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Meet the argonaut, an octopus that traps air to float in open water

Believing argonauts used their long front tentacles as sails, early naturalists named the cephalopods after the navigators of Greek mythology. In truth, these sexually dimorphic creatures, also called paper nautiluses, trap air in their shells as ballast and propel themselves across the ocean’s surface. This allows them to live in open water rather than on the seafloor, making them one of the few pelagic octopuses.

GREATER ARGONAUT
Argonauta argo

RANGE Tropical and subtropical oceans worldwide
HABITAT Open water
LIFESPAN One year
SURVIVING SPECIES At least four
DIET Mollusks, crustaceans, jellyfish

Female

MANTLE Main body, about 5 inches long
RADULA Rows of tiny teeth look like a conveyor belt
EGG CASE Translucent and spiraled, resembling a nautilus; houses and protects eggs
FRONT TENTACLES Produce calcite to form a delicate egg case

Female argonauts can be eight times larger and 600 times heavier than the males.

Male

BODY About 3/4 of an inch and shell-less
HECTOCOTYLUS Ridged, armlike tentacle used to transfer sperm to females; stored in a pouch under the animal’s left eye

Researchers didn’t discover male argonauts until the 19th century. Before that, they believed that both sexes looked the same and that hectocotyli were parasitic worms.

Scientists still haven’t observed a living male argonaut in the wild.

DEATH AND DISMEMBERMENT

Females dwarf males in size, making mating complicated. Males hold sperm in their hectocotylus, which they detach from their body and give to the female. She stores it until the eggs are ready to be fertilized, and the male likely dies.