Tuesday, 4 September 2012

The Malacological Marvel of the Nacreous Nautilus

                            I looked at the Paper Nautilus yesterday and began with a quote from Aristotle’s History of Animals, so it only seems right to repeat that action with another quote from his book. It is obvious from this passage that he was aware of the two different types of the animal: -
“There are two other kinds which dwell in shells, which some persons call nautilus (and nauticus), and others call it the egg of the polypus … this animal generally feeds near the land; when it is thrown upon the shore by the waves, after its shell has fallen off, it cannot escape, and dies upon the land … and there is another, which inhabits a shell like a snail. This animal never leaves its shell, but remains in it, like the snail, and sometimes stretches out its tentacula.” 
Aristotle History of Animals, Book IV, Chap. 1, Para. 16

Nautilus Pompilius

The first described is the Paper Nautilus (Argonauta argo) and the second is the Pearly Nautilus (Nautilus Pompilius), which is the creature that most people will recognise – examples can be found in almost every sea-side shop that sells sea-shells, although recently concern has been expressed at the declining numbers in the wild and the nautilus could soon be placed on the CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) list. It is so well-known that the American magazine The Conchologists' Exchange, founded in 1886, changed its name after the second issue and became The Nautilus, a quarterly magazine devoted to the study of malacology, which, in conjunction with the Biodiversity Heritage Library, has back issues available in PDF and other formats, right back to the first issue (a truly wonderful resource). And of course, it is the name of Nemo’s submarine in Jules Verne’s 20,000 Leagues Under the Sea.

Richard Owen - Memoir of the Pearly Nautilus - 1832

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

Richard Owen - Memoir of the Pearly Nautilus - 1832

Richard Owen - Memoir of the Pearly Nautilus - 1832

Richard Owen - Memoir of the Pearly Nautilus - 1832

 The nautiloids (six species in two genera) possess chambered shells, hence the alternate name the Chambered Nautilus, which grow with the animal. As it becomes too large for the current chamber (camera), it builds a ‘wall’ (septa) behind itself and extends the front of the shell to accommodate its body, often up to thirty times. The camerae are connected by a duct, the siphuncle, in the centre of each septum, and liquid is passed into the camerae which the nautilus regulates by osmosis pressure as an aid to buoyancy, not unlike of the swim bladder in fishes. The Chambered Nautilus can descend to depths of 700 metres (at about 800 metres hydrostatic pressure will cause the shell to implode, resulting in instant death). 

Internal spiral showing camerae

The nautilus is the only cephalopod with an external bony structure (others, like cuttle-fish, have the internal ‘bone’ familiar to budgerigar fanciers, whilst some, like squid, have a cartilaginous ‘quill’), and the shell grows in a logarithmic spiral, a spiral that the mathematician Jacob Bernoulli called the spira mirabilis ‘miraculous spiral’. The spira mirabilis increases in size without altering in shape, and can also be found in other natural forms – in the heads of sunflowers, for instance. It is not however, as some have claimed, a Golden Spiral, in which the growth factor is equal to φ (phi) – a ratio of 1:1.618  i.e. (1 + sqrt[5])/2)  – which is to say, the spiral increases by a factor of φ from the point of origin for each quarter turn. Very roughly, the spiral of a Chambered Nautilus shell triples in radius with each full turn; the golden-ratio spiral increases by a factor of approximately 6.85 for each full turn. I’ll come back to the Golden Ratio and the Golden Number another day – it is a fascinating subject (honest!).

The distinctive irregular stripes camouflage the nautilus shell, seen from above they break up the shape in dabbled sunlight whereas the almost white underside make the shell indistinguishable from brighter lights at the surface.

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

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

Like other cephalopods, they have tentacles, often in excess of ninety – Owen, in the Memoir, counts ninety-two, which draws two exclamation marks from him in his description – but unlike octopi, the tentacles do not have suckers but are ridged instead, which give the nautilus a very strong grip. They can live for over twenty years but do not reach sexual maturity until about fifteen years, which compounds the problems caused by over-fishing, as the population is denied the opportunity to sustain itself. As I have mentioned before, taking shells from naturally dead specimens does not affect population numbers but deliberately taking live animals from the wild is indefensible. 

Do we, as a species, really want to cause the extinction of another species that has survived for 500 million years just because we can?

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