What is a Shark?
What is a shark? What makes a shark a shark?
Here are some of the commonalities of sharks.
Cartilaginous vs. Bone:
Sharks are a unique fish which fall in a category known as Elasmobranchs. Elasmobranchs are cartilaginous fish. This means they have no bones - no skeleton made of hard bone. Their body frame consists of material called "cartilage" similar to that of a human ear or nose!
Sharks are not alone in the category of cartilaginous fish. Others in this category are Skates, Rays and Chimaeras (sometimes referred to as ratfishes). The class that these cartilaginous fish fall into is Chondrichthyes. This term is a combination of two Greek words which mean "cartilage" and "fish."
All sharks have the characteristic of pectoral and pelvic paired fins on the underside of their bodies. They also share a strong tail with a caudal (tail) fin. However, not all sharks have the typically thought of first dorsal fin. (You know, the fin you see in scary movies that breaches the ocean's surface just before an attack.) Examples of those sharks without a first dorsal fin are: Cowsharks, the Bluntnose Sixgill Shark and the Frilled Shark.
Gills and Gill Slits:
Fish have gills - organs which take in oxygen from the water. Sharks have between 5 and 7 "Gill Slits." This differentiates the shark from other fish.
Gill slits are openings between the between the gill arches that allows water to pass through their mouths and over their gills thus providing oxygen to the shark. This is why it was commonly thought that sharks must continually swim, as they need to pump water through their gill slits in order to breathe.
It has been found however that several species of shark can "rest" motionless. They are able to pump water through their gill slits by merely opening and closing their mouths making it appear as though they are "swallowing" water. A some species' spiracles can also introduce oxygenated water into the gill cavity allowing the shark to be motionless.
Sharks have a unique "6th sense" called a Lateral Line. This is a pair of sensory tubes which run the entire length of the shark on each side of it's body. The lateral line also crosses over the top of the head above the eyes, below the eyes. The lateral line is stimulated by movement and pressure changes. So if there was another fish swimming near by or a fish or Pinniped in distress... the shark would "sense" the movement in the water and thus find it's next meal.
Electrosense: Ampullae of Lorenzini:
The ampullae of Lorenzini are small jelly-filled pours which appear over the shark's snout, around the lips and eyes. These pours are located just below the skin and make the shark aware of even weak electric fields. This too helps guide the shark to it's next meal. Every living thing gives off bioelectric stimuli. Because of this, sharks can even find prey that is buried in the sand on the ocean floor.
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