Argonauts (genus Argonauta, the only genus in the Argonautidae family) are a kind of pelagic octopus that live close to the surface of warm seas rather than on the sea floor, as nearly all other octopuses do. The female produces a paper-thin egg case resembling a shell into which she deposits her eggs. Compared with the females, the males are tiny, and while the females have been known since ancient times, the males were not described until the 19th Century. Females grow up to 10 cm and make shells up to 45 cm, while male body size is often less than 2 cm.

Anatomy and Reproduction

Like most octopuses, the argonaut has a rounded body, eight tentacles, and no fins. The males lack the dorsal tentacles used by the females to create their egg cases. The males have a modified arm called a "hectocotylus" that is used for sperm transfer to the female. When fertilizing the female, the hectocotylus is inserted into a cavity in the female's mantle, known as the pallial cavity, and detached. The females are only able to mate once in their lifetime, whereas the male is able to reproduce multiple times.

Egg case

The most curious thing about the argonauts are the egg cases they produce, which resemble the shells of extinct ammonites. The case is secreted by the tips of the female's two greatly expanded dorsal tentacles prior to egg laying. After she deposits her eggs in the floating egg case, the female takes shelter in it herself, 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 egg cases 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 (see Monks and Palmer). However, this is uncertain and it is unknown whether this is the result of direct or convergent evolution.

Feeding and defense

When feeding Argonauts use their tentacles to grab prey and drag it toward the mouth. It then bites the prey, injecting it with poison made in the salivary gland. If the prey is shelled, the argonaut uses its radula, a tongue-like appendage, to drill into a shelled organism. Then it will inject the poison into its prey.

Argonauts have the capability to alter their color. They can effectively blend in with their surroundings to avoid predators. They also make ink, which is ejected when the animal is being attacked. This ink is used to paralyze the sense of smell of the attacker, providing time 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.

Species

• Genus Argonauta
  • Greater Argonaut, Argonauta argo
  • Argonauta bottgeri
  • Argonauta cornuta
  • Winged Argonaut or Muddy Argonaut, Argonauta hians
  • Knobby Argonaut or Knobbed Argonaut, Argonauta nodosa
  • Argonauta nouryi
  • Argonauta pacifica

Trivia

Argonauts are featured in Twenty Thousand Leagues Under the Sea, noted for their ability to use their tentacles as sails.

Reference

• Monks, Neale and Palmer, Philip. Ammonites. Smithsonian Institution Press, Washington D.C. 2002.