Sharks
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics so the shark can move faster. They have several sets of replaceable teeth. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs.
Intelligence
Contrary to the common wisdom that sharks are instinct-driven “eating machines”, recent studies have indicated that many species possess powerful problem-solving skills, social skills and curiosity. The brain- to body-mass ratios of sharks are similar to mammals and other higher vertebrate species.
In 1987, near Smitswinkle Bay, South Africa, a group of up to seven great white sharks worked together to move a partially beached dead whale to deeper waters to feed.
Sharks can engage in playful activities. Porbeagle sharks have been seen repeatedly rolling in kelp and chasing an individual who trailed a piece of kelp behind it.
Speed
In general, sharks swim (”cruise”) at an average speed of 8 kilometers per hour (5.0 mph) but when feeding or attacking, the average shark can reach speeds upwards of 19 kilometers per hour (12 mph). The shortfin mako may range upwards of 50 kilometers per hour (31 mph). The shortfin mako shark is the fastest shark and one of the fastest fish. The great white shark is also capable of bursts of speed. These exceptions may be due to the “warm-blooded”, or homeothermic, nature of these sharks’ physiology.
Sleep
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish’s spinal cord rather than its brain, coordinates swimming, so it is possible for spiny dogfish to continue to swim while sleeping.
It is also possible that sharks sleep in a manner similar to dolphins, one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.
Teeth
The teeth of sharks are embedded in the gums rather than directly fixed to the jaw, and are constantly replaced throughout the shark’s life. Multiple rows of replacement teeth are grown in a groove on the inside of the jaw and moved forward in a “conveyor belt”; some sharks can lose some 30,000 teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species teeth are replaced one at a time, while in the cookie-cutter sharks the entire row of teeth is replaced simultaneously.
The shape of a shark’s tooth depends on its diet: those that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are greatly reduced and non-functional.
Types of Sharks
Types of shark teeth
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers.[44] When a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish’s spinal cord rather than its brain, coordinates swimming, so it is possible for spiny dogfish to continue to swim while sleeping.[44]
It is also possible that sharks sleep in a manner similar to dolphins,[44] one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.
