Chapter one: Sharks
Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii), and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.
Since then, sharks have diversified into over 470 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep seaspecies of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), and the largest fish in the world, which reaches approximately 12 metres (39 ft). Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive in both seawater and freshwater.[2] They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth.
Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain
Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy.Because sharks do not have rib cages, they can easily be crushed under their own weight on land.
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Sea Animals
Science FictionThe sotry is about sea animals that tell us if they are extinct endangerd or rare what type of animal it is and to what type of spiece it is what it eat and where it eat when it eat and how it eat.
