The river dolphin that chose the open ocean

Nature · Published 17 October 2026 Swipe more like this →

In short: Most river dolphins are confined to freshwater systems, but the La Plata dolphin defies this rule. As the only member of its group to inhabit saltwater estuaries and coastal oceans, it occupies a unique ecological niche, balancing a life between riverine origins and the vast Atlantic.

The La Plata dolphin (Pontoporia blainvillei), also known as the Franciscana or toninha, occupies a distinct evolutionary space. While it belongs to the river dolphin group, it is the only species within that lineage that lives in the ocean and saltwater estuaries. Its distribution spans the coastal Atlantic waters of southeastern South America, ranging from the Tropic of Capricorn near Ubatuba, Brazil, down to Península Valdés, Argentina. Interestingly, while some individuals venture into the open ocean, many spend their entire lives within river systems, never venturing into the sea.

Physically, the species is defined by extreme proportions. It possesses the longest beak as a percentage of body size of any extant cetacean, reaching up to 15% in older adults. It is a relatively small mammal, with males growing to 1.6 meters (5.2 ft) and females to 1.8 meters (5.9 ft), weighing up to 50 kilograms (110 lb).

Its anatomy is specialized for its lifestyle: it is a bottom feeder with homodont, conical-shaped teeth, and large, broad flippers with serrated trailing edges. It is also notably inconspicuous, moving slowly and smoothly, which may be an adaptation to its status as prey for killer whales and various shark species. Conservation status for the La Plata dolphin is critical. It is listed as "Vulnerable" on the IUCN Red List and is categorized as being in danger of extinction throughout significant portions of its range under the Convention on the Migratory Species of Wild Animals (CMS). The species faces severe threats from habitat degradation caused by agricultural runoff and industrial pollution, as well as incidental capture in gillnets. In some areas, the impact is devastating; for instance, 90% of the population in Guanabara Bay was lost over just three decades.

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