While franciscanas are river dolphins that are at home in sea water, tucuxis are the opposite - oceanic dolphins that have evolved to live in fresh water as well as in the ocean. Some tucuxis never swim in the sea. They live thousands of miles from the ocean in the headwaters of the Amazon River that flow down the foothills of the Andes in Peru and Colombia. Conversely, some tucuxis spend their whole lives at sea, ranging from the Caribbean coast of Mexico to Argentinian waters. Research suggests that this dolphin exists in two subspecies. Marine tucuxis are larger than riverine tucuxis. They also have a more bluish coloration, to help them blend in with the deep, clear waters of coastal regions.

Tucuxis are also known as grey dolphins, although their underside is a pale pink. They are small dolphins, especially those that live in the headwaters of rivers. The forehead (melon) is quite rounded, and the longish beak contains 140 teeth.

Tucuxis feed mainly on fish and shrimps. They usually hunt in small groups of two to seven dolphins, although larger groups of up to 20 in fresh water and 50 in the ocean are sometimes seen. The dolphins swim and breathe in synchrony so that they can all attack at the same time when they spot a shoal of fish or other supply of food. Tucuxis sometimes swim upside-down to trap fish against the surface of the water. In fact, individuals often have bare patches on their dorsal fin, where it has been scraped on the river bottom or seabed while swimming inverted.

Tucuxis are energetic swimmers and often leap out of the water, but they are quite timid and tend to keep away from boats.

Distribution: Amazon River system, and Atlantic coastline of Central and South America.

Habitat: Fresh and salt water.

Food: Fish.

Size: 1.4 - 1.9 m (4.5 - 6.25 ft); 40 - 53 kg (88 - 117 lb).

Maturity: 3 years.

Breeding: Single young born in summer every two years.

Life span: Unknown.

Status: Data deficient.