PORT PHILLIP BAY


Knobbed Argonaut 

Argonauta nodosus Lightfoot, 1786

View scientific description and taxonomy

Scientific Details

Eight circumoral appendages; eight arms, ventrolateral tentacles absent. Mantle cavity with three openings to exterior. Funnel-locking apparatus present. Arms with numerous suckers, without horny, toothed rims. Fins absent. Dorsal border of mantle fused with head. Extreme sexual size dimorphism, females large (to 90 mm mantle length), males tiny (to 14 mm mantle length). Female mantle roughly conical, posterior mantle with sharp upward torsion (most pronounced in animals preserved within shells). Head small, embedded within mantle. Eyes large, protruding, separated from level of head by distinct constrictions at base. Arms flattened from aboral to oral side, unequal length (1>2>3>4). Dorsal arms thickest, longest, with conspicuous, wing-like web used for secreting the female's shell. Second arms over twice mantle length, exceeding length of third and forth arms. Web deepest between dorsal arms. Suckers biserial; enlarged suckers absent. Gills with approximately 14-16 lamellae per demibranch. Funnel organ conspicuous, I I-shaped, outer limbs elongate ovate pads, shorter than medial limbs. Funnel locking apparatus present; small, ovate, cartilaginous, depressions on funnel corners corresponding to rounded nodules on inside of mantle wall. Shell large (diameter to 266 mm), laterally flattened. "Horns" may be present in smaller individuals, becoming less conspicuous. Centre of spiral not pressed in. Ribs comprised of chains of separate tubercles or nodules, radiating out in concentric circles. Ribs numerous, close together, often branching once or twice as they radiate towards the keel. Dark staining on keel tubercles of first third of shell. Third right arm of males hectocotylised; develops in a sac under the right eye giving the male argonaut the appearance of having only seven arms. Detached male hectocotylus bears 60-78 suckers in two rows. Female mantle length to 90 mm, total length to 267 mm, shell diameter to 266 mm.
Male mantle length to 14 mm, total length to 30 mm.

Taxonomy

Phylum:
Mollusca
Class:
Cephalopoda
Subclass:
Coleoidea
Order:
Octopoda
Suborder:
Incirrata
Family:
Argonautidae
Genus:
Argonauta
Species:
nodosus

Other Names

  • Southern Argonaut
  • Knobby Argonaut

General Description

The males and females of this free-swimming octopus look very different. The female lives in a white paper-thin shell (often called a "paper nautilus") that reaches around 27 cm long. The female reaches about 27 cm long. The common name of this octopus comes from the knobbed ridges across the sides of the shell. The female has a round body and eight arms, each with two rows of suckers. The first arm pair has two wide webs at the ends. These webs both secrete the white shell and are spread over the outside of the shell when the animal is swimming. The webs can change colour from silver to dark maroon red. The male is much smaller, being less than 3 cm long. It has no shell, a rounded body and large eyes. It has eight arms each with two rows of suckers. The third left arm is much longer and highly modified. It develops in a pouch that reaches around the same size as the body. Female mantle length to 9 cm, total length to 27 cm, shell diameter to 27 cm. Male mantle length to 14 mm, total length to 3 cm.

Biology

This pelagic octopus spends its entire life in open-ocean. Very little is known about the biology of this species. Like other argonaut species it is presumed to feed on pelagic molluscs (particularly heteropods and pteropods), small fishes and crustaceans. Mating in argonauts is very unusual. When a tiny male argonaut encounters a female he ruptures a pouch containing a special reproductive arm. He loads this arm with sperm, detaches it and passes it to the female. Once the arm is shed it is believed that the male argonaut dies. The detached arm crawls into the gill cavity of the female where it attaches to her gills and is stored until it is required for fertilisation. Single females have been found with the arms of multiple males attached to their gills. Eggs are laid in strings attached to the inner core of the female's shell and protected until they hatch. Hatchlings are transparent and are often full of yolk, providing their first meals as they disperse and start fending for themselves. Large swarms of this argonaut wash ashore at irregular intervals along the southern Australian coast. There appears to be no set cycle in these mass strandings with currents, winds, krill schools and lunar cycles all being speculated as the causes. Beach washed shells are popular in the shell trade. Word quickly spreads when strandings occur with some collectors picking up hundreds of shells in a night.

Habitat

Surface waters of open-ocean, may be found in shallow coastal waters or stranded on the beach following periods of onshore winds. Shelled females to approximately 10 m, males and juvenile females to at least 150 m.

Open water

Distribution guide

Southern Hemisphere waters. Southern Australia including Victoria.

Species Group

Octopuses and allies Argonauts

Depth

Shore (0-1 m)
Shallow (1-30 m)
Deep ( > 30 m)

Water Column

Surface Midwater

Max Size

3 cm

Diet

Carnivore

Harmful

Potential to bite, especially if handled. Venom status unknown.

Commercial Species

No

Global Dispersal

Native to Australia

Identify

Conservation Status

  • DSE Advisory List : Not listed
  • EPBC Act 1999 : Not listed
  • IUCN Red List : Not listed

Author

article author Finn, J.K.

Dr. Julian Finn is a Senior Curator of marine invertebrates at Museum Victoria.

Author

article author Norman, M.

Dr. Mark Norman is Head of Sciences at Museum Victoria.

citation

Cite this page as:
Finn, J.K. & Norman, M., 2011, Knobbed Argonaut, Argonauta nodosus, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 23 Nov 2017, http://portphillipmarinelife.net.au:8098/species/7832

Text: creative commons cc by licence