Argonauta nodosa – Paper Nautilus

ArgonautaNodosaNot exactly Maatsuyker Island botany, but … recently I have been studying mathematical patterns in plants  e.g. the Fibonacci sequence and the Golden Ratio. The image often used to illustrate this pattern in nature is the Chambered Nautilus shell.  My mind went to some shells that I keep as a souvenir and which I always thought, mistakenly as it turns out, were members of the Nautilus family. This photo is of the largest of my shells, measuring 11 cm wide.

CottageIn June 2008 Marcus and I spent 3 glorious weeks at another lighthouse, on Gabo Island, about 30 kms off the coast of Mallacoota near the NSW and Victorian border. We stayed in the Assistant Lighkeeper’s Residence, a fully-equipped 3 bedroom house, built of the same solid beautiful pink granite as the lighthouse.LightFromAir As with Maatsuyker Island, you need to take all food and supplies for the duration of the stay because access (by air or sea) is weather-dependent. Amongst many natural and man-made destinations accessible by walking tracks, the island has a small, protected beach on the north-western coast. Beach The cold June sea was not often calm enough to entice us to have a dip, but we did spend some very happy hours there watching visiting humpback whales. To this day, I still have a very precious memory of the tranquil bay one sunset with a pair of whales so close we could hear them “exhaling”.

But back to the shells… A regular visitor to the island had witnessed a mass “wash-up” of these shells at the bay and when we were leaving the island, I was given 3 shells as a souvenir of our visit.

Since these shells are the best easy-to-hand example, I have been using them to learn how to accurately draw the logarithmic spiral described by the Golden Ratio, or phi. But during my research, I realised that my shells did not look like the photos of Nautilus. So what exactly are my shells? I used reference books and found that my shells are known by the common name Paper Nautilus but are not Nautilus shells at all. Specifically they belong to the genus Argonauta, a group of pelagic octopuses. This species, A. nodosa, aka Knobby Argonaut, is one of 6 extant species (many other species now extinct). Apparently it is common in Australian waters. The thing I found really fascinating is that unlike the Nautilus which grows within its shell, the female Argonaut builds this calcareous secretion on the end of one of its tentacles as a container for its eggs. Who would have thought?!?!

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