Some squids school when young and become solitary later in life. The male octopus uses a specialized arm called a hectocotylus to transfer sperm to the mantle cavity of a receptive female. The female lays strings of fertilized eggs on the roof of her den. She guards, cleans and aerates the eggs with water expelled from her siphon until hatching anywhere from 30 days to a year, depending on the species. The female builds a wall of rocks to seal off the den and remains in the den until just before she dies, after the eggs have hatched.
Squids often mate in large groups, and attach their egg capsules to the ocean floor or to seaweed.
Most adult octopuses and squid die after reproducing. Their bodies are recycled in the food web, nourishing other animals, and ultimately providing food for their young when they hatch. Most poisons produced by octopuses and squids are too weak to harm humans. The blue ringed octopus is an exception. This Australian native produces enough poison to paralyze and kill a human.