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Argonauta nodosa (Family Argonautidae)

Knobbed Argonaut

Distribution

Distribution:

Indian and Pacific Oceans, most common around southern Australia, New Zealand and South Africa (WA, NT, QLD, NSW, VIC, TAS, SA).

Features:

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 25 cm long. The female reaches about 40 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 4 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.

Ecology/Way of Life:

This pelagic octopus spends its entire life in open ocean. Little is known of its behaviour in the wild. It feeds on schooling fish and shrimp (krill). The webs seem to play a role in feeding. Captive females will shoot out a whip-like arm to seize prey that bump into the webs. Swarms of females have sometimes been encountered in open ocean and it is unclear whether they spend all their time in schools or just gather to breed.

Mating in this octopus is very strange. When a small male finds a female he ruptures the pouch containing the special reproductive arm. He inserts sperm into this arm and then detaches it. The male probably dies while the arm crawls into the female's gill cavity attaching near the gills. Single females have been found containing up to six male arms. She uses these arms to fertilise her eggs. Eggs are laid in strings from the inner core of the shell and protected by the female 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.

Preferred Image

Interaction with Humans/Threats:

Beach washed shells are popular in the shell trade, selling for up to $AU50 each. Word quickly spreads when strandings occur with some collectors picking up hundreds of shells in a night.

Further Reading:

Tree of Life website

Norman, M.D. and A. Reid. 2000. A guide to the squid, cuttlefishes and octopuses of Australasia. Gould League/CSIRO Publishing, Melbourne. 96 pp.

Norman, M.D. 2000. Cephalopods: A world guide. ConchBooks, Hackenheim, Germany.

Acknowledgments:

Text by Mark Norman, photographs by David Paul and B. Reid.

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Images and Multi-media:  
Attached Image  
image/jpeg 138465 bytes Female Knobbed Argonaut
Attached Image  
image/jpeg 108825 bytes Shell of female Knobbed Argonaut
Attached Image  
image/jpeg 187302 bytes Pygmy male Knobbed Argonaut
Distribution Map  
image/jpeg 44266 bytes Distribution of Knobbed Argonaut, Argonauta nodosa
Video  
video/x-msvideo 1143874 bytes Video of Knobbed Argonaut

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