Sharks are a diverse group of carnivorous species, ranging in size from the tiny dwarf shark (Squaliolus laticaudus), which matures at less than fifteen centimeters in length, to the enormous whale shark (Rhincodon typus), which reaches fifteen meters or more in length and represents the largest fish species of any kind. Curiously, the whale shark and the nearly-as-large basking shark (Cetorhinus maximus) are plankton feeders. They capture their tiny food organisms by swimming open-mouthed through the water and straining out the plankton with fine comblike structures in their gills, called gill rakers. Most sharks, however, have sharp, bladelike teeth, suitable for attacking and feeding on more active prey. The white shark (Carcharodon carcharias) is a voracious roving predator that may grow to twelve meters in length. It has been implicated inmore fatal shark attacks than any other species. In addition to well-developed eyes, inner ears, and olfactory (smell) organs, sharks possess a lateral line system, as do most bony fishes. This is a sense organ consisting of a canal beneath the skin, on each side of the body, connected to the surface by numerous pores. It is sensitive to vibrations in the water, giving sharks a sense of "distant touch" that enables them to navigate and hunt their prey in murky water. Another sensory feature of sharks and other elasmobranchs is an electroreception system, consisting of receptors, called ampullae of Lorenzini, on the surface of the snout. Apparently, this system is useful in hunting, since it allows the weak electric fields produced by the muscle contractions of prey species to be detected. Itmayalso function in intraspecific communication (communication with others of the same species), since many elasmobranchs possess electric organs. Sharks are typically torpedo-shaped and slightly depressed in form-that is, flattened from top to bottom. They swim by means of rhythmic undulations of the body, which are produced by sequential contraction of the myomeres (body-muscle segments). The tilt of the shark's pectoral fins (the paired fins toward the front of the body) and heterocercal tail (the upper lobe of the tail fin being larger than the lower lobe) enable it to maintain its relative depth position as it swims forward, despite the fact that the shark lacks a swim bladder. Also improving the buoyancy of sharks are their cartilage skeletons, which are lighter than bone, and their large, oily livers. Some shark livers contain a unique low-density oil called squalene. Sharks and other cartilaginous marine fishes regulate the concentration of solutes (dissolved substances) in the body in a manner very different from that of the bony fishes, which either retain salt (freshwater bony fishes) or secrete salt (marine bony fishes). Sharks maintain a concentration close to or higher than that of seawater by retaining urea and trimethylamine oxide, two relatively nontoxic nitrogenous waste products. Reproduction in the sharks and other cartilaginous fishes is characterized by internal fertilization. A pair of intromittent, or copulatory, organs called claspers are located on the pelvic fins (the paired fins nearer the tail region) of the male. These are used to transfer sperm to the female genital opening. Embryos remain in the body or are released in egg cases, for a long gestation, or development, period. A small number of young either are born alive or hatch from an egg case in active, well-developed form.
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