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cymbium

Argonaut (animal)

The argonauts (genus Argonauta, the only extant genus in the Argonautidae family) are a group of pelagic octopuses. They are also called paper nautiluses, referring to the paper-thin eggcase that females secrete. This 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 Argonauta.

Argonauts are found in tropical and subtropical waters worldwide; they live in open ocean. Like most octopuses, they have a rounded body, eight arms and no fins. However, unlike most octopuses, argonauts live close to the sea surface rather than on the seabed. Argonauta species are characterised by very large eyes and small distal webs. The mantle-funnel locking apparatus is a major diagnostic feature of this taxon. It consists of knob-like cartilages in the mantle and corresponding depressions in the funnel. Unlike the closely allied genera Ocythoe and Tremoctopus, Argonauta species lack water pores.

Of its names, "argonaut" means "sailor on the Argo", and "nautilus" is Greek ναυτίλος = "sailor", because it was formerly supposed to use its shell-secreting arms as sails when it was on the surface.

The chambered nautilus was later named after the argonaut.

Physical 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 only mate once in their short lifetime, whereas the females are iteroparous. In addition, the females have been known since ancient times while the males were only described 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, then is detached from the male. The hectocotylus 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 of the eggcase 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 along with 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 3-layered structure and a higher proportion of magnesium carbonate (7%) than other cephalopod shells.

The egg case contains a bubble of gas used for buoyancy similar to shelled cephalopods, although it does not have a chambered phragmocone as in other shelled cephalopods.

Most other octopuses lay eggs in caves; it is speculated that, before ammonites died out during the Cretaceous–Tertiary extinction event, the argonauts may have evolved to use 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 bottgeri, 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 2-2.4. 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 poison 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 poison.

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 salps, although little is known about this relationship.

Classification

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

Argonauta absyrtus
Argonauta argo (type)
Argonauta bottgeri
Argonauta cornuta*
Argonauta hians
Argonauta itoigawai
Argonauta joanneus
Argonauta nodosa
Argonauta nouryi
Argonauta pacifica*
Argonauta tokunagai

*Species status questionable.

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:

Binomial name and author citation Current systematic status Type locality Type repository
Argonauta arctica Fabricius, 1780 Undetermined Unresolved; ?Tullukaurfak, Greenland Unresolved
Argonauta bibula Roding, 1798 Undetermined Unresolved Unresolved
Argonauta compressa Blainville, 1826 Undetermined Mer de Indes Unresolved; [other Blainville types at MNHN] [not reported by Lu et al. (1995)]
Argonauta conradi Parkinson, 1856 Species of uncertain status [fide Robson (1932:200)] "New Nantucket, Pacific Ocean" Unresolved
Argonauta cornu Gmelin, 1791 Undetermined Unresolved Unresolved; LS?
Argonauta cymbium Linne, 1758 Non-cephalopod; foraminiferous shell [fide Von Martens (1867:103)
Argonauta fragilis Parkinson, 1856 Species of uncertain status [fide Robson (1932:200)] Not designated Unresolved
Argonauta geniculata Gould, 1852 Species of uncertain status [fide Robson (1932:200)] Near Sugarloaf [Mountain] Rio [de] Janeiro [Brazil] Type not extant [fide Johnson (1964:32)]
Argonauta maxima Dall, 1871 Nomen nudum
Argonauta navicula Lightfoot, 1786 Species dubium [fide Rehder (1967:11)] Not designated Unresolved
Argonauta rotunda Perry, 1811 Non-cephalopod; Carcinaria sp. [fide Robson (1932:201)]
Argonauta rufa Owen, 1836 Incertae sedis [fide Robson (1932:181)] "Indian seas" ["South Pacific ocean" fide Owen (1842:114)] Unresolved; Museum of the Royal College of Surgeons? Holotype
Argonauta sulcata Lamarck, 1801 Nomen nudum
Argonauta tuberculata f. aurita Von Martens, 1867 Undetermined Unresolved ZMB
Argonauta tuberculata f. mutica Von Martens, 1867 Undetermined Coast of Brazil ZMB Holotype
Argonauta tuberculata f. obtusangula Von Martens, 1867 Undetermined Not designated ZMB Syntypes
Argonauta vitreus Gmelin, 1791 Undetermined Not designated Unresolved; LS?
Octopus (Ocythoe) raricyathus Blainville, 1826 Undetermined [Argonauta?] Not designated MNHN Holotype; specimen not extant [fide Lu et al. (1995:323)]
Ocythoe punctata Say, 1819 Argonauta sp. [fide Robson (1929d:215)] Atlantic Ocean near the North American coast (from stomach of dolphin) Unresolved; ANSP? Holotype [not traced by Spamer and Bogan (1992)]
Tremoctopus hirondellei Joubin, 1895 Argonauta or Ocythoe [fide Thomas (1977:386)] (Atlantic Ocean) MOM Holotype [station 151] [fide Belloc (1950:3)]

In design

The argonaut was 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 Minoan art from Crete. A variation known as the double argonaut design was also found in Minoan jewelry.

In literature and etymology

References

External links

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