Greater Argonaut and Shell, Argonauta argo

Greater Argonaut Shell, Argonauta argo. Collected off the beach in Las Barriles, Baja California Sur, December 2017. Larger shell size: 12.2 cm (4.8 inches) x 8.1 cm (3.2 inches). Collection courtesy of Mike Rousseau, Maple Ridge, British Columbia, Canada. First two photographs courtesy of Brad Murakami, Surrey, British Columbia, Canada.

Greater Argonaut Shell, Argonauta argo. Collected by a shallow-water diver in coastal waters off Los Barilles, Baja California Sur. Size: 17 cm (6.7 inches) x 11.6 cm (4.6 inches) x 6.0 cm (2.4 inches).

Greater Argonaut and Shell, Argonauta argo. Collected made at night off the ocean surface in coastal waters off  Loreto, Baja California Sur, June 2004. Size: 12 cm (4.9 inches). Shell 5.2 cm (2.0 inches) x 3.5 cm (1.4 inches). Collection and photographs courtesy of Beto Davis, Loreto.

Phylogeny: The Greater Argonaut, Argonauta argo (Dall, 1869), is a Cephalopod member of the Argonautidae Family of Argonaut Shells. The genus Argonauta is the only genus in this family, and there are four species in this genus. They are also known as the Pacific Argonaut and in Mexico as Argonauta Grande.

Morphology: Greater Argonauts are a part of a group of pelagic octopuses that collectively are known as Paper Nautili. The females can be up to 20 times larger than the males, and they secrete a paper-thin egg case (“shell”) that is utilized to rear families and for camouflage and defense. The males  do not have “shells”.  Argonauts have a rounded body with eight tentacles and no fins. The male can be differentiated from the female by the existence of an altered third tentacle (the hectocotylus) that it keeps in a pouch until it is needed for reproduction. Two of the tentacles are webbed and used for food collection. They have very large eyes. The live animals have a blue sheen on the first pair of arms and around the eyes. The “shells” are made of calcite and 7% magnesium carbonate, with a three-layer construction. These shells are thin, brittle. Unlike nautilus shells, they lack the gas-filled chamber, and are not a true cephalopod shell. The chamber traps surface air to maintain buoyancy and is utilized as a brood chamber. The “shell” is compressed with an elliptical profile with two rows of small sharp tubercles along the narrow finely tuberculate-based keel and prominent ribs. They are more rounded than other species in the family having a depth that is 62% to 66% of its length. The shell is white colored with black along the oldest portion of the keel. The female Greater Argonaut shell is the largest of the Argonaut’s and can be up to 22.0 cm (8.7 inches) in length, 12.6 cm (5.0 inches) in height, and 6.3 cm (2.5 inches). The Greater Argonaut can be easily confused with the Noury’s Argonaut, Argonauta nouryi (smaller; elongated and longer than it is deep).

Habitat and Distribution: The Greater Argonaut is a pelagic species that spends most of its life at, or near, the surface of the open sea. They are vertical migrators that move deeper during the day and move toward the surface at night for feeding. They are found at depths of up to 200 m (656 feet), though some sources extend this depth to 300 m (984 feet).  They are found worldwide in warm waters and along the entire eastern and western coastlines of Mexico in the Atlantic and Pacific Oceans.

Diet: Greater Argonauts are suspension feeders that feed primarily on jellyfishes, salps and planktonic mollusks and crustaceans, which they capture with their tentacles.

Predation: Greater Argonauts are prey for many organisms including including dorado, marlin, sailfish, spearfish and tuna, as well as penguins and seals.

Reproduction: Greater Argonauts are gonochoric ( male or female for life) and reproduce sexually, with internal fertilization. Mating occurs when the male inserts his hectocotylus into the female’s mantle.  The hectocotylus breaks off inside the female, where it fertilizes her eggs. Females are known to contain more than one hectocotylus at any one time obtained from different males. Females may also maintain eggs in three stages of development at any one time. Unlike most octopods, the females does not die after reproducing, but the male does.

Ecosystem Interactions: Greater Argonauts are often associated with, or living on, drift algae, salps, larger jellies, and other large planktonic species. In some cases it appears that they are merely doing so for camouflage and for protection from predators. In other cases it seems they are targeting other epibionts as a food source. It appears that these relationships are generally commensal.

Human Interactions: Greater Argonauts are edible and collected, primarily in the Mediterranean Sea, as a food source. They are not generally targeted as a fishery in other locations. Greater Argonaut shells are prized and deemed to be of great interest and value to shell collectors although they are extremely fragile. From a conservation perspective they considered to be of Least Concern.

Synonym:  Ocythoe argonautae.