The Atlantic white-sided dolphin Lagenorhynchus acutus
reaches up to 2.8 m in length. It has moderately long and slender flippers and small tail flukes. The dorsal fin is tall, large and located on the middle of the back. The head is smoothly sloping with a distinct but small snout. It has a complex clearly demarcated colour pattern. It is black to dark grey on the back from the upper beak to the tail, changing sharply to light grey on the sides. The lower jaw and belly, as far as the anus, are white. A distinct white band is visible on the flanks under the dorsal fin between the black and light grey colouration, which turns into an olive-yellow stripe towards the rear at the lower margin of the dark dorsal colouration. A dark line runs from the upper beak to, and surrounding, the eye.Atlantic white-sided dolphins are usually found in large pods of up to several thousand individuals. Mixed schools with other species including white-beaked dolphins, Lagenorhynchus albirostris
, have been recorded. Their surface behaviour is typical of dolphins with acrobatic leaps but bow-riding uncommon. Dive duration is unknown (Kinze, 2002).