The South Asian river dolphins are two species of toothed whales in the genus Platanista, both of which live in fresh water habitats in northern South Asia. Both species in the genus are endangered.
|South Asian river dolphin|
|P. gangetica leaping out of the water|
|Size compared to an average human|
|Delphinus gangetica |
|Ranges of the Ganges river dolphin and of the Indus river dolphin|
Until 1998, they were regarded as two separate species; in 1998, the classification was changed from two separate species to subspecies. More recent studies have recovered them as distinct species, and they have thus been reclassified as such.
Taxonomy and evolutionEdit
Until the 1970s, the South Asian river dolphin was regarded as a single species. The two subspecies are geographically separate and have not interbred for many hundreds if not thousands of years. Based on differences in skull structure, vertebrae, and lipid composition scientists declared the two populations as separate species in the early 1970s.
In 1998, the results of these studies were questioned and the classification reverted to the pre-1970 consensus of a single species containing two subspecies until the taxonomy could be resolved using modern techniques such as molecular sequencing. The latest analyses of mitochondrial DNA of the two populations did not display the variances needed to support their classification as separate species. However, a 2021 study re-analyzed the two populations and found significant genetic divergence and major differences in the structure of their skulls; this, when combined with the fact that the Ganges and Indus basins had not been contiguous for over 40 million years (indicating that the event that split the two species was likely a chance event with no gene flow between the split populations afterwards) led to the conclusion that both were indeed distinct species.
- blind river dolphin, side-swimming dolphin
An assessment of divergence rates in mitochondrial DNA of the two species indicates that they diverged from a common ancestor around 550,000 years ago, likely in a case of stream capture that briefly connected the Indus and Ganges basins, allowing dolphins to travel through both, up until the rivers reverted to their previous state and isolated the dolphins in new habitats. The ancestor of the genus Platanista is thought to have been a marine platanistid inhabiting the epi-continental seas in South Asia during the sea level rises in the middle Miocene. The earliest fossil identified as belonging to the genus only 12,000 years old.
The South Asian river dolphins have the long, pointed nose characteristic of all river dolphins. Their teeth are visible in both the upper and lower jaws even when the mouth is closed. The teeth of young animals are almost an inch long, thin and curved; as animals age, the teeth undergo considerable changes and in mature adults become square, bony, flat disks. The snout thickens towards its end. Navigation and hunting are carried out using echolocation. They are unique among cetaceans in that they swim on their sides. The body is a brownish color and stocky at the middle. The species has only a small, triangular lump in the place of a dorsal fin. The flippers and tail are thin and large in relation to the body size, which is about 2-2.2 meters in males and 2.4–2.6 m in females. Mature females are larger than males. Sexual dimorphism is expressed after females reach about 150 cm (59 in); the female rostrum continues to grow after the male rostrum stops growing, eventually reaching approximately 20 cm (7.9 in) longer.
Births may take place year round, but appear to be concentrated between December to January and March to May. Females gestate on average once every two years; gestation is thought to be approximately 9–10 months. After around one year, juveniles are weaned and they reach sexual maturity at about 10 years of age. During the monsoon, South Asian river dolphins tend to migrate to tributaries of the main river systems. Occasionally, individuals swim along with their beaks emerging from the water, and they may "breach"; jumping partly or completely clear of the water and landing on their sides.
South Asian river dolphins rely on echolocation to find prey due to their poor eyesight. Their extended rostrum is advantageous in detecting hidden or hard-to-find prey items. The prey is held in their jaws and swallowed. Their teeth are used as a clamp rather than a chewing mechanism.
The genus feeds on a variety of shrimp and fish, including carp and catfish. They are usually encountered on their own or in loose aggregations; the dolphins do not form tight interacting groups.
The genus lacks a crystalline eye lens and has evolved a flat cornea. The combination of these traits makes the eye incapable of forming clear images on the retina and renders the dolphin effectively blind, but the eye may still serve as a light receptor. The retina contains a densely packed receptor layer, a very thin bipolar and ganglion cell layer, and a tiny optic nerve (with only a few hundred optic fibers) that are sufficient for the retina to act as a light-gathering component. Although its eye lacks a lens (this species is also referred to as the "blind dolphin"), the dolphin still uses its eye to locate itself. The species has a slit similar to a blowhole on the top of the head, which acts as a nostril.
The dense pigmentation in the skin overlying the eye prevents light from reaching the retina from any entrance except for a pinhole sphincter-like structure. This structure is controlled by a cone-shaped muscle layer that extends from the posterior eye orbit to the overlying eye skin layer. The sphincter-like structure is capable of sensing light and may be able to sense the direction from where the light was emitted. The muddy waters and low light conditions that Platanista inhabits negate the use of the little vision that remains.
Distribution and habitatEdit
The South Asian river dolphin is native to freshwater river systems in Nepal, India, Bangladesh and Pakistan. They are found in the Indus River and connected channels, in the Beas River, in the Ganges and Brahmaputra rivers and their tributaries in Pakistan, India, Bangladesh and Nepal. They live in water with high abundance of prey and reduced flow. They migrate seasonally downstream in colder conditions with lower water levels and upstream in warmer conditions with higher water levels.
Two assessments in 2014 and 2015 estimated fewer than 5,000 individuals for the genus as a whole, of which about 3,500 belong to the Ganges river dolphin and about 1,500 to the Indus river dolphin.
International trade is prohibited by the listing of the South Asian river dolphins on Appendix I of the Convention on International Trade in Endangered Species. It is protected under the Indian Wildlife Act, although these legislations require stricter enforcement. It is listed as Endangered on the Red List of Threatened Species. The Indus river dolphin is listed as endangered by the US government National Marine Fisheries Service under the Endangered Species Act.
The genus is listed on Appendix I and Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals.
This article incorporates text from the ARKive fact-file ""Ganges river dolphin"" under the Creative Commons Attribution-ShareAlike 3.0 Unported License and the GFDL.
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