Octopuses are semelparous animals, which means they reproduce once and then they die. After a female octopus lays a clutch of eggs, she quits eating and wastes away; by the time the eggs hatch, she dies. ... Females often kill and eat their mates; if not, they die a few months later, too).

Why do octopus kill their mates?

The strangulation seems to happen when "an octopus wraps at least one arm around the base of the mantle of the competitor" (or mate), Huffard wrote in 2010. This constriction then keeps the octopus from taking in fresh water to run past its gills—starving the animal of its oxygen source.

Why do male octopus die after mating?

Unfortunately, when female octopuses lay their eggs they also begin a slow march toward death. They stop eating, dedicate themselves entirely to guarding their clutch, and eventually waste away. The males don't get off any easier—they die shortly after mating.