet. Some experts consider this shark the most dangerous you are likely to see in the Bahamas (“Sharks;” Abernethy).

Other commonly sighted sharks of the Bahamas include (in no particular order):

1. The Nurse shark (Ginglymostoma cirratum) is very common throughout the Bahamas, especially in coral reefs. These are benthic creatures with almost imperceptible eyes which vary between 2 and 13 feet in length. They are virtually harmless to humans, unless provoked. And, unlike most sharks, this one is tan in color (“Sharks”).

2. Lemon sharks (Negaprion brevirostris) are common along coastlines. They prefer shallower water, especially mangrove bays for basking during daytime, but venturing into deeper water to feed at dusk and evening. They are named for their comparably light coloring and dark yellow back. Females tend to be bulkier than males and can reach over 10 feet in length. It eats mainly fish, but has been known to attack people. It is not considered very dangerous, but should be treated with caution (“Sharks”).

3. The Blacktip Reef shark (Carcharhinus melanopterus) is rather common to the Bahamas and is similar in appearance to the Caribbean Reef shark. Some of its fins, however, are tipped with black (hence its name), it has white stripes along each side, and it has a longer, pointier snout than a Caribbean. It can grow to about 6 feet in length. It generally hunts in small groups, feeding on smaller reef fish. It is not thought to be very dangerous to humans (Abernethy).

4. The Oceanic Whitetip (Carcharhinus longimanus) shark can also be found near the Bahamas. They range in size from 10 to 13 feet. Like the Blacktip, they have fins which look like they have been dipped in paint (in white paint, instead of black). They do not live near the coast, but instead tend toward more open water, swimming with Shortfin Pilot whales whenever possible (the reason is not known). Their behavior is unpredictable, but known to be extremely aggressive at times so it is best to avoid them when possible (“Sharks;” Benchley).

5. Silky sharks (Carcharhinus falciformis) can also be found in large numbers in the Bahamas. They tend to stay in deep water, not too far from the shore. They do look much smoother than other sharks, and their coloring varies from silver to bronze (“Sharks”).

6. Great Hammerhead (Sphyrna mokkaran) sharks can be sighted in Bahamian waters, but they are rarer than most of the aforementioned species and are usually only spotted in the fall and winter months before migrating to cooler waters in the summer. They, as most sharks do, begin feeding at dusk. They range in size from 10 feet to 20 feet. It is easily identifiable by its trademark rectangular, or hammer-shaped, head. It eats octopuses, other sharks (including other Hammerheads), crustaceans, squid, rays, and other fish. This shark can at times be aggressive, but generally does not attack people (“Sharks”).

7. Bonnethead sharks (Sphyrna tiburo) are a type of Hammerhead which can be found in reefs of the Bahamas. It is a smaller, shy shark with a semicircular head-shape. It reaches about 3.5 feet in length on average. It has a variety of teeth, suited to eat hard-shelled crustaceans and small soft-skinned fish. It is not considered dangerous to humans due to its timid nature and small size (“Sharks”).

8. The Blue shark (Prionace glauca) can be found worldwide, like most pelagic sharks which roam the oceans freely. It is named for its very vibrant blue coloring. It has a graceful, sleek body and large eyes and is thought to be among the fastest fish in the ocean. They can reach up to 12.5 feet in length. Generally, the Blue shark eats squid and fish. They are often found in large groups of individuals nearly the same size and the same sex. It is classified as nearly endangered, and has been known to attack people, however infrequently, and is not considered a high risk predator (“Sharks”).

9. The Sawshark (Pristiophorus schroederi) is also common to the Bahamas. It is brownish in color, grows to almost 3.5 feet, and has a very distinguishable, long saw-like snout. It eats a variety of small ocean-dwelling creatures. It is virtually harmless to people (Abernethy).

10. The Whale shark (Rhincodon typus) is easily identified by its unique patterning and its extremely large size. It is the biggest fish living in the oceans. It ranges from 25 feet to 50 or more in length. It has thousands of tiny teeth, which, like a whale, it uses to siphon plankton out of the water. It is not harmful to humans, nor afraid of them. But if provoked in some way, can provide a crushing blow with its muscular tail (“Sharks;” Tricas, 151).

There are also other sharks which are common to the Bahamas, but which are not commonly seen due to their living at such great depths, such as the Bluntnose Sixgill shark (Hexanchus griseus), the Mako (Isurus oxyrinchus), the Spined Pygmy shark (Squaliolus laticaudus), and the Common Thresher (Alopias vulpinus), which sometimes surfaces and may even jump into the air.

Even though we tend to find these creatures of the deep (and shallows) to be utterly terrifying, we must conserve them. They keep countless oceanic populations in check, which in turn balances the whole ecosystem. If we overfish to the point of extinction, the creature the next trophic level down will rise to the top of the food chain. With barely any creature, especially living in the ocean, reproducing as slowly and unfruitfully as the sharks, the animal that would move into the shark’s niche would inevitably reproduce (overproduce) unchecked and therefore overeat its own prey, throwing the whole system off balance, until finally reaching its carrying capacity where it would begin to die out again. Not to mention that killing off a shark population kills some of our own needs and wants. The delicacy of shark fin soup, the shark to human cornea transplant; the squalene oil (which hopefully can be synthesized someday soon) will all become extinct, too. (The United States recently protected 39 species of shark in quest for biodiversity and an obligation to, if not give back, at least to replace what we have taken from our earth. Most conservation attempts are privately funded and operated; national governments generally tend to avoid such issues.) Regardless of all these problems, with a creature who directly affects our daily lives so little and so rarely, and from whom we gain so much, why should we bother to obliterate its entire existence? The sharks have been around for nearly 400 million years, do we really want to diminish them to only memories within less than a couple hundred?

Bibliography

"1749-2005 Map of the Bahamas' and the Antilles' Confirmed Unprovoked Shark Attacks." International Shark Attack File. 10 May 2006. Florida Museum of Natural History, University of Florida. .

Abernethy, Jim. "Bahamas Shark Encounters." JASA. .

Benchley, Peter. Shark Trouble. New York: Random House, 2002. 54-68.

Rodriguez, Melissa. "Shark Attack." About. .

"Sharks." Enchanted Learning. WGBH Educational Foundation. .

Springer, Victor G., and Joy P. Gold. Sharks in Question. Washington, DC: Smithsonian Institution P, 1989. 16.

Tricas, Timothy C., Kevin Deacon, Peter Last, John E. McCosker, Terence I. Walker, and Leighton Taylor. The Nature Company's Guide to Sharks and Rays. Time-Life Books, 1997.

For Further Info on this Topic, Check out this WWW Site: http://scuba.about.com/od/sharks/a/sharksattack.htm.
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