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.
The chambered nautilus was later named after the argonaut.
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.
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.
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 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.
Male argonauts have been observed residing inside salps, although little is known about this relationship.
Argonauta argo (type)
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)]|