Types of Dolphin
All the typical dolphins and porpoises belong to one family, Delphinidae; but the freshwater dolphins (4 species) are listed as a separate family Planistidae. The reason for this separation becomes apparent when a closer look at the four freshwater species is taken.
First of all, these dolphins are found in fresh water, sometimes as much as 1,000 miles from the sea! The four species live in the Amazon, Orinoco, Ganges and Yangtse rivers. They are rather primitive forms of dolphin, with a definite neck region and a poorly developed dorsal fin. Freshwater or river dolphins may have as many as 200 teeth in their long, bird like beaks. They are successful hunters, but almost certainly they detect their prey by sense of hearing and the use of sensory bristles which pick up the vibrations of the prey, as their eyes are very tiny and these dolphins are probably almost blind. As they live in waters which become extremely muddy at times, the eyes would be of little help to them anyway. Despite this apparent handicap, the river dolphin of the Amazon region is known to kill and eat the dreaded piranha fish!
The species of dolphin known to most people is the Atlantic bottle-nosed dolphin, as this is the one normally seen in dolphinariums. Bottle-nosed dolphins respond readily to training and seem to like the company of humans. They are frequently observed in dolphinariums demonstrating the extraordinary power of the tail by "standing" high out of the water and propelling themselves backwards by rapid movement of the tail.
The common dolphin is a deep water species and it has been known to dive to a depth of 609m (2,000ft) and return immediately to the surface again without any apparent discomfort or injury. This is achieved because of the remarkable changes that take place during the dive. As the dolphin commences its dive the heartbeat slows down and blood is transferred from the muscles to the brain. The nerve centres in the brain that control breathing, unlike those in the human brain, are almost insensitive to the increase in carbon dioxide in the blood and as a result of this, the dolphin is able to hold its breath without the need to "gasp" as the carbon dioxide level increases. This applies to all whales.Read More: Communication and Intelligence