Greater Argonaut
Encyclopedia
Argonauta argo, also known as the greater argonaut, is a species
of pelagic octopus
belonging to the genus
Argonauta. The female of the species, like all argonaut
s, creates a paper-thin eggcase that coils around the octopus much like the way a nautilus
lives in its shell
, hence the name paper nautilus. The Chinese name for this species translates as "white sea-horse's nest". A. argo was the first argonaut species to be described and is consequently the type species
of the genus. A. argo is the largest species in the genus and also produces the largest eggcase. Live animals have a characteristic blue sheen on the first arm
pair and around the eye
s. The eggcase is characterised by two rows of small, sharp tubercle
s running along a narrow keel, smooth ribs across the walls of the shell, and a thickening along the shell aperture, which forms distinct protrusions or 'horns' on either side. Argonauta cygnus Monterosato, 1889 was described based on a shell which lacked these protrusions, although it is now considered a junior synonym of A. argo. The greatest recorded size of an A. argo eggcase is 300.0 mm.
A. argo is cosmopolitan, occurring in tropical and subtropical waters worldwide. A dwarf form exists in the Mediterranean Sea
, which was described as Argonauta argo mediterranea Monterosato, 1914, although this taxon
is now regarded as invalid.
A. argo is thought to feed primarily on pelagic molluscs. The species is preyed on by numerous predators. It has been reported in the stomach contents of Alepisaurus ferox from the south-western Pacific.
Males of this species reach sexual maturity
at a mantle
length (ML) of 8 mm. Females mature at about double the size of Argonauta bottgeri and Argonauta hians. They begin to secrete an eggcase at 6.5–7 mm ML. Eggs are usually laid when females reach 14–15 mm ML, although the size at which this takes place differs across the animal's range. A small A. argo residing in an 88 mm long eggcase was estimated to be carrying 48,800 embryos. Females grow to 100 mm ML, while males do not exceed 20 mm ML.
In the open ocean, A. argo has been observed attached to jellyfish
. This behaviour has been known for a long time, although little was understood about the relationship prior to the work of Heeger et al. in 1992. In "Predation on jellyfish by the cephalopod Argonauta argo", Heeger et al. describe their observations of a female A. argo found atop a host jellyfish. The argonaut was seen holding on to the aboral
(exumbrellar) surface of the jellyfish using its lateral and ventral arms. The authors found that about half of the animal's aboral surface was damaged and large pieces of mesoglea
were missing, presumably removed by the argonaut. Additionally, two holes, apparently bite marks, were found in the center of this area with channels leading from these holes into the gastral cavity of the jellyfish. The argonaut presumably used these channels to suck food particles from the gastral cavity. Heeger et al. suggested that "the association provided shelter or camouflage for the argonaut".
Observations of captive A. argo females suggest that the expanded webs of the dorsal arms may aid the animal in feeding. Mark Norman
mentions that "when food was touched against the spread webs, an arm shot out of the shell in a sweeping action, grabbing the prey". It is speculated that argonauts do not actively hunt, but employ this method to catch animals that bump into them in the open ocean.
A. argo is occasionally involved in mass strandings along the South Africa
n and southern Australia
n coastlines. The strandings are seasonal and generally occur between April and August, towards the end of the animals' spawning season.
A damaged beak of a female A. argo (ML = 40.0 mm; caught at 20°56′N 175°33′W), measuring 4.30 mm in hood length and 7.80 mm in crest length, is mentioned in A Handbook for the Identification of Cephalopod Beaks.
The type specimen of A. argo was collected in the Mediterranean Sea
and is deposited at the Linnean Society of London
.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of pelagic octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
belonging to the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Argonauta. The female of the species, like all argonaut
Argonaut (animal)
The argonauts are a group of pelagic octopuses. They are also called paper nautiluses, referring to the paper-thin eggcase that females secrete...
s, creates a paper-thin eggcase that coils around the octopus much like the way a nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
lives in its shell
Mollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
, hence the name paper nautilus. The Chinese name for this species translates as "white sea-horse's nest". A. argo was the first argonaut species to be described and is consequently the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
of the genus. A. argo is the largest species in the genus and also produces the largest eggcase. Live animals have a characteristic blue sheen on the first arm
Cephalopod arm
A cephalopod arm is distinct from a tentacle, though the terms are often used interchangeably.Generally, cephalopod arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. Octopuses have eight arms and no tentacles, while squid and cuttlefish...
pair and around the eye
Cephalopod eye
Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye, which consists of a lens projecting an image onto a retina. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface , and...
s. The eggcase is characterised by two rows of small, sharp tubercle
Tubercle
A tubercle is generally a wart-like projection, but it has slightly different meaning depending on which family of plants or animals it is used to refer to....
s running along a narrow keel, smooth ribs across the walls of the shell, and a thickening along the shell aperture, which forms distinct protrusions or 'horns' on either side. Argonauta cygnus Monterosato, 1889 was described based on a shell which lacked these protrusions, although it is now considered a junior synonym of A. argo. The greatest recorded size of an A. argo eggcase is 300.0 mm.
A. argo is cosmopolitan, occurring in tropical and subtropical waters worldwide. A dwarf form exists in the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
, which was described as Argonauta argo mediterranea Monterosato, 1914, although this taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
is now regarded as invalid.
A. argo is thought to feed primarily on pelagic molluscs. The species is preyed on by numerous predators. It has been reported in the stomach contents of Alepisaurus ferox from the south-western Pacific.
Males of this species reach sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
at a mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
length (ML) of 8 mm. Females mature at about double the size of Argonauta bottgeri and Argonauta hians. They begin to secrete an eggcase at 6.5–7 mm ML. Eggs are usually laid when females reach 14–15 mm ML, although the size at which this takes place differs across the animal's range. A small A. argo residing in an 88 mm long eggcase was estimated to be carrying 48,800 embryos. Females grow to 100 mm ML, while males do not exceed 20 mm ML.
In the open ocean, A. argo has been observed attached to jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
. This behaviour has been known for a long time, although little was understood about the relationship prior to the work of Heeger et al. in 1992. In "Predation on jellyfish by the cephalopod Argonauta argo", Heeger et al. describe their observations of a female A. argo found atop a host jellyfish. The argonaut was seen holding on to the aboral
Aboral
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
(exumbrellar) surface of the jellyfish using its lateral and ventral arms. The authors found that about half of the animal's aboral surface was damaged and large pieces of mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
were missing, presumably removed by the argonaut. Additionally, two holes, apparently bite marks, were found in the center of this area with channels leading from these holes into the gastral cavity of the jellyfish. The argonaut presumably used these channels to suck food particles from the gastral cavity. Heeger et al. suggested that "the association provided shelter or camouflage for the argonaut".
Observations of captive A. argo females suggest that the expanded webs of the dorsal arms may aid the animal in feeding. Mark Norman
Mark Norman (marine biologist)
Mark Douglas Norman is a marine biologist living in southern Australia where he works through the University of Melbourne and Museum Victoria. For over a decade, Norman has been working exclusively with cephalopods and he is one of the leading scientists in the field, having discovered over 150...
mentions that "when food was touched against the spread webs, an arm shot out of the shell in a sweeping action, grabbing the prey". It is speculated that argonauts do not actively hunt, but employ this method to catch animals that bump into them in the open ocean.
A. argo is occasionally involved in mass strandings along the South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
n and southern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n coastlines. The strandings are seasonal and generally occur between April and August, towards the end of the animals' spawning season.
A damaged beak of a female A. argo (ML = 40.0 mm; caught at 20°56′N 175°33′W), measuring 4.30 mm in hood length and 7.80 mm in crest length, is mentioned in A Handbook for the Identification of Cephalopod Beaks.
The type specimen of A. argo was collected in the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
and is deposited at the Linnean Society of London
Linnean Society of London
The Linnean Society of London is the world's premier society for the study and dissemination of taxonomy and natural history. It publishes a zoological journal, as well as botanical and biological journals...
.