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Oceanic dolphins are the members of the Delphinidae family of cetaceans. These marine mammals are related to whales and porpoises. They are found worldwide, mostly in the shallower seas of the continental shelves. As the name implies, these dolphins tend to be found in the open seas, unlike the river dolphins, although a few species such as the Irrawaddy Dolphin are coastal or riverine.
Six of the larger species in the Delphinidae, the Orca and the Pilot, Melon-headed, Pygmy Killer and False Killer Whales, are commonly called whales, rather than dolphins; they are also sometimes collectively known as "blackfish".
The Delphinidae are the most diverse of the cetacean families, with numerous variations between species. They range in size from 1.2 meters and 40 kg (Haviside's Dolphin), up to 9 meters and 10 tonnes (the Orca). Most species weigh between approximately 50 and 200 kg. They typically have a curved dorsal fin, a clear 'beak' at the front of the head, and a forehead melon, although there are exceptions to all of these rules. They have a wide range of colors and patterns.
Most delphinids primarily eat fish, along with a smaller number of squid and small crustaceans, but some species specialise in eating squid, or, in the case of the Orca, also eat marine mammals. All, however, are purely carnivorous. They typically have between 100 and 200 teeth, although a few species have considerably fewer.
Delphinids travel in large pods, which may number a thousand individuals in some species. Each pod forages over a range of a few dozen to a few hundred square miles. Some pods have a loose social structure, with individuals frequently joining or leaving, but others seem to be more permanent, perhaps dominated by a male and a 'harem' of females. Individuals communicate by sound, producing low frequency clicks, and also produce high frequency ultrasound whistles of 80-220 kHz, which are primarily used for echolocation. Gestation lasts from ten to twelve months, and results in the birth of a single calf.
Recent molecular analyses indicate that several delphinid genera (especially Stenella and Lagenorhynchus) are not monophyletic as currently recognized. Thus, the coming years will likely see significant taxonomic revisions within the family.