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Cnidarians

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Sharks

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SHARKS: Predators with a Purpose

 

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The Whale shark (Rhincodon typus) is the largest fish on Earth, possibly reaching 60 feet in length, or about as long as two school buses! This whale shark has a remora riding on it's head. (The remora is a kind of fish that acts like a suction cup, attaching to larger animals to get a free ride).

Check out a video clip of a whale shark (2 minutes in length, 4 Mb file size, requires a quicktime viewer).More pictures of sharks can be seen here.

 

The shark is a fascinating creature, surrounded by myth and misconception. To many, sharks symbolize the very essence of ruthlessness, representing the ultimate savages of the natural world. Though many people would rather not intentionally socialize with these fearsome predators of the depths, I find it thoroughly captivating to swim among sharks. Sharks and their direct predecessors have been swimming in the world's oceans for well over 300 million years, and were going about their business long before dinosaurs walked the Earth. The fact that sharks have survived for so long without changing very much is a real tribute to the effectiveness of their anatomy.

Sharks are fishes, contained within the taxonomic class called Chondrichthyes (meaning "cartilage-fish"). Sharks and other cartilaginous fishes (rays, skates, and ratfishes) differ from the bony fishes in that they have a cartilaginous skeleton, and lack a swim bladder. This class of fishes contains over 600 species worldwide, including over 300 species of sharks.

Sometimes sharks are referred to as primitive creatures. They are an ancient group of animals, so it might seem correct to assume that they are primitive. Unfortunately, this assumption is wrong. Recent studies have shown that sharks are, in fact, quite sophisticated. Most sharks have an incredible sense of smell. These sharks can detect one drop of blood dissolved in as much as one million gallons of water. Many sharks can detect the extremely minute electrical currents generated by the muscles of swimming fish. Some sharks can sense at a great distance the tiny pressure variations generated by an injured fish struggling to swim. Contrary to popular opinion, most sharks have excellent low light vision, thanks to a mirror located behind the retina. This mirror reflects light through the retina a second time. A shark may have many rows of teeth. When an old tooth breaks or becomes too dull, a new one rotates into place. Are these the attributes of a primitive animal?

Sharks come in many shapes and sizes. The largest fish in the ocean is, in fact, the tremendous whale shark, reaching about 60 feet in length. The smallest known shark is only a few inches long when fully grown. While many sharks do have conspicuous teeth, many of these animals eat only small invertebrates. Other sharks have no teeth at all, feeding by straining plankton from the water much like the balleen whales do.

The myth that sharks are maneaters has been established throughout history with startling regularity. The fact is, you are much more likely to be hit by a car, bitten by a dog, or even struck by lightning than you are to be attacked by a shark, except under the most unusual of circumstances. Certainly sharks can eat people, but the simple fact is that they rarely do. They are no more dangerous to people than any other large predator, like a tiger or a lion. Why do we label the shark a killer, while we call the lion magnificent?

Sharks are very important in the ocean ecosystem. Like most top predators, sharks feed on the sick and weak, thereby keeping the schools of fish on which they feed healthy. Lions and tigers serve the same role in their respective ecosystems, removing the weaker animals from the herds, and keeping the gene pool strong. Although it has long been said "The only good shark is a dead one," sharks have a very important role in the ocean ecosystem, and they most certainly are not better off dead. Believe it or not, we need sharks in the oceans.

Shark populations are dwindling due to heavy commercial fishing and the general attitude that they are nothing more than a nuisance. This shortsighted view of the ocean ecosystem is dangerous--a shortage of sharks could be disastrous to the health of ocean food chains, including but certainly not limited to the ones we rely upon for food resources. We humans are already placing enough strain on these food chains as it is, without adding the shark to the equation. We must respect these animals and give them the space they need to live. Sharks really are beautiful animals...once you get to know them.

Here is a video about the biology of sharks!

Scalloped Hammerhead

The Scalloped Hammerhead in the Galapagos islands. This shark is extremely frightened of divers bubbles. Watch a cool video about Hammerheads!

The Grey Reef shark (Carcharhinus amblyrhynchos) is common around Pacific coral reefs, feeding mostly on fish and invertebrates. They reach a little over 7 feet in length. These sharks tend to swim together in groups, unlike most other sharks, making them dangerous, even though they are small. Photo by Jonathan Bird

 

The Basking Shark (Cetorhinus maximus) , is a filter feeder, and eats only zooplankton and small fishes. The filter-feeding scheme works well: the Basking Shark can grow to 45 feet, making it the world's second largest fish. This shark inhabits the cooler temperate regions of the world's oceans. It is frequently mistaken for a whale as it cruises just at or below the surface straining plankton. Photo by Jonathan Bird

One of the most dangerous sharks in the world, the Great White (Carcharodon carcharias) is perhaps best known for its role as the antagonist in the film "Jaws." This large, aggressive shark has indeed been known to attack people on occasion, although attacks are quite rare. Great Whites may reach 21 feet in length and weigh over 2,500 pounds. They inhabit all of the temperate (cool) oceans. Photo by Jonathan Bird

 

The parts of a shark. More pictures of sharks can be seen here.


How do you tell a male from a female shark? The picture above shows a MALE. The male has a pair of claspers at the base of the pelvic fins.

This is a female, showing a lack of claspers on the pelvic fins.

 

Watch the new free internet TV series about the underwater world:


JONATHAN BIRD'S BLUE WORLD

More Wonders of the Sea Lessons....

NEW! Jonathan Bird's book ADVENTURES WITH SHARKS!

   

 

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update 2/10/13