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The argonauts (genus Argonauta, the only extant taxon genus in the family Argonautidae) are a group of pelagic octopus. They are also called paper nautili, referring to the paper-thin eggcase that females secrete; however, as octopuses, they are only distant relatives of true Nautilus. Their structure lacks the gas-filled chambers present in chambered nautilus shells and is not a true cephalopod shell, but rather an evolutionary innovation unique to the genus. It is used as a brood chamber, and to trap surface air to maintain buoyancy. It was once speculated that argonauts did not manufacture their eggcases but utilized shells abandoned by other organisms, in the manner of . Experiments by pioneering marine biologist Jeanne Villepreux-Power in the early 19th century disproved this hypothesis, as Villepreux-Power successfully reared argonaut young and observed their shells' development.
Argonauts are found in Tropics and subtropical waters worldwide. They live in the open ocean, i.e. they are pelagic. Like most octopuses, they have a rounded body, eight cephalopod limb (arms) and no . However, unlike most octopuses, argonauts live close to the surface rather than on the seabed. Argonauta species are characterised by very large cephalopod eye and small webs between the arms. The funnel–mantle locking apparatus is a major diagnostic feature of this taxon. It consists of knob-like in the mantle and corresponding depressions in the funnel. Unlike the closely allied genera Ocythoe and Tremoctopus, Argonauta species lack water pores.
The males lack the dorsal 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.
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 ; Neale Monks and C. Phil Palmer speculate that, before 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.
Argonauts are capable of altering their color. They can blend in with their surroundings to avoid . They also produce cephalopod 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 , , and . Shells and remains of argonauts have been recorded from the stomachs of Lancetfish and Mahi-mahi.
Male argonauts have been observed residing inside aggregate (Pegea socia), although little is known about this relationship.
Four extant species are widely considered valid:
Several additional taxa are either treated as valid species or regarded as nomina dubia:
A number of extinct species have also been described:
The extinct species Obinautilus awaensis was originally assigned to Argonauta, but has since been transferred to the genus Obinautilus.
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 genus Ocythoe and Glass Octopus. 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 , , jellyfish and . If the prey is shelled, the argonaut uses its radula to drill into the organism, then inject the toxin.
Classification
The genus Argonauta contains up to seven extant taxon species. Several extinction species are also known.
species Argonauta joanneus (lateral and keel views)]]
Dubious or uncertain taxa
The following taxon associated with the family Argonautidae are of uncertain taxonomic status:
Unresolved Unresolved Unresolved; other not Unresolved Unresolved; LS? Unresolved Type not extant fide Unresolved Unresolved; Museum of the Royal College of Surgeons? Holotype ZMB ZMB Holotype ZMB Syntypes Unresolved; LS? MNHN Holotype; specimen not extant fide Unresolved; ANSP? Holotype not MOM Holotype station fide
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 Greece contexts, as seen both at Mycenae and the beehive tomb at Volo.
In literature and etymology
External links
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