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River dolphinFrom Wikipedia, the free encyclopedia

River dolphins

Temporal range: Oligocene – Recent 

PreЄ 

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O 

S 

D 

C 

P 

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J 

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Pg 

N 

Baiji ( Lipotes vexillifer )

Scientific classification

Kingdom: Animalia 

Phylum: Chordata

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Class: Mammalia 

Order: Cetacea 

Suborder: Odontoceti

Superfamilies 

  Inioidea 

  †Lipotoidea 

  Platanistoidea 

See text 

River dolphins are the five living 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 superfamilyPlatanistoidea, but molecular studies show that river dolphins do not form aclade. 

Four species live in fresh water  rivers. The fifth species, the La Plata dolphin, lives in saltwater  estuaries and

near-shore marine environments. However, it is scientifically classed in the river dolphin group rather than

theoceanic dolphin family.

Contents

[hide] 

  1 Characteristics 

  2 Taxonomy 

  3 Differences between marine and river dolphins 

  4 Extinction of the baiji 

  5 In literature 

  6 Facultative freshwater cetaceans and non-river dolphins in riverine environments 

  7 Notes 

  8 References 

Characteristics[edit] 

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.[1]

 

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Taxonomy[edit] 

The four  families of river dolphins are classified by Rice, 1998[1]

 as belonging to the superfamily Platanistoidea.

Formerly, Platanistidae was listed as the only extant family of the Platanistoidea superfamily. The previously

accepted classification treated all four families as belonging to this family and treated the Ganges and IndusRiver dolphins as separate species. One of those families has since become extinct.

Five lineages of dolphin have evolved to live in big, muddy rivers. 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  Lipotidae (commonly known as the Yangtze 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 Platanistaand

other river dolphins.

  Superfamily Platanistoidea

  Family Platanistidae

  Genus Platanista 

  South Asian river dolphins, Platanista gangetica, with two subspecies

  Ganges River dolphin (susu), P. g. gangetica 

  Indus River dolphin (bhulan), P. g. minor  

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  Family † Allodelphinidae (Miocene) 

  Family †Squalodelphinidae (Oligocene to Miocene)

  Family †Squalodontidae (Oligocene to Miocene)

  Family †Waipatiidae (Oligocene to Miocene)

  Superfamily Inioidea

  Family Iniidae 

  Genus Inia 

   Amazon river dolphin (boto), Inia geoffrensis 

  Inia geoffrensis geoffrensis 

  Inia geoffrensis humbotiana 

   Araguaian river dolphin, Inia araguaiaensis 

  Bolivian river dolphin, Inia boliviensis 

  Genus †Meherrinia (late Miocene)

  Family Pontoporiidae

  Genus Pontoporia 

  La Plata dolphin (Franciscana), Pontoporia blainvillei  

  Superfamily †Lipotoidea 

  Family †Lipotidae 

  Genus †Lipotes 

  Baiji  (or Chinese river dolphin), †Lipotes vexillifer  (functionally extinct, since December 2006)

In 2012 the Society for Marine Mammalogy[7]

 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,[8]

 consider

the boliviensis population to be a subspecies of Inia geoffrensis.

Differences between marine and river dolphins[edit] 

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.[1]

 

Extinction of the baiji[edit] 

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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.[5]

 The last verified sighting was in September

2004.[9]

 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.[10]

 

Overfishing, damming and subaquatic sonar pollution (which interfered with the dolphins' sonar-based method

of locating food), is believed to have led to their extinction. Reuters news reported this as their first record of an

aquatic mammalian extinction in 50 years.

In literature[edit] 

  The Hungry Tide, by Amitav Ghosh,[11]

 centres around a character studying Irrawaddy dolphins in

Bangladesh.

Facultative freshwater cetaceans and non-river dolphins in riverine

environments[edit] 

River dolphin is considered a taxonomic description  – suggesting an evolutionary relationship among the

group. 'True' river dolphins are an ancient evolutionary linage evolved in freshwater environments.

Some species of cetacean live in rivers and lakes, but are more closely related to oceanic

dolphins or  porpoises, and more recently entered freshwater. 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 asiaeorientalis 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.