What Makes a Shark a Shark?
Most people have little difficulty recognizing a shark when they see one. Yet sharks are an incredibly diverse group of fishes, and unifying them all under a single definition is not as simple as many imagine.
Sharks are, of course, most famous (and infamous) for their teeth. What makes shark teeth special is that they are produced and shed serially, rotating forward as though riding on a kind of dental conveyer belt. With this remarkable tooth replacement system, an individual shark may produce, use, and shed as many as 6,000 teeth each year. Over its entire lifetime, a shark may deposit tens of thousands of teeth into the sediment. No wonder sharks' teeth are among the most abundant of vertebrate fossils. Many people have also heard of, or experienced first-hand, sharks' sandpapery skin. This unusual texture is due to countless thousands of tiny, tooth-like scales — called dermal denticles (literally, "tiny skin teeth") — with sharp enameloid crowns poking through the tough dermis. Multiple gill slits — typically located on the side of the head - are another familiar feature of sharks. Most species of sharks have five pairs of gill slits, but a few have six and even seven pairs. Sharks are also characterized by having a cartilaginous skeleton, strengthened at strategic locations with unique hexagonal plates called "tesserae". Tesserae are composed of crystals of calcium salts and are found in parts of the skeleton — such as the jaws and vertebral centra — where strength is particularly important.
In addition to these more familiar features, sharks are characterized by some additional peculiarities. For instance, the upper jaw is not fused to the cranium, allowing the jaws to be partially protruded from the head. Like most fishes, sharks have two sets of paired fins (pectoral and pelvic, corresponding to arms and legs in humans) but - unlike those of bony fishes — they are supported by soft, unsegmented rays called "ceratotrichia" (used in making shark-fin soup). All extant sharks practice internal fertilization. This is achieved via paired, sausage-shaped intromittant organs called "claspers", which are located along the inner margins of the pelvic fins of males. Sharks lack any trace of a swim bladder (the gas-filled buoyancy-control organ of many bony fishes), and rely instead on a large, oil-filled liver to reduce their over-all density and provide some measure of hydrostatic lift. An enormous liver dominates the shark body cavity, leaving little space for other organs; as a result, the intestine of sharks is characteristically short and partitioned in any of three ways: either spiraled, scrolled, or ringed. Partitioning of the intestine slows the passage of food and increases the amount of surface area for more efficient absorption of nutrients. And lastly, rather than simply excreting nitrogenous wastes as do most vertebrates, sharks retain high tissue concentrations of urea and trimethylamine oxide. These products of protein metabolism play an important role in regulating the internal salt-to-water balance of sharks.
This complex suite of characteristics is shared to some degree by all sharks, as well as their 'pancake' cousins, the sawfishes, guitarfishes, electric rays, skates, and stingrays. Therefore, all these fishes are regarded as members of the same group within the class Chondrichthyes, the subclass Elasmobranchii ("strap gills"). Perhaps more startlingly, skates and rays are as much sharks as is the Great White.