Sperm whale and octopus. Illustration of the skeleton and internal organs of a sperm whale (Physeter macrocephalus) with an octopus at left. Sperm whales can reach 18 metres in length, while the largest octopus has an arm span of over 4 metres. Sperm whales are toothed whales, though the teeth are thought to be used in aggression between males, rather than eating. Sperm whales dive deep (over 1000 metres) to hunt their main prey, giant squid.