The Malacological Marvel of the Nacreous Nautilus

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n I looked at the Paper Nautilus yesterday and began with anquote from Aristotle’s History of Animals, so it only seems right tonrepeat that action with another quote from his book. It is obvious from thisnpassage that he was aware of the two different types of the animal: –

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n“There are two other kindsnwhich dwell in shells, which some persons call nautilus (and nauticus), andnothers call it the egg of the polypus … this animal generally feeds near thenland; when it is thrown upon the shore by the waves, after its shell has fallennoff, it cannot escape, and dies upon the land … and there is another, whichninhabits a shell like a snail. This animal never leaves its shell, but remainsnin it, like the snail, and sometimes stretches out its tentacula.”

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nAristotle History of Animals, Book IV, Chap. 1, Para. 16

Nautilus Pompilius

nThe first described is the PapernNautilus (Argonauta argo) and the second is the Pearly Nautilus (NautilusnPompilius), which is the creature that most people will recognise –nexamples can be found in almost every sea-side shop that sells sea-shells,nalthough recently concern has been expressed at the declining numbers in thenwild and the nautilus could soon be placed on the CITES (Convention onnInternational Trade in Endangered Species of Wild Fauna and Flora) list. It isnso well-known that the American magazine The Conchologists’ Exchange,nfounded in 1886, changed its name after the second issue and became ThenNautilus, a quarterly magazine devoted to the study of malacology, which,nin conjunction with the Biodiversity Heritage Library, has back issuesnavailable in PDF and other formats, right back to the first issue (a trulynwonderful resource). And of course, it is the name of Nemo’s submarine in JulesnVerne’s 20,000 Leagues Under the Sea.

Richard Owen – Memoir of the Pearly Nautilus – 1832

nRichard Owen’s first major workn(and the one that established his reputation) is his Memoir on the PearlynNautilus (1832), an astonishing and impressive work. Regardless of how younmay feel about Owen the man, his erudition and methodology is without question,nand during the early part of his career he was a scientist of global standing.nThe Memoir is a masterpiece of comparative anatomy and dissection, notnleast because of the quality of the illustrations done by Owen himself, but itnis not really what you’d call bedside reading.

Richard Owen – Memoir of the Pearly Nautilus – 1832

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Richard Owen – Memoir of the Pearly Nautilus – 1832

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Richard Owen – Memoir of the Pearly Nautilus – 1832

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n The nautiloids (six species inntwo genera) possess chambered shells, hence the alternate name the ChamberednNautilus, which grow with the animal. As it becomes too large for the currentnchamber (camera), it builds a ‘wall’ (septa) behind itself andnextends the front of the shell to accommodate its body, often up to thirtyntimes. The camerae are connected by a duct, the siphuncle, in thencentre of each septum, and liquid is passed into the cameraenwhich the nautilus regulates by osmosis pressure as an aid to buoyancy, notnunlike of the swim bladder in fishes. The Chambered Nautilus can descend tondepths of 700 metres (at about 800 metres hydrostatic pressure will cause thenshell to implode, resulting in instant death).

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Internal spiral showing camerae

nThe nautilus is the onlyncephalopod with an external bony structure (others, like cuttle-fish, have theninternal ‘bone’ familiar to budgerigar fanciers, whilst some, like squid, havena cartilaginous ‘quill’), and the shell grows in a logarithmic spiral, a spiralnthat the mathematician Jacob Bernoulli called the spira mirabilisn‘miraculous spiral’. The spira mirabilis increases in size withoutnaltering in shape, and can also be found in other natural forms – in the headsnof sunflowers, for instance. It is not however, as some have claimed, a GoldennSpiral, in which the growth factor is equal to φ (phi) – a ratio ofn1:1.618 i.e. (1 + sqrt[5])/2) – which is to say, the spiral increases by anfactor of φ from the point of origin for each quarter turn. Very roughly, thenspiral of a Chambered Nautilus shell triples in radius with each full turn; thengolden-ratio spiral increases by a factor of approximately 6.85 for each fullnturn. I’ll come back to the Golden Ratio and the Golden Number another day – itnis a fascinating subject (honest!).

nThe distinctive irregular stripesncamouflage the nautilus shell, seen from above they break up the shape inndabbled sunlight whereas the almost white underside make the shellnindistinguishable from brighter lights at the surface.

nNautilidae appeared in the late Triassic,nand have remained largely unchanged for the last 500 million years, hence thencommon description as ‘living fossils’, although some extinct relatives grew tona size in excess of 8 ft. diameter. They are opportunistic predators but feednmostly on carrion, which they eat with a beak and their nine teeth.

Richard Owen – Memoir of the Pearly Nautilus – giving it two exclamation points

nLike otherncephalopods, they have tentacles, often in excess of ninety – Owen, in the Memoir,ncounts ninety-two, which draws two exclamation marks from him in hisndescription – but unlike octopi, the tentacles do not have suckers but arenridged instead, which give the nautilus a very strong grip. They can live fornover twenty years but do not reach sexual maturity until about fifteen years,nwhich compounds the problems caused by over-fishing, as the population isndenied the opportunity to sustain itself. As I have mentioned before, takingnshells from naturally dead specimens does not affect population numbers butndeliberately taking live animals from the wild is indefensible.