The single species, H. leptonyx, occurs along the coasts and pack ice of Antarctica and also is regularly present on most subantarctic islands. Wandering individuals have reached South Africa, southern Australia, Tasmania, New Zealand, Lord Howe Island, the Cook Islands, Tierra del Fuego, and the Atlantic coast of South America nearly to Buenos Aires. Adult males, which average smaller than females, are about 300 cm long and weigh about 270 kg. Adult females are 290-380 cm long and weigh up to 500 kg.
Coloration is dark gray to almost black dorsally, paler on the sides, and silvery below.
There are gray spots on the throat, shoulders, and sides. The body is streamlined but massive, the head seems disproportionately large and reptilian, and the foreflippers are comparatively long. The skull and canine teeth are unusually long, and the postcanine teeth, having three prominent tubercles with narrow clefts between them, are second in complexity to those of Lobodon. Mature animals normally occur in the outer fringe of the pack ice. Animals about 3-9 years old may reach the Antarctic coast in summer.