Argonaut (animal)

Description

Sexual dimorphism and reproduction

Argonauts exhibit extreme sexual dimorphism in size and lifespan. Females grow up to 10 cm and make shells up to 30 cm, while males rarely surpass 2 cm. The males mate only once in their short lifetime, whereas the females are iteroparous, capable of having offspring many times over the course of their lives. In addition, the females have been known since ancient times, while the males were described only in the late 19th century.

The males lack the dorsal tentacles used by the females to create their eggcases. The males use a modified arm, the hectocotylus, to transfer sperm to the female. For fertilization, the arm is inserted into the female's pallial cavity and then becomes detached from the male. The hectocotylus when found in females was originally described as a parasitic worm.

Eggcase

Female argonauts produce a laterally compressed calcareous eggcase in which they reside. This "shell" has a double keel fringed by two rows of alternating tubercles. The sides are ribbed with the centre either flat or having winged protrusions. The eggcase curiously resembles the shells of extinct ammonites. It is secreted by the tips of the female's two greatly expanded dorsal tentacles (third left arms) before egg laying. After she deposits her eggs in the floating eggcase, the female takes shelter in it, often retaining the male's detached hectocotylus. She is usually found with her head and tentacles protruding from the opening, but she retreats deeper inside if disturbed. These ornate curved white eggcases are occasionally found floating on the sea, sometimes with the female argonaut clinging to it. It is not made of aragonite as most other shells are, but of calcite, with a three-layered structure and a higher proportion of magnesium carbonate (7%) than other cephalopod shells.

The eggcase contains a bubble of air that the animal captures at the surface of the water and uses for buoyancy, similarly to other shelled cephalopods, although it does not have a chambered phragmocone. Once thought to contribute to occasional mass strandings on beaches, the air bubble is under sophisticated control, evident from the behaviour of animals from which air has been removed under experimental diving conditions. This system to attain neutral buoyancy is effective only at the relatively shallow depths of the upper 10 meters of the water column. Young females with mantle lengths less than 9 millimeters are shell-less like the males, with both having been found in waters between 50–200 meters.

Most other octopuses lay eggs in caves; Neale Monks and C. Phil Palmer speculate that, before ammonites died out during the Cretaceous–Paleogene extinction event, the argonauts may have evolved to use discarded ammonite shells for their egg laying, eventually becoming able to mend the shells and perhaps make their own shells. However, this is uncertain and it is unknown whether this is the result of convergent evolution.

Argonauta argo is the largest species in the genus and also produces the largest eggcase, which may reach a length of 300 mm. The smallest species is Argonauta boettgeri, with a maximum recorded size of 67 mm.

Beak

The beaks of Argonauta species are distinctive, being characterised by a very small rostrum and a fold that runs to the lower edge or near the free corner. The rostrum is "pinched in" at the sides, making it much narrower than in other octopuses, with the exception of the closely allied monotypic genera Ocythoe and Vitreledonella. The jaw angle is curved and indistinct. Beaks have a sharp shoulder, which may or may not have posterior and anterior parts at different slopes. The hood lacks a notch and is very broad, flat, and low. The hood to crest ratio (f/g) is approximately 2.0–2.4 . The lateral wall of the beak has no notch near the wide crest. Argonaut beaks are most similar to those of Ocythoe tuberculata and Vitreledonella richardi, but differ in "leaning back" to a greater degree than the former and having a more curved jaw angle than the latter.

Feeding and defense

Feeding mostly occurs during the day. Argonauts use tentacles to grab prey and drag it toward the mouth. It then bites the prey to inject it with venom from the salivary gland. They feed on small crustaceans, molluscs, jellyfish and salps. If the prey is shelled, the argonaut uses its radula to drill into the organism, then inject the toxin.

Argonauts are capable of altering their color. They can blend in with their surroundings to avoid predators. They also produce ink, which is ejected when the animal is being attacked. This ink paralyzes the olfaction of the attacker, providing time for the argonaut to escape. The female is also able to pull back the web covering of her shell, making a silvery flash, which may deter a predator from attacking.

Argonauts are preyed upon by tunas, billfishes, and dolphins. Shells and remains of argonauts have been recorded from the stomachs of Alepisaurus ferox and Coryphaena hippurus.

Male argonauts have been observed residing inside aggregate salps (Pegea socia), although little is known about this relationship.

Classification

The genus Argonauta contains up to seven extant species. Several extinct species are also known.

Four extant species are widely considered valid:

• Argonauta argo Linnaeus, 1758
• Argonauta hians Lightfoot, 1786
• Argonauta nodosus Lightfoot, 1786
• Argonauta nouryi Lorois, 1852

Several additional taxa are either treated as valid species or regarded as nomina dubia:

• Argonauta boettgeri Maltzan, 1881
• Argonauta cornutus Conrad, 1854
• Argonauta pacificus Dall, 1871

A number of extinct species have also been described:

• †Argonauta absyrtus Martill & Barker, 2006
• †Argonauta biarmata Ponzi, 1876
• †Argonauta itoigawai Tomida, 1983
• †Argonauta joanneus Hilber, 1915
• †Argonauta oweri Fleming, 1945
• †Argonauta sismondai Bellardi, 1872
• †Argonauta tokunagai Yokoyama, 1913

The extinct species Obinautilus awaensis was originally assigned to Argonauta, but has since been transferred to the genus Obinautilus.

Dubious or uncertain taxa

The following taxa associated with the family Argonautidae are of uncertain taxonomic status:

In design

The argonaut was the inspiration for a number of classical and modern art and decorative forms including use on pottery and architectural elements. Some early examples are found in Bronze Age Minoan art from Crete. A variation known as the double argonaut design was also found in Minoan jewelry. This design was also transposed and adapted in both gold and glass in contemporary Mycenaean contexts, as seen both at Mycenae and the Tholos at Volo.

In literature and etymology

• Argonauts are featured in Twenty Thousand Leagues Under the Seas, noted for their ability to use their tentacles as sails. Although it is false natural history, this was a long-standing and widespread myth, apparently originating with Aristotle.
• A female argonaut is also described in Marianne Moore's poem "The Paper Nautilus".
• "Argonauta" is the name of a chapter in Anne Morrow Lindbergh's Gift from the Sea.
• Paper nautiluses were caught in the novel The Swiss Family Robinson.
• Argonauts gave their name to an Arabidopsis thaliana mutation and by extension to Argonaute proteins.