The Argonaut

A model of Argonauta argo from the American Museum of Natural History

The paper nautilus, aka the argonaut (family Argonautidae) bears an uncanny resemblance to the majestic nautiloids of yore.   This resemblance becomes bizarre when one realizes that the resemblance is superficial.  The paper nautilus is no nautiloid at all.* It is in fact an octopus.  Its shell is not a true shell manufactured within an internal shell sack (as in other mollusks), but is rather an egg case secreted and delicately assembled by the female.  It is as though, instead of having a protective skeleton from birth, you were expected to build one and hatch your young inside of it.

Argonauts are pelagic predators, hunting planktonic mollusks, minnows, and small octopi and squid.  Like other cephalopods, they can change colors and alter their shape.  They move by means of jet propulsion.  Argonauts exemplify one of the most formidable traits of the cephalopod: intelligence. To quote Marcie Orenstein’s Marine Animals of Bermuda, “These animals often form associations with gelatinous marine species, utilizing them for food, locomotion, and protection.”  For example Argonauts are often found clutching the top of jellyfish and steering the latter around the ocean.  The Argonaut eats through the jellyfish’s mantle into its digestive tract.  It can thereafter rely on the jellyfish’s tentacles as a sort of fishing apparatus with which to catch prey (which it takes from inside the jellyfish’s gastral cavity).  The jellyfish further serves as a protective shield, for the argonaut’s predators, such as tuna and dolphins, tend to shun such hydrozoans.  Additionally the Argonaut gets a free ride for as long as its jellyfish remains alive.

We have pictures of this behavior! Argonauta argo atop the jellyfish, Phyllorhiza punctata. Photographs Copyright ©, Thomas Heeger, University of San Carlos, Philippines.

I mentioned above that only female argonauts build shells.  The male is a strange armorless dwarf, a tenth the size of the female.  One of the male argonaut’s arms, the hectocotylus, is specialized for mating.  The Tree Of Life Argonaut webpage describes this process, “At mating, the hectocotylus, which carries one large spermatophore, breaks out of its sac and then from the male body. The free hectocotylus invades, or is deposited in, the female’s mantle cavity, where it remains viable and active for some time.”  Georges Cuvier first discolvered the hectocotylus but mistakenly described the organ as a worm parasitic on the female argonaut.

Georges Cuvier's original illustration of the Hectocotylus of an Argonaut (Iconographie du règne animal de G. Cuvier, 1829)

The paper-thin calcareous shell of the female argonaut is truly an egg case.  It is shaped in a Fibonacci spiral for reasons which are unclear, however, like a nautiloid’s shell, it contains a bubble of air which the adult female nautilus manipulates for buoyancy.  Aristotle wrote that the Argonaut used its egg case and its flattened tentacles to catch the wind and sail, however there is no current evidence of such behavior (although cephalopods travel all sorts of strange ways).  When the eggs grow large in the shell case and hatch, the female is pushed out of her egg case and she dies.  Marianne Moore very beautifully described the female argonaut’s maternal solicitude in her poem The Paper Nautilus (while additionally cross-referencing the hydra, which we love at this blog).  Moore describes the female argonaut as a metaphor for creativity:

for she is in
a sense a devil-
fish, her glass ram’shorn-cradled freight
is hid but is not crushed;
as Hercules, bitten

by a crab loyal to the hydra,
was hindered to succeed,
the intensively
watched eggs coming from
the shell free it when they are freed

(From The Complete Poems of Marianne Moore. Copyright © 1961 Marianne Moore)

An Argonaut Illustration by the gifted Debby Mason

*Of the shelled nautiloids which once ruled earth, which teemed in countless numbers through the oceans of the Paleozoic and the Mesozoic, exactly one species is left–the magnificent pearly nautilus, concerning which, more anon.