river dolphins have two flippers and a tail fin. these flippers contain four digits. although river dolphins do not possess fully developed hind limbs, some possess discrete rudimentary appendages, which may contain feet and digits. river dolphins are slow swimmers in comparison to oceanic dolphins, which can travel at speeds up to ; the tucuxi can only travel at about . unlike other cetaceans, their neck vertebrae are not fused together, meaning they have greater flexibility than other non-terrestrial aquatic mammals, at the expense of speed. this means they can turn their head without actually moving their entire body. when swimming, river dolphins rely on their tail fins to propel themselves through the water. flipper movement is continuous. river dolphins swim by moving their tail fins and lower bodies up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. all species have a dorsal fin.