Argonauts are inhabitants of open ocean. They spend their lives swimming in blue water, feeding on pelagic molluscs and crustaceans, and avoiding the many oceanic predators. They swim by jet propulsion, using powerful jets of water squirted through their funnel.Morphology
Argonauts are octopuses. Their body organs are enclosed in a sac known as a mantle. Their gills hang free in this sac. Water is drawn into the mantle and over the gills, being ejected through the argonaut's funnel. The head is attached to the mantle. Eight arms surround a central mouth with a beak and radula. All arms have two sucker rows.
The first (dorsal) pair of arms of females are modified with large webs for secretion of a shell. Females develop a shell very early in life at a size of approximately 7 mm mantle length. Females can grow to around 40 cm long (depending on the species) adding to their shells as they grow.
Female argonaut (Argonauta nodosus) with one of her two shell-secreting webs spread out on the glass of an aquarium. Photo: © R. Kuiter
Male argonauts never grow larger than a few centimetres. They do not form a shell. Their third left arm is modified into a 'hectocotylus' for transferring sperm to the female. This arm develops in a sac prior to being passed to the female.
Three preserved male argonauts (Argonauta nodosus), each at a varied stage of everting its specialised third left arm. Photos: © Julian Finn
Argonauts are epipelagic animals, inhabiting the near surface waters of tropical and temperate oceans of the world. Female argonauts have a tendency to cling to any living or dead objects floating or drifting on the water surface. They have been observed 'riding' jellyfish, attached to floating weed and attached to each other forming large chains of up to 20-30 individuals. Male and juvenile female argonauts have been observed riding inside salps (Pegea socia).Diet
Female argonauts are specialised consumers of pelagic molluscs, chiefly heteropods and pteropods. Remains of pteropods, heteropods, octopods and crustaceans have been recovered from the stomachs of female argonauts. Argonauta hians has also been observed to feed on comb jellies.Predators
In Australian waters argonauts have been reported in the diets of Southern Bluefin Tuna (Thunnus maccoyii), Australian Fur Seal (Arctocephalus pusillus doriferus), Little Penguin (Eudyptula minor) and the Shy Albatross (Thalassarche cauta). Elsewhere in the world argonauts have been reported in the diets of fast-moving, streamlined, oceanic fishes including lancet fishes, dolphinfishes, mackeral, tunas, swordfish, marlins, sailfishes and whaler sharks. They have also been recorded in the diets of catsharks, dolphins, small toothed whales, fur seals, penguins, terns, petrels, albatrosses and tropicbirds.
Very little is known about the reproduction of argonauts or the process by which the male's hectocotylus is deposited inside the female's mantle cavity. Female argonauts have been found with up to three hectocotyli (each from a different male) wrapped around their gills inside their mantle cavities. Occasionally mature males have been found riding inside the female's shell.