A juvenile octopus grows at a rapid rate, perhaps because of its short life span. Extremely effective at turning the food it eats into body mass, a young octopus increases its weight by 5 percent each day. By the end of its life, an octopus will weigh one-third as much as all the food it has eaten. Should a larval octopus be fortunate enough to survive this difficult period, it enters into the next stage of its life: it grows into an adult octopus.
Around the age of 1 or 2 years old, the full-grown octopus is ready to mate. When octopuses reproduce, males use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. The hectocotylus in benthic octopuses is usually the third right arm. Males die within a few months of mating. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature.
After the eggs have been fertilized, the female lays about 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals). The female hangs these eggs in strings from the ceiling of her lair, or individually attaches them to the substrate depending on the species. The female cares for the eggs, guarding them against predators, and gently blowing currents of water over them so that they get enough oxygen.
The female does not eat during the two to ten month period spent taking care of the unhatched eggs (Incubation period varies according to species and water temperature). At around the time the eggs hatch, the mother dies.
Then, we are back to the beginning of the life cycle of an octopus.