From Wikipedia, the free encyclopedia
Grey reef shark
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 (orSelachii), and are the sister group to the rays. However, the term “shark” has also been used for extinct members of the suborder 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.[1]




Until the 16th century,  sharks were known to mariners as “sea dogs”.   The etymology of the word “shark” is uncertain. One theory is that it derives from theYucatec Maya word xok, pronounced ‘shok’.  Evidence for this etymology comes from the OED, which notes the name “shark” first came into use after Sir John Hawkins‘ sailors exhibited one in London in 1569 and used the word “sharke” to refer to the large sharks of the Caribbean Sea.

An alternate etymology states that the original sense of the word was that of “predator, one who preys on others” from the German Schorck, a variant ofSchurke “villain, scoundrel” (cf. card sharkloan shark, etc.), which was later applied to the fish due to its predatory behaviour.



General anatomical features of sharks







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.


Jaws of sharks, like those of rays and skates, are not attached to the cranium. The jaw’s surface (in comparison to the shark’s vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called “tesserae“, which are crystalblocks of calcium salts arranged as a mosaic.  This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull sharktiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers.  In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.


Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hairand feathers.  Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.

What is your favorite shark and why?

Also check out Portland Aquarium’s latest educational display features new sharks that are known as juvenile epaulette sharks, also known as “Walking Sharks.”

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