The Scientist - USA (2020-11)

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ZOOGRAPHY; OR, THE BEAUTIES OF NATURE DISPLAYED. IN SELECT DESCRIPTIONS FROM THE ANIMAL, AND VEGETABLE, WITH ADDITIONS FROM THE MINERAL KINGDOM. SYSTEMATICALLY ARRANGED

BY JEF AKST

The Paper Nautilus, 300 BC–present

T

hey were said to float atop the

sea surface using their shells

as gracile vessels, and to lift

two of their eight arms as sails that

would catch the wind and propel them

through the water while the other

arms served as oars. This legend,

which traces back to Aristotle around

300 BC, describes unique octopuses

called argonauts, referencing the

sailors of the Argo from Greek

mythology. But while argonauts are

one of a handful of octopus groups

that have left the seafloor to adopt a

pelagic lifestyle, they do not, in fact,

sail the water’s surface.

From Aristotle’s time on, “the story

got more and more embellished,”

says Julian Finn, a curator of marine

invertebrates at Museums Victoria in

Australia and an expert on argonaut

taxonomy, biology, and lore. “And then

people started drawing them based on

the story,” eventually getting to “this

argonaut sailing high above the surface.”

Myths about argonauts (family

Argonautidae) continued to flourish

thanks in part to the fact that the

octopuses are elusive, fostering

misconceptions about their biology and

leaving many fundamental questions

unanswered for millennia. In the early

19th century, for example, zoologist

Georges Cuvier mistook the detached

arms of tiny (1- to 2-centimeter-long),

shell-less male argonauts for parasitic

worms when he found them inside

female octopuses. In fact, researchers

later learned that when a male

encounters a potential mate, it releases

an arm loaded with sperm that can

crawl into the female’s mantle cavity.

In the 1830s, French marine biologist

Jeanne Villepreux-Power put to bed an

age-old rumor that female argonauts

steal their shells from other animals,

confirming that they make their own

and use it as a brood case for their eggs.

Adding to the aura of mystery around

argonauts is the fact that, every so often,

thousands or even tens of thousands of

them wash up on a beach. Some people

had speculated that air gets trapped in the

animals’ shells and causes them to float

and wash ashore, but no one had ever

documented the argonauts’ behavior in their

natural environment to find out whether air

bubbles in shells present an issue.

So in December 2005 and January

2006, Finn, then a PhD student, went

with fishermen off Shimane Peninsula

in the Sea of Japan each morning in

the hopes that they would see a female

greater argonaut (Argonauta argo).

Before long, Finn had scooped up a

total of three female argonauts. He

transported them to a nearby harbor,

where he would remove all air from the

shell of one of the argonauts before he

released it just above the seafloor and

observed its behavior.

“Straight aw ay, it floundered around

and you could see that it just didn’t have

stability underwater,” Finn says. But then

the argonaut would jet up to the surface and

rock back and forth with its shell sticking

out of the water to capture air. The octopus

would use one arm to contain the air in the

shell before it dove to a depth where the

animal became neutrally buoyant (Proc Roy

Soc B, 277:2967–71, 2010). “And then it just

shot off.... It went from being this animal

that was struggling underwater to being a

perfect submarine—it was so graceful.”

So air in an argonaut’s shell is

probably not what leads to the animal’s

untimely demise, Finn reasons. The

cause of the mass argonaut strandings,

along with much else about these storied

creatures, remains a mystery. g

SETTING SAIL: This illustration of Argonauta argo by William Daniell comes from William Wood’s

1807 natural history text Zoography. Wood writes that the bizarre octopus, often called a paper

nautilus, “is supplied with eight arms, two of which are furnished at their extremity with an oval

membrane that serves for a sail, while the other six, hanging over the sides of the boat, are employed as

oars, and occasionally serve to steer by.” This illustration was likely copied from an earlier one,

according to Julian Finn of Museums Victoria; both illustrations incorrectly show the octopus’s

“sails”—actually membranous webs on its top arms—projecting outward from the arms like flags. In

reality, the argonaut’s webs do not have an open end, but are fully surrounded by the animal’s arms,

and often stretch out over the sides of their shells.