The Tucuxi (Sotalia fluviatilis) is a dolphin found both in the rivers of the Amazon Basin and in the coastal waters to the north and east of South America. The word "tucuxi" (pronounced too-koo-shi) is dervied from the Tupi (the language of the Mayanas Indians ) word tuchuchi-ana and has now been adopted as the species' common name. Despite being found in similar geographic locations to 'true' river dolphins such as the Boto, the Tucuxi is not closely related to them genetically. Instead it is classed in the oceanic dolphin family (Delphinidae). Physically the species, particularly the marine variety, resembles the Bottlenose Dolphin. However, it is not related close enough, and is given its own genus, Sotalia.

Physical description

The Tucuxi is frequently described (see three references below) as looking similar to the Bottlenose Dolphin. However it is typically smaller, in particular the riverine ecotype (150cm) which is smaller than its marine counterpart (up to 210cm). The dolphin is coloured light to bluish grey on its back and sides. The ventral region is much lighter, often pinkish in the riverine ecotype and a lighter gray in the marine. The dorsal fin is typically slightly hooked, particularly amongst freshwater species. The beak is well-defined and of moderate length.

Distribution

The marine Tucuxi is found close to estuaries, inlets and other protected shallow water areas around the east to north South America coast. It has been reported as far south as Southern Brazil and round as far as Nicaragua. One report exists of the animal reaching Honduras. The riverine Tucuxi exists along much the length of the Amazon River and many of its tributaries, and is found in Peru, south-east Colombia, eastern Ecuador. Plenty of examples have been seen in the Orinoco River further north, though it is not clear whether these are riverine or lost marine individuals.

Behaviour

Both marine and riverine ecotypes exists in small groups of about 10-15 individuals, occasionally up to 30 in marine environments and swim in tight-knit groups, suggesting a highly developed social structure. Tucuxis are quite active and may jump clear of the water, somersault, spy-hop or tail-splash. They are unlikely however to approach boats.

Tucuxis have been observed to feed with other river dolphins. They feed on a wide variety of fish. Studies of growth layers suggest that the species can live up to 30 (marine) to 35 (river) years.

Conservation

The Tucuxi is endemic to the regions described above and, although no precise estimates of population are available, it is common. Possible natural predators are the Orca and Bull Shark. Such predation has not been observered. A significant human problem are fishing nets. Reliable figures do not exist on the number of Tucuxis accidentally killed each year due to entanglements. Deliberate hunting in the Amazon Basin, for food or for use as shark bait at sea, has also been reported. Pollution, in particular mercury poisoning of water due to gold-mining, is a particular concern for this species, which exists only close to shore.

Tucuxis are observed not to maintain good health and attitude in captive environments. A few Tucuxis remain in captivity in European aquaria.

References

• National Audobon Society Guide to Marine Mammals of the World ISBN 0375411410
• Encyclopedia of Marine Mammals ISBN 0125513402
• Whales, Dolphins and Porpoises, Mark Carwardine, ISBN 0751327816