The greater argonaut is sometimes known as the paper nautilus, due to the spiral shell that females grow. This shell is actually an egg case! It’s much more delicate than a chambered nautilus’s shell, but it can be repaired or re-grown if it gets damaged.
Argonauts can swim just fine, but they often prefer to hitch rides on jellies, floating clumps of seaweed, and even each other!
Female argonauts trap air in their shells, which pressurizes as they dive. That lets them go extra-deep!
This family (Argonautidae) contains a single genus (Argonauta) and four species (A. argo, A. hians, A. nodosus and A. nouryi).
Among the 4 known species belonging to this genus, A. argo is the largest, specifically the female. Argonauts are one of the most sexually dimorphic cephalopods. The females are considerably larger than the dwarf males and males do not construct a shell.
Finally, the ladies in the animal Kingdom are the big, beautiful ones! Female mantle length is to at least 97 mm (4 inches) with a total length to 438 mm (17 inches). Her shell length is to at least 300 mm (12 inches). Male total length can be up to 15 mm (1/2 inch!), excluding the hectocotylus.
Females are thought to live longer and reproduce many times while the tiny males are thought to only reproduce once and have much shorter lifespans.
The greater argonaut is found worldwide in tropical-subtropical waters. Their distribution extends the northern and southern hemispheres (between approximately 40°N and 40°S), and are found in the waters of the Atlantic Ocean, Indian Ocean, and Pacific Ocean. They are known as far south as South Africa, Western Australia, southern Peru and Brazil, and as far north as the Mediterranean Sea, Japan, and California.
Generally, argonauts inhabit open waters in the epipelagic zone (<100 m or 300 ft in depth). Adult females tend to hangout at the surface, while males and juvenile females have been reported from the surface to at least 200 m and 300 m (650 and 985 ft), respectively.
Argonauts form associations with gelatinous marine species such as jellies and salps; utilizing them as food or as an aid for the octopus to obtain foods such as amphipods or other small crustaceans.
The greater argonaut’s color can vary from deep maroon to silver. The dorsal side of their mantle has large chromatophores and smaller chormatophores on the ventral side. The darker color on their dorsal surface and reflective silver on their ventral surface produces a method of camouflage known as countershading, an effective predator avoidance tactic for these near-surface dwellers.
Predators of the greater argonaut include tunas, lancelets, and seabirds. Other tactics to avoid predators include inking followed by jet propulsion, having a sting-y acquaintance, and having a gas-filled shell to mediate buoyancy. In addition to food, the salp/jellyfish association could provide shelter, camouflage, or protection, since many predators avoid animals that can sting.
This group of pelagic octopuses are commonly referred to as paper nautiluses because the adult females produce a paper thin white calcareous shell or “paper nautilus”. This shell is actually a calcareous brood case secreted by the female’s webbed dorsal arms. The shell is large, has thin walls with one chamber (not like the chambered nautiluses) where egg strings are anchored to the innermost coil of the shell and protected by mom. Moms can fix anything and everything. If the paper nautilus is damaged, a female can repair it or can completely rebuild it.
Male argonauts never develop a shell, but don’t worry, they have another super power. These tiny males have to find a female in the open ocean, and when they do, they have to make sure they’ve got a good shot at mating and passing on their genes! Males have a modified sex arm called the hectocotylus. Old news, right? Well, male Argonauts third left arm is the modified hectocotylus that develops in a sac under the left eye and as a result the male appears asymmetrical!
Males usually have 12-13 suckers on their normal arms, but they have approximately 95 suckers on the hectocotylized arm! Mature female Argonauts are often found with multiple male hectocotyli (each from a different male) wrapped around the gills inside their mantle cavities. Intact males also have been found within female shells. Eggs can mature in up to five batches at different stages of development.
In general, female argonauts manage their neutral buoyancy by first filling their paper nautilus with gas. By rocking the shell back-and-forth at the surface, the female captures a large volume of air and then seals it with her arms. This rockin’ shell technique allows her to capture a larger volume of air than would be possible with a passive shell at the surface.
Next, the female forces the volume of air to a depth where the water pressure sufficiently compresses the air within the shell. At this targeted depth, the buoyancy of the captured air counteracts the submerged weight of the Argonaut and neutral buoyancy is achieved. By capturing a larger volume of air, female Argonauts can increase the depth at which they are neutrally buoyant, which may allow them to avoid surface wave action and predation from above.
It’s a different story for males and juvenile females who do not construct a shell. Tiny males and small females appear to inhabit a greater depth range and must use alternative strategies to maintain their position in the water column.
In general, argonauts are free swimming, but have a tendency to cling to floating objects in water. Female Argonauts have been observed hitching-a-ride on jellyfishes, attached to floating seaweed, or attached to each other forming a “cephalo-chain” of up to 20-30 individuals! Males and juvenile females have also been observed riding salp and associated with the football octopus also known as the tuberculate pelagic octopus (Ocythoe tuberculate).
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