Oceanic dolphins are the members of the Delphinidae family of cetaceans. These aquatic 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".

Characteristics

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 (Heaviside's Dolphin), up to 7 meters and 4.5 tonnes (the Orca). Most species weigh between about 50 and about 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 much less.

Delphinids travel in herds, or 'pods', which may number a thousand individuals in some species. Each herd forages over a range of a few dozen to a few hundred square miles. Some herds 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.

Taxonomy

ORDER CETACEA

• SUBORDER MYSTICETI
• SUBORDER ODONTOCETI
• Superfamily Platanistoidea: All river dolphins
• Family Platanistidae: Indus and Ganges river dolphins
• Family Lipotidae: Baiji (Yangtze River Dolphin)
• Family Iniidae: Boto (Amazon River Dolphin)
• Family Pontoporiidae: Franciscana (La Plata Dolphin)
• Family Delphinidae
• Genus Peponocephala
• Melon-headed Whale, Peponocephala electra
• Genus Orcinus
• Orca (Killer Whale), Orcinus orca
• Genus Feresa
• Pygmy Killer Whale, Feresa attenuata
• Genus Pseudorca
• False Killer Whale, Pseudorca crassidens
• Genus Globicephala
• Long-finned Pilot Whale, Globicephala melas
• Short-finned Pilot Whale, Globicephala macrorhynchus
• Genus Delphinus
• Long-beaked Common Dolphin, Delphinus capensis
• Short-beaked Common Dolphin, Delphinus delphis
• Genus Lissodelphis
• Northern Rightwhale Dolphin, Lissodelphis borealis
• Southern Rightwhale Dolphin, Lissodelphis peronii
• Genus Sotalia
• Tucuxi, Sotalia fluviatilis
• Genus Sousa
• Pacific Humpback Dolphin, Sousa chinensis
• Indian Humpback Dolphin, Sousa plumbea
• Atlantic Humpback Dolphin, Sousa teuszii
• Genus Stenella
• Atlantic Spotted Dolphin, Stenella frontalis
• Clymene Dolphin, Stenella clymene
• Pantropical Spotted Dolphin, Stenella attenuata
• Spinner Dolphin, Stenella longirostris
• Striped Dolphin, Stenella coeruleoalba
• Genus Steno
• Rough-toothed Dolphin, Steno bredanensis
• Genus Tursiops
• Common Bottlenose Dolphin, Tursiops truncatus
• Indo-Pacific Bottlenose Dolphin, Tursiops aduncus
• Genus Cephalorhynchus
• Chilean Dolphin, Cephalorhynchus eutropia
• Commerson's Dolphin, Cephalorhynchus commersonii
• Heaviside's Dolphin, Cephalorhynchus heavisidii
• Hector's Dolphin, Cephalorhynchus hectori
• Genus Grampus
• Risso's Dolphin, Grampus griseus
• Genus Lagenodelphis
• Fraser's Dolphin, Lagenodelphis hosei
• Genus Lagenorhynchus
• Atlantic White-sided Dolphin, Lagenorhynchus acutus
• Dusky Dolphin, Lagenorhynchus obscurus
• Hourglass Dolphin, Lagenorhynchus cruciger
• Pacific White-sided Dolphin, Lagenorhynchus obliquidens
• Peale's Dolphin, Lagenorhynchus australis
• White-beaked Dolphin, Lagenorhynchus albirostris
• Genus Orcaella
• Irrawaddy Dolphin, Orcaella brevirostris
• Australian Snubfin Dolphin, Orcaella heinsohni
• Family Phocoenidae: porpoises
• Family Monodontidae: Beluga and Narwhal
• Family Physeteridae: Sperm Whale
• Family Kogiidae: Pygmy Sperm Whale and Dwarf Sperm Whale
• Family Ziphiidae: beaked whales

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.

References

• LeDuc, R.G., Perrin, W.F., Dizon, A.E. (1999). Phylogenetic relationships among the delphinid cetaceans based on full cytochrome b sequences. Marine Mammal Science 15, 619–648.
• May-Collado, L., Agnarsson, I. (2006). Cytochrome b and Bayesian inference of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344-354.
• Site sur les differents orques et leur mode de reproduction évitant la consanguinité.