Ganges River Dolphin

The South Asian river dolphin (Platanista gangetica) is an endangered freshwater or river dolphin found in the region of South Asia which is split into two subspecies, the Ganges river dolphin (P. g. gangetica) (≈3,500 individuals) and the Indus river dolphin (P. g. minor) (≈1,500 individuals). The Ganges river dolphin is primarily found in the Ganges and Brahmaputra Rivers and their tributaries in Bangladesh, India and Nepal, while the Indus river dolphin is now found only in the main channel of the Indus River in Pakistan and active channels connected to it between the Jinnah and Kotri barrages as well as in the Beas river in India. From the 1970s until 1998, they were regarded as separate species; however, in 1998, their classification was changed from two separate species to subspecies of a single species (see taxonomy below). The Ganges river dolphin has been recognized by the government of India as its National Aquatic Animal and is the official animal of the Indian city of Guwahati. The Indus river dolphin has been named as the National Mammal of Pakistan and state aquatic animal of Punjab, India. The species was described by two separate authors, Lebeck and Roxburgh, in 1801, and it is unclear to whom the original description should be ascribed. 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. Thus, at present, a single species with two subspecies is recognized in the genus Platanista, P. g. gangetica (Ganges river dolphin) and P. g. minor (Indus river dolphin). An assessment of divergence rates in mitochondrial DNA of the two subspecies indicates that they diverged from a common ancestor around 550,000 years ago. This ancestor 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 species is only 12,000 years old. The South Asian river dolphin has 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; however, 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. The oldest recorded animal was a 28-year-old male, 199 cm in length. 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. 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. The South Asian river dolphin relies 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 jaw and swallowed. Their teeth are used as a clamp rather than a chewing mechanism. The species feeds on a variety of shrimp and fish, including carp and catfish. The Ganges subspecies may take birds and turtles. They are usually encountered on their own or in loose aggregations; the dolphins do not form tight interacting groups. The species 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.