Porpoises are a group of fully aquatic marine mammals, that are sometimes referred to as mereswine, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). There are eight extant species of porpoise. They are small toothed whales that are very closely related to oceanic dolphins. The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade-shaped teeth distinct from the conical teeth of dolphins. Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates, and their closest living relatives are the hippopotamuses, having diverged from them about 40 million years ago.
Porpoises range in size from the 1.4 metres (4.6 ft) and 54 kilograms (119 lb) vaquita, the smallest cetacean to be discovered, to the 2.3 metres (7.5 ft) and 220 kilograms (490 lb) Dall's porpoise. Several species exhibit sexual dimorphism in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Dall's porpoise is one of the fastest cetaceans discovered, with the ability to travel at 40 knots. Porpoises have the ability to produce biosonar and it is their primary sensory system. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep them warm in cold water.
Porpoise may refer to:
Porpoise is a tradename for scuba developed by Ted Eldred in Australia and made there from the late 1940s onwards. It included:
This rebreather was developed by Ted Eldred in the late 1940s and early 1950s. Some were made and used for sport diving, but they did not go into full production. It was worn on the back without a casing. The bag was between the canister and valve assembly and the diver's back. It had two wide breathing tubes like an aqua-lung or an IDA71, running to a mouthpiece, which had a shutoff button to keep water out when it was not in the mouth. It appeared in three versions: MK1, MK2, MK3. It had a special valve that both breathing tubes ran through: it was activated by surrounding pressure; when he got deeper than was safe for pure oxygen it restricted (not stopped) the supply of breathing gas, to tell him to go to shallower depth. (Before open-circuit scuba was readily available in Australia, many Australian sport divers used war-surplus Siebe Gorman Salvuses.)
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