Argonaut is a type of octopus most commonly found in tropical and subtropical seas. Its presence in temperate waters of Southern California indicate that warm water currents from the South are prevalent. Argonauts are pelagic (usually found far offshore in the open ocean) and demonstrate a dramatic sexual dimorphism (male is about one inch long and females up to 18 inches). The male argonaute has one of it's eight arms (the third) highly modified to carry sperm to the female. The modified arm of the male is called a hectocotylus and during mating this arm breaks off and crawls into the female to remain there until the female is ready to fertilize her eggs. When first discovered in the early 1800s the hectocotylus arm was thought to be a parasitic worm and was given the species name of Hectocotylus (therefore the origin of the term hecotocotylus arm of cephalopods).
The female argonaut makes a thin, laterally compressed calcareous shell with one chamber that is used as a brood pouch for eggs. The first of the eight arms of the female bears a wide sail-like lobe that is used to secrete and form the shell. The shell looks like an extinct ammonid shell or resembles shells from the extant cephalopod group called nautiluses. Thus, argonautes are commonly referred to as "paper nautiluses", however, they are octopuses and not closely related to nautiluses. Due to their beauty and delicate nature, argonaute shells are highly prized by collectors
Argonauts eat plankton like krill, shrimp and pelagic snails. A recent paper describes an argonaut positioned on the top of a jellyfish and eating away the bell to be able to send arms down through the jelly to remove plantonic food in the jelly's stomach.
Argonauts are eaten by tuna, billfish and blue sharks