River dolphin

River Dolphin
Chinese River Dolphin
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Cetacea
Suborder:	Odontoceti
Superfamily:	Platanistoidea
Families
See text

River dolphins are four living species of dolphin which reside in freshwater rivers and estuaries. They are classed in the Platanistoidea superfamily of cetaceans. Three species live in fresh water rivers. The fourth species, the La Plata Dolphin, lives in salt-water estuaries and near-shore marine environments. However, it is scientifically classed in the river dolphin group rather than the oceanic dolphin family.

Ecology

River dolphins are in danger of extinction due to habitat loss, hunting by humans, and naturally low numbers. Also, many river dolphins possess very poor eyesight -- some are considered blind -- which can lead to unfortunate encounters with humans or human-made objects (boats or fishing nets, for example).

Taxonomy

The four families of river dolphins are classified by Rice, 1998^[1] as belonging to 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 Indus River Dolphins as separate species. 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. A December 2006 survey found no members of Lipotidae (commonly known as the Yangtze River dolphin) and declared the species functionally extinct.^[4][5]

Current classification by Rice (1998)

• Superfamily Platanistoidea
  • Family Platanistidae
    • Genus Platanista
      • Ganges and Indus River Dolphin, Platanista gangetica with two subspecies
        • Ganges River Dolphin (or Susu), Platanista gangetica gangetica
        • Indus River Dolphin (or Bhulan), Platanista gangetica minor
• Superfamily Inioidea
  • Family Iniidae
    • Genus Inia
      • Amazon River Dolphin (or Boto), Inia geoffrensis
        • Inia geoffrensis geoffrensis
        • Inia geoffrensis boliviensis
        • Inia geoffrensis humbotiana
  • Family Pontoporiidae
    • Genus Pontoporia
      • La Plata Dolphin (or Franciscana), Pontoporia blainvillei
• Superfamily †Lipotoidea
  • Family †Lipotidae
    • Genus †Lipotes
      • Chinese River Dolphin (or Baiji), †Lipotes vexillifer (functionally extinct, since December 2006)

Previous classification

• Family Platanistidae
  • Ganges River Dolphin (or Susu), Platanista gangetica
  • Indus River Dolphin (or Bhulan), Platanista minor
  • Amazon River Dolphin (or Boto), Inia geoffrensis
  • Chinese River Dolphin (or Baiji), Lipotes vexillifer (presumed extinct as of 2006)
  • La Plata Dolphin (or Franciscana), Pontoporia blainvillei

Extinction

On December 13, 2006, the Yangtze River Dolphin, or Baiji, was declared "functionally extinct", after a 45-day search by leading experts in the field failed to find a single specimen ^[4][6]. The last verified sighting of the beak-nosed dolphin was in September 2004.^[7] However, 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.^[8]

It is believed that overfishing, damming and sub-aquatic sonar pollution (which interfered with the dolphin's sonar-based method of locating food), led to the extinction. Reuters news reported this their first record of a mammalian extinction in 50 years.

Notes

References

• Reeves, Randall R. et al. (2002). National Audubon Society guide to marine mammals of the world. Alfred A. Knopf. 527 pp.