RIVER DOLPHINS AND OCEANIC DOLPHINS

River dolphins are a nontaxonomic category including five living species and one recently extinct species of dolphin that reside in freshwater rivers and estuaries. They inhabit areas of Asia and South America. All members of the group were formerly classified in the cetacean superfamily Platanistoidea, but molecular studies show that river dolphins do not form a clade. Four species live in fresh water rivers, as did the recently extinct fifth species, the baiji or Chinese river dolphin. The sixth species, the La Plata dolphin, lives in saltwater estuaries and near-shore marine environments. However, it is scientifically classed with South American river dolphins rather than in the oceanic dolphin family. Conversely, Sotalia fluviatilis is classified with marine dolphins even though it is only found within the Amazon Basin.

Characteristics

River Dolphin usually grow up to 8 feet (2.44 m) but most are smaller. River dolphins may be white, pink, yellow, brown, gray, or black.^[1]

Taxonomy

Four families of river dolphins (Iniidae, Pontoporiidae, Lipotidae and Platanistidae) are currently recognized, comprising three superfamilies (Inioidea, Lipotoidea and Platanistoidea). Platanistidae, containing the two subspecies of South Asian river dolphin, is the only currently accepted extant family of Platanistoidea. The four families were classified by Rice (1998)^[1] as all belonging to Platanistoidea. Previously, many taxonomists had assigned all river dolphins to a single family, Platanistidae, and treated the Ganges and Indus River dolphins as separate species. With the disappearance of the baiji or Chinese river dolphin in 2006, one of the recently accepted superfamilies, Lipotoidea, has become extinct.
River dolphins are thought to have relictual distributions. Their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages.^[2][3] Many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution; thus, a grouping of all river dolphins is paraphyletic. Non-South Asian river dolphins are actually more closely related to marine dolphins than to South Asian river dolphins.^[4]
A December 2006 survey found no members of Lipotes vexillifer (commonly known as the baiji, or Chinese river dolphin) and declared the species functionally extinct.^[5]
The current classification of river dolphins is as follows:^[1][6]

Phylogeny of cetaceans based on cytochrome b gene sequences, showing the distant relationship between Platanista and other river dolphins.

Superfamily Platanistoidea



Oceanic dolphins are members of the cetacean family Delphinidae. 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 killer whale, pilot (long-finned and short-finned), melon-headed, pygmy killer and false killer whales, are commonly called whales, rather than dolphins; they are also sometimes collectively known as "blackfish".

Characteristics

Atlantic Spotted Dolphin

The Delphinidae are the most diverse of the cetacean families, with numerous variations between species. They range in size from 1.2 m (3.9 ft) and 40 kg (88 lb) (Haviside's dolphin), to 9 m (30 ft) and 10 tonnes (killer whale). Most species weigh between 50 and 200 kg (110 and 440 lb). They typically have curved dorsal fins, clear 'beaks' at the front of their heads, and forehead melons, although exceptions to all of these rules are found. They have a wide range of colors and patterns.^[1]
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 killer whale, also eat marine mammals and birds. 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.^[1] Individuals communicate by sound, producing low-frequency whistles, and also produce high-frequency broadband clicks of 80-220 kHz, which are primarily used for echolocation. Gestation lasts from 10 to 12 months, and results in the birth of a single calf.

Taxonomy