What is a Shark?
Sharks are highly evolved living representatives of a large and unique group of fishes, or class Chondrichthyes (a term combining the Greeks words for Cartilage and fish).Cartilaginous fishes have a long evolutionary history, diverging nearly 500 millions years ago from the bony fishes, from which humanity has it ultimate origin.
Characterizing Sharks
Sharks are jawed fishes.They are aquatic,water breathing vertebrates with a brain and spinal cord;fins;plate-like pairs of internal gills; and paired sense organs, but they don't have swim bladders or lungs.
Sharks Jaw are simple,and armed with transerve row of hard teeth that are replace slowly but continuously, attached to the jaws by soft tissue.
All sharks have paired fins, pectoral (or breast fin) and pelvic , on the underside of the body. Most sharks have a cyclindrical or slightly depressed head or body; a strong tail with a caudal, or tail, fin; one or two dorsal fins on their back and an anal fin on the underside of the tail behind the vent, the common reproductive and excretory opening.
The skeleton of Cartilaginous fishes are composed of cartilage instead of bone, which is support by calcified nodules (tesserae) bones or bonelike tissue may be present in the scales and teeth of some shark like fishes. The prominent bony scales bones of the head, and bony fins rays of familiar bony fishes are absent in sharks. instead their body is covered with small tooth like placoid scales.
Variation of Sharks
When one thinks of sharks, they tend to picture a large aggressive white shark, with fierce some teeth and gaping jaws, cutting through water like a torpedo. While there are numbers of restless predatorial sharks that do conform to this stereotype, they are only part of the story. Sharks vary enormously in both shapes and sizes, with more than 80% of sharks being smaller than human beings
A marvelous Diversity
Sharks have a long evolutionary history during which they have fine-tuned their design to suit the varied habitats in which they lives. Today, sharks includes tiny deep sea dwarves; flattened ray like bottom dwellers; graceful, streamlined, ocean swimmers; and huge slow cruising filter feeders with caverous mouths.
Male sharks tend to be smaller than female sharks of the same species. Although a number of cat sharks reverse this rule. Roughly 8% are dwarves, the smallest being the spined pygmy shark, which reaches 15cm at maturity, and the largest being the whale shark, being up to 14m.The 'average' might have a mature size of 90cm long, hardly the monster of Jaws infamy.
Rays
700 or so of living species of rays are essentially flattened shark derivatives-"flat sharks" or "winged sharks" Rays are a diverse group. some at less than 10cm long are among the smallest of all cartilaginous fishes, while the manta, with a maximum "wingspan" of more than 6m,is one of the largest. Rays are similiar to sharks in basic body plan but have modifications related to their specialized form and their largely bottom dwelling life.
Termed silver sharks in Japan and ghost sharks in Australia, Chimaeras are realtively obscure cartilaginous fishes. Shallow water chimaeras are more silvery than sharks, but deep water chimaeras tend to have the somber dark hues of deep water sharks and rays.The split between /elasmobranches(sharks and rays) and holocephalans (chimaeras) occured early in the evolution of cartilaginous fishes some 4oo milliojns years ago, and today Chimaeras are the renmant of a mighty host of fossil holocephalans that challenged the sharks for dominace during the carboniferous.
Shark is ferocious animal. I did't know so much about sharks and their types. The pictures you have shared are awesome.