Sharks, Rays and Skates
by Robert Churney Jr.
Class Chondricthyes contains over 600 species of sharks, rays, skates, and ratfish. The ratfish of Subclass Holocephali are very small in number, only 30 species. The majority of Chondricthyes is made up of the sharks, rays and skates of Subclass Elasmobranchii.
Key Features of Chondricthyes
* Jaws, paired appendages, deep visceral arches next to the
pharynx, an inner ear containing 3 semicircular canals, a paired
nasal cavity. Well developed electroreceptors.
* Cartilaginous endoskeleton. Skin covered by placoid scales and mucous glands; Teeth are modified placoid scales.
* No lungs or swim bladder - bouyancy is maintained mainly by large reserves of liver oil.
* Caudal fin is heterocercal. (a tail with an upper lobe)
* Seperate sexes, internal fertilization and direct development.
Sharks are some of the oldest creatures on the planet, true examples of an evolutionary masterpiece. These streamlined works of art range in size from the tiny 8 inch gypsy shark to the 45 foot whale shark. In contrast to the bad name sharks have, only a handful of the 350 known species are dangerous to humans. In fact, many sharks will stay far from the surface if there are humans above. Only the Great White Shark has been known to rise to the surface and "look around."
Sharks are covered by a layer of tough skin covered with small tooth-like scales called placoid scales. Unlike fish scales, placoid scales are embedded within the dermis of the shark's skin. The presence of these scales makes the skin tough like sandpaper. Placoid scales are also a clever modification for swimming. In theory, as a shark swims, a sereis of whirlpools forms behind each scale.The spinning water reduces wtaer drag and allows the shark to swim fast and efficiently. Placoid scales are also called denticles because they are thought to be miniature versions of the shark's main teeth.
Shark teeth are some of the most envied teeth in the animal kningdom. Sharks have the amazing ability to replace teeth infinitely during their lifetime. Shark teeth are not set into the jaws, like humans. Instead, teeth are set into the gums. In addition, below each tooth (inside of the gums) are layers of replacement teeth. If a tooth falls out, a brand new tooth rises from the storage layer within a few days. Some species of sharks have been known to shed as many as 30,000 teeth in one lifetime.
The shark is truly the ultimate predator. It is perfectly adapted in every way to locate, hunt and kill prey. The carnivorous diet of the shark consists of fish, other sharks, young seals and whatever else it feels like eating.
Sharks are fully-oaded carnivores that utilize an impressive array of sensing organs. The most obvious is the mouth. The tongue is used for tasting the water, as well as for tasting food. Sharks are quite intelligent too. They often feel and investigate objects by gently biting them. In fact, many shark attacks were provoked (by people) when a shark simply wanted to investigate a person. The next sensory feature is the nose. Two super-sensitive nostrils lie near the front of the head. They are so powerful they can sense a drop of blood from over a mile away. Shark eyes are also very powerful - they can see 10 times better than humans in darkness. But shark eyes are also very sensitive, and many species have developed a special membrane called a nictitating membrane to cover the eyes when needed. Sharks also have excellent hearing and are especially attracted to low frequency sounds. The less visible sensory structure is the lateral line that runs the length of the shark. It allows the shark to sense vibrations and movement underwater. Finally the most powerful sensory organ sharks have is located on the nose in the form of two little black "dots." This organ has the ability to sense electric impulses produced by an animal. Sharks use this organ to find prey and sense if the animal is dying. To illustrate another human-provoked shark attack, the faster your heart beats, the more a shark thinks you are a dying animal.
In the back of every shark's mouth is a set of parallel splits in the skin. These gills are lined with filaments that that contain blood vessels. In order to breathe, sharks must keep a constant flow of water through these gills slits. It was once thought that sharks could never stop swimming - even to sleep, but recent studies show that some sharks can lower their metabolism, allowing them to fall into a meditation-like state of sleep.
Mating and Reproduction
Sharks are sexually dimorphic, meaning there are differences between male and female bodies. Mating behavior is very complex. A male shark will select a female and follow her very closely. He may even encircle the female. At a point he will quickly bite the female's pectoral fin to stimulate her. After a while, the female will give in to the male who then positions his ventral surface along the female's ventral surface. The male then inserts his claspers into the female's cloaca. Fertilization occurs internally.
Sharks reproduce in three ways:
Oviparity: The female lays eggs that develop over a period of a few months. Young hatch at sea and are on their own.
Oviviparity: The eggs are hatched in the oviduct of the female.The embryos develop in the uterus until fully grown. Then they are released into the sea.
Viviparity: The embryo is nourished inside of the female by a placenta. The gestation period lasts almost a year before the newborn is released.
Rays and Skates
Rays and skates make up the second order of Subclass Elasmobranchii. Order Batoidei contains about 300 to 350 different species. Rays, like sharks, are found worldwide in every ocean. They are mainly docile animals. Most are slow-moving bottom dwellers that spend most of their time resting on the ocean floor. Most features of rays are true to skates as well. In fact, the two names are almost interchangable.
The main feature that differentiates a ray from a shark is the flat rough-skinned body, with the gill slits and the mouth both found on the underside of the animal. The pectoral fins of rays have been greatly modified into large wings used for locomotion. The tail has also been greatly modified into a long and slender whip-like tail. Ray tails may have one or a few small spines containing venom.
Rays, like sharks, feed on a variety of seafood. Rays eat mostly fishes and invertebrates with their blunt teeth. Some even consume plankton and small organisms. Between the shark and the ray, the two animals prey on nearly everything that swims.
A flat body allows skates and rays to settle in with the sand at the bottom of the ocean floor and wait for prey to pass overhead. Special colorations on the dorsal surface provide some camoflage with the ray's surroundings. Most rays lash out with the poison tail barb to catch unsuspecting animals. And the electric ray uses a piar of special electricity-generating organs to shock and kill prey. The pectoral wings act much like the wings of a bird, gently "flapping" to propel the ray forward. Actually, the wings move by waves of dorsoventral muscle contractions passing from the anterior end of the wing to the posterior.
There are 5 gill openings on the underside of a ray's body. Most rays breathe by taking in water through large openings on the upper portion of the head. Oxygen is taken in through the gills.
Ferilization and Reproduction
Rays display internal fertilization. Most species will give birth to live young. Skates lay flat, rectangular eggs that are enveloped by a leathery shell. Empty egg cases have been found on beaches and have come to be called mermaid's purses.
Ten Underwater Photos of Sharks, Rays and Skates
Aaron's marine Haven http://www.batcave.net/science/animals/thefintale/body.htm
Class Chondricthyes http://www-biol.paisley.ac.uk/courses/tatner/biomedia/units/fish5.htm
Physical Shark Biology http://www.geocities.com/RainForest/Canopy/3018/biology.html
Britannica Online http://www.britannica.com/
Homeworkhelp.com - Class Chondricthyes http://www.homeworkhelp.com/homeworkhelp/freemember/text/bio/high/private/ch12/1203/main.htm
Placoid Scales http://www.austmus.gov.au/fishes/students/scales/placoid.htm
Maryland Geological Survey: Miocene Sharks Teeth of Calvert County http://mgs.dnr.md.gov/esic/brochures/sharks.html
Infoplease.com - Ray reproduction and Distribution http://www.infoplease.com/ce6/sci/A0860640.html
Shark Reproduction http://www.mar.dfo-mpo.gc.ca/science/shark/english/reproduction.htm
Shark Reproduction http://web.singnet.com.sg/~sharkegg/reproduc.htm
Skates and Rays http://www.comptons.com/encyclopedia/ARTICLES/0150/01685760_A.html
The Great White Shark http://library.thinkquest.org/J001458/GreatShark.htm
Miller, Stephen A. and Harley, John P. ZOOLOGY: 3rd edition, 1996 McGraw-Hill Companies, Inc.
McLachlan, Dan H. and Ayres Jak FIELDBOOK OF PACIFIC NORTHWEST SEA CREATURES, 1979 Mc Lachlan and Ayres
For additional information on Class Chondricthyes, search below on Google.
Underwater Images (c) and Courtesy Of http://www.fotoclipart.com/ and http://www.corbis.com/.
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