Temporal range: Oligocene–Recent
|Baiji (†Lipotes vexillifer)|
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.
The largest river dolphins usually grow up to 2.4 meters (8 feet) long, but most of the animals are smaller. River dolphins may be white, pink, yellow, brown, gray, or black.
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) 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. 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.
- Superfamily Platanistoidea
- Family Platanistidae
- Family †Allodelphinidae (Miocene)
- Family †Squalodelphinidae (Oligocene to Miocene)
- Family †Squalodontidae (Oligocene to Miocene)
- Family †Waipatiidae (Oligocene to Miocene)
- Superfamily Inioidea
- Family Iniidae
- Family Pontoporiidae
- Superfamily †Lipotoidea
In 2012 the Society for Marine Mammalogy began considering the Bolivian (Inia geoffrensis boliviensis) and Amazonian (Inia geoffrensis geoffrensis) subspecies as full species Inia boliviensis and Inia geoffrensis, respectively; however, much of the scientific community, including the IUCN, continue to consider the boliviensis population to be a subspecies of Inia geoffrensis.
Differences between marine and river dolphins
Both river dolphins and marine dolphins belong to a group of mammals called cetaceans, but they differ somewhat in appearance. For example, the snout of a river dolphin measures about 58 centimeters (2 ft) long, approximately four times as long as that of most marine dolphins. River dolphins have smaller eyes than marine dolphins, and their vision is poorly developed because they live in dark, muddy water. This environment also makes river dolphins less active than marine dolphins. River dolphins feed primarily on fish.
Extinction of the baiji
On December 13, 2006, the baiji (Lipotes vexillifer) was declared "functionally extinct", after a 45-day search by leading experts in the field failed to find a single specimen. The last verified sighting was in September 2004. In August 2007, reports surfaced that a man saw and videotaped what appears to be a baiji in the Yangtze River. A team of scientists attempted to verify the sighting beginning in September 2007.
Overfishing, damming and subaquatic sonar pollution (which interfered with the dolphins' sonar-based method of locating food), is believed to have led to their disappearance. Reuters news reported this as their first record of an aquatic mammalian extinction in 50 years.
- The Hungry Tide, by Amitav Ghosh, centres around a character studying Irrawaddy dolphins in Bangladesh.
Facultative freshwater cetaceans and non-river dolphins in riverine environments
River dolphin has been considered a taxonomic description – suggesting an evolutionary relationship among the group, although it is now known that they form two distinct clades. 'True' river dolphins are ancient evolutionary lineages evolved in freshwater environments.
Some species of cetacean live in rivers and lakes, but are more closely related to oceanic dolphins or porpoises, and entered freshwater more recently. Such species are considered facultative freshwater cetaceans as they can use both marine and freshwater environments. These include species such as the Irrawaddy dolphin Orcaella brevirostris, found in the Mekong, Mahakam, and Irrawaddy Rivers, and the Yangtze finless porpoise Neophocaena phocaenoides asiaeorientalis.
The tucuxi (Sotalia fluviatilis) in the Amazon River is another species descended from oceanic dolphins; however, it does not perfectly fit the label of 'facultative' either, as it occurs only in freshwater. The tucuxi was until recently considered conspecific with the costero (Sotalia guianensis), which inhabits marine waters. It may also be true for Irrawaddy dolphin and finless porpoise that the species might be found in both freshwater and marine environments, but the individual animals found in rivers may not be able to survive in the ocean, and vice versa.
The Franciscana (Pontoporia blainvillei) has shown a converse evolutionary pattern, and is descended from the 'true' river dolphins, but inhabits estuarine and coastal waters.
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