Paper Shells and Surfing Sea Creatures
August 16, 2019

by Brittany Arendse

What are winters in Nature’s Valley if not the perfect time to comb the coastline, at sunrise, for the most delicate paper nautilus shells? Argonauta argo or the ‘paper nautilus’, so called for its thin shell and similarity to the true nautilus are actually very different in form, biology and evolution when compared. For one the chambered nautilus is the only living cephalopod (molluscs like squid, octopus, etc.) with a real external shell. What then are these paper treasures that wash up on our beaches if not a shell?

Well, the debate around the form and function of these ‘shells’ has been a longstanding one. Aristotle, a Greek philosopher and great mind of his time and contributing in many ways today still to Western philosophical thinking, had his own ideas about the functioning of such a creature.  Around 300 BC he suggested that the paper nautilus used its ‘shell’ as a boat, sailing at the water’s surface, using its tentacles as oars and sails to navigate. What a wonderful thought!

“The nautilus is a polypus peculiar both in its nature and its actions; for it sails upon the surface of the sea, rising up from the depths of the waters. It is brought to the surface with its shell inverted, in order that it may go out more easily and navigate in an empty shell. When it reaches the surface, it turns its shell over. There is a membrane extended between two of its tentacula similar to the web feet of birds, except that theirs is thick and that of the nautilus thin and like a spider’s web. This it uses for a sail when the wind blows, and it extends two of its tentacula for rudders. If alarmed, it fills its shell and sinks in the sea.” Aristotle History of Animals Book IX Chap 25 Para 12
 

Early scientists suggested the ‘shell’ was perhaps made by another… Much like the hermit crab, who peruses the ocean floor, picking out the finest of them all and wearing it proudly as its new home, at least till the shell is outgrown.

It is only in the early 1800s that Jeanne Villepreux-Power, a female Marine Biologist (there weren’t many of these at the time), conducted the first ‘aquaria’ study, using wooden chambers placed in the local Bay, to observe the animals in more natural condition. She, like us, wanted to know how these ‘shells’ come to be. So, she damaged the ‘shells’ of her aquaria specimens to see what would happen. Astonishingly, during her observation periods, she witnessed these argonauts repairing their shells using their two unique, membranous tentacles (once thought to function as sails and oars). Of course, this result was met with immense pushback as it was a ridiculous notion, brewed up in the mind of a woman no less.

One of Villepreux-Power’s cages for the argonauts. (RMN Muséum national d’Histoire naturelle (MNHN))

Today we know that this ‘shell’ is a calcite egg case, which houses and protects unborn eggs – essentially functioning as a cosy little nest. The females produce these egg cases when young, and they live in them until death. The males do not have an egg case and are microscopic in comparison to the female. She can grow to be around 30 cm and he is only the size of a fingernail, approximately 1cm – sexual dimorphism at its finest. Also, the reason why the males were only described in the late 19th century; they are so small and were so scarcely encountered that it was assumed males and females looked the same. This raises questions, I can tell. Questions around reproduction… well the males have a specialised arm, called a hectocotylus (once thought to be a parasitic worm… ouch!), that carries the sperm. The arm is inserted into the female’s pallial cavity and detaches from the male.

Paper nautilus life cycle by BrushbabyArt

The egg case has an additional function, which not many know about. In 2010 it was discovered that the argonaut’s ‘shell’ controls buoyancy. The octopus swims up to the water’s surface and traps air in its case by rocking back and forth. The amount of air taken up equals the weight of the animal at a certain depth. So, the argonaut will sink down until it reaches neutral buoyancy, where the air is trapped, and the animal’s weight is equal, allowing it to float in the water column with very little energy expenditure. What an amazing mechanism!

http://blogs.discovermagazine.com

 

There is so much more to these animals than pretty paper shells! Don’t you think?

 

Sources:

http://from-bedroom-to-study.blogspot.com/2012/09/the-aristotelian-anecdote-on-sailing.html

https://www.sciencefriday.com/articles/the-seamstress-and-the-secrets-of-the-argonaut-shell/

Scientists solve millennia-old mystery about the argonaut octopus 

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