Argonaut (animal)
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 Eggcases of various Argonauta species
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Argonauta argo (type) |
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The argonauts (genus Argonauta, the only genus in the Argonautidae family) are a group of pelagic octopuses. They are also called paper nautiluses, by analogy with chambered nautiluses and 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.[1]
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.
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[edit] Physical description
[edit] 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.[2]
[edit] Eggcase
Female argonauts produce a laterally compressed, calcareous eggcase in which they reside. This "shell" possesses 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) prior to egg laying. After she deposits her eggs in the floating eggcase, the female herself takes shelter in it, often along with the male. 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 surface, sometimes with the female argonaut still clinging to it. It is not made of aragonite as most other shells are, but of chitin.
While most other octopuses lay eggs in caves, it is speculated the argonauts may have evolved to use ammonite shells for their egg laying, eventually becoming able to mend the shells and perhaps even make their own.[3] However, this is uncertain and it is unknown whether this is the result of direct or 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.[4][5] The smallest species is Argonauta bottgeri, with a maximum recorded size of 67 mm.[4][6]
[edit] 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.[6]
[edit] 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, but also 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.[6]
Male argonauts have been observed residing inside salps, although little is known about this relationship.[7]
[edit] Trivia
Argonauts are featured in Twenty Thousand Leagues Under the Sea, noted for their ability to use their tentacles as sails. There is no evidence for this.
A female argonaut is also described in Marianne Moore's poem "The Paper Nautilus."
[edit] References
- ^ (German) Naef, A. (1923). "Die Cephalopoden, Systematik". Fauna Flora Golf. Napoli (35) 1: 1-863.
- ^ (Italian) Delle Chiaje, S. (1825). Memorie sulla storia e notomia degli animali. Senza Verlebre del Regno di Napoli. I..
- ^ Monks, N. & P. Palmer (2002). Ammonites. Smithsonian Institution Press, Washington D.C..
- ^ a b Pisor 2005, p. 12
- ^ Nesis 1982
- ^ a b c Clarke, M. R. (1986). A Handbook for the Identification of Cephalopod Beaks. Oxford University Press, 273 pp.
- ^ Banas, P. T., D. E. Smith & D. C. Biggs (1982). "An association between a pelagic octopod, Argonauta sp. Linnaeus 1758, and aggregate salps". Fish. Bull. U.S. 80: 648-650.
- (Russian) Nesis, K. N. 1982. Abridged key to the cephalopod mollusks of the world's ocean. Light and Food Industry Publishing House, Moscow, 385+ii pp. [Translated into English by B. S. Levitov, ed. by L. A. Burgess (1987), Cephalopods of the world. T. F. H. Publications, Neptune City, NJ, 351 pp.]
- Sweeney, M. J. Taxa Associated with the Family Argonautidae Tryon, 1879
[edit] External links
- CephBase: Argonauta
- Information on the genus Argonauta
- http://www.nhm.ac.uk/hosted_sites/tcp/Argo.html
