The Tucuxi dolphin (pronounced 'too-koo-shee') (Sotalia fluviatilis), is "the world's only exclusively freshwater delphinid" (IUCN) - a freshwater member of the family of Oceanic Dolphins within the order of cetaceans. It is closely related to the Costero (Guiana Dolphin - Sotalia guianensis). Tucuxi is the form of Sotalia in the Amazon region and Costero occurs in the marine and estuarine waters of eastern South and Central America and the Caribbean. Threats to the Tucuxi include water pollution, hydroeclectric facilities, noise pollution and habitat destruction.
|Tucuxi. Source: IISD|
|Tucuxi size comparison. Source: Chris Huh/Wikipedia|
Kingdom: Anamalia (Animals)
Gray river dolphin
The Tucuxi dolphin quite closely resembles the Bottlenose dolphin, but is slightly smaller with a mean body length of 1.7 meters (m) for the marine tucuxi and 1.4 m for the riverine Tucuxi. The dorsal fin is triangular in shape, curved to the back, and extends approximately 11 to 13 centimeters (cm) in height. The flippers are slightly broader in comparison to other dolphins. The eyes are about 18 millimeters in diameter. The iris is brown with a longitudinal oval pupil. In general, the Tucuxi is dark gray on its dorsal side and pale pink on its ventral side with a distinct line separating these two regions. An area of pale gray starts behind the pectoral fin and another gray area starts from the mid-body and runs to the anus. The tip of the beak and tip of the dorsal fin are occassionally white.
Sotalia fluviatilis is often confused with a smaller version of bottlenose dolphin. However, its internal structure provides distinguishing diagnostic features: undivided nasal sacs and partially unfused neck vertebrae. Sotalia manifests 26 to 35 teeth on each side of the jaw. (da Silva. 1996; Nowak. 1999)
The breeding system is polyandrous (where each female has more than one male partner), with males exhibiting aggression toward other males in the breeding season.
The average body length at which the marine Sotalia fluviatilis reaches sexual maturity is between 160 cm to 170 cm. The gestation period of the Sotalia fluviatilis is approximately 11 to 12 months. The birth size of the marine Tucuxi is 60 to 65 cm, and for the riverine subspecies, 71 to 83 cm. Birth occurs during October or November when the water levels are low. (da Silva and Best. 1996)
Relatively little is catalogued regarding the species lifespan in the wild; however, one individual in captivity lived to the age of about 32.
Sotalia is found to be most active in early morning or late afternoon. It is a slow swimmer and rarely jumps from the water. However, in the wild populations this species displays acrobatics and behaviors such as vertical and lateral jumps, somersaults, surfing in the waves of the boats, and rolling on the surface of the water. Captive tucuxi do not display aerial stunts voluntarily.
The two types of tucuxi have similar social groupings with 1-30 individuals. Groups often swim in perfect synchrony.
Sotalia fluviatilis produces echolocation at 8-15 kHz, 30 kHz, and 95 kHz. The rates of the clicks are 10-70 per second when animal is moving. The short pulses and high rates of repetition allow distinction of objects at distances less then 15 cm. Whistling has also been reported in the wild riverine tucuxi, and a higher frequency whistle has been reported when the animals are agitated in captivity.
Aggressive behavior occurs amongst males during breeding season. (Nowak 1999,da Silva and Best 1996).
Geographic range of the Tucuxi dolphin. Source: IUCN
Sotalia fluviatilis, commonly known as the Tucuxi or river dolphin, includes two subspecies, the riverine S. f. fluviatilis, and the marine S. f. guianensis. The riverine tucuxi is found in the main tributaries of the Amazon Basin in northern Brazil, Peru, Colombia, and Ecuador. The marine tucuxi is found along the Atlantic coastlines of South and Central America. (da Silva and Best 1996, Nowak 1999)
Occurring in the river systems of the Amazon and the Orinoco, as well as along the coasts from Brazil to Nicaragua, the tucuxi dolphin is split into two subspecies. The freshwater subspecies, Sotalia fluviatilis fluviatilis, inhabits only fresh water and is found as much as 250 km up the Orinoco River system and as much as 2,500 km up the Amazon River system. The marine subspecies, Sotalia fluviatilis guianensis, is found in the coastal estuaries and bays of the east coast of South America as far south as the Brazilian city of Florianópolis.
Tucuxi are found in the main channels of tributaries as well as in large lakes, but they do not enter flooded forest and avoid rapids. (da Silva and Best 1996, Nowak 1999)
The seasonal fluctuation in river water levels has a great influence on the freshwater subspecies. It enters lakes during high water but leaves as the waters begin to fall to avoid being trapped. A shy dolphin, the Tucuxi tends to be most active during the early morning and late afternoon, but is usually a slow swimmer that jumps infrequently. It dives for approximately 30 seconds, and uses echolocation to communicate as well as to catch fish and shrimp. Group size varies, but can be up to 20 in freshwater or 50 in the marine subspecies.
In addition to a diet of chiefly fish and shrimp, the following contents have also been found in the stomach of the Tucuxi: pelagic lupeids, demersal scianids, neritic cephalopods and anchovies.
The superstition of some fishermen, who believe the Tucuxi dolphin to be a sacred animal that brings the bodies of drowned people back to the shore, has ensured that it has rarely been targeted as a food item. In 1994, the International Whaling Commission's (IWC) Scientific Committee urged member states to reduce by-catch and monitor populations. The IWC had previously started the Sotalia Project with the organisation Brasil's Biologists, which set out to study the behavior and habitat needs of the Tucuxi dolphin, and has managed to build a significant collection of photo identifications. The IUCN lists the species as Data Deficient.
The greatest danger is incidental capture in the fishing nets belonging to large commerical fisheries. Other concerns are water pollution and hydroelectric dams. The species has been listed under Appendix I since 1982. Sotalia is considered "insufficiently known" by the IUCN. In Brazil, the taxon is protected under the Federal Fisheries Law. An organization in Brazil, the IWC/Brasil's Biologist, started the Sotalia Project in 1991 to study the behavior and habitat needs of Sotalia fluviatilis. Furthermore, the group aims to gain more knowledge on how humans impact the Tucuxi's habitat in Brazil. There have already been hundreds of photo-identifications of the dolphins. (Nowak. 1999; da Silva and Best; 1996)
Sotalia fluviatilis has been sometimes intentionally captured to be used as bait for sharks in Brazil. (da Silva and Best 1996)
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- Tucuxi dolphin. EOL
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