Chambered & Paper Nautilus
“I was fascinated by their extraordinary beauty and as I did not know very much about them, I decided that I would see what I could find out“ (Philip Hall 1997 Journal of the Marine Life Society of South Australia). The words of a Curious Naturalist and my sentiments entirely. Nautililoids have long been on the periphery of my interests as a terrestrial biologist. The relationship between the chambered nautilus and the moon piqued my curiosity and the possibility of finding a paper nautilus on Bonza Bay beach has long kept my nautiloid interest alive. This year's 2012 episode of beached paper nautilus fullfilled that wish.
Given their rarity as treasures to be found on the beach, both chambered nautilus and paper nautilus are collectors' items. While many beach-combers have collected them, there is a paucity of information recording date and place where specimens have been found. The biology of nautiloids, pelagic animals living in tropical oceans, is not well known. There are however several web-sites, for example : those of Hall 1997, Mangold et al 2010, Norman 2007, Orenstein & Wood 2008 and Wikipedia, which summarise that knowledge .
I have little to add to that knowledge beyond documenting my observations, tabulating measurements of paper nautilus specimens and speculating. The first speculation is about the circumstances giving rise to the paper nautilus beaching episodes. The second is about the spiral shape of nautiloids and its accord with “spira mirabilis”.
The chambered nautilus Nautilus sp. and the paper nautilus Argonauta argoare two marine molluscs in the class Cephalopoda. Although they are both called nautilus and have tentacles like an octopus, the two are not closely related. They also have completely different life-styles as Argonautais pelagic and floats around the oceans while Nautilusis benthic capable of descending to depths of 800 m.
The word nautilus derives from the Greek word for a sailor, as in nautical, which describes their seafaring way of life.
The chambered nautilus is a living fossilas the genus Nautilushas persisted in the same form for 500 million years. There are only three extant species: Nautilus macromphalus, N. pompiliusand N. scrobiculatus. Their world-wide distribution is the tropical Indo-Pacific ocean between 30°N-30°S and 90°E-175°E , so outside the limits of South African waters. They live where the water temperature is below 25°C.
The chambered nautilus is a very rare find on Eastern Cape beaches. There are five specimens in the East London Museum from Gonubie in 1989, Kaysers Beach in 1963 and the others, collected prior to 1960 from Cove Rock and Kidds Beach. The only recent report is one from Hamburg per John Goldsmid. He reports that in 2010 he found a specimen in the dunes which judging by its state had washed up many years ago. He recalled another specimen found about 20 years ago at Bira lagoon that had also been there for a long time.
Nautilus have up to 100 tentacles arranged in two rows. Unlike the tentacles of most Cephalopods, those of the chambered nautilus lack suckers. To swim the nautilus uses jet-propulsion to draw water in and squirt it out.
The nautilus shell is divided by septae into chambers. At birth there are four chambers and the animal lives in the largest outer chamber. The other chambers are used to regulate buoyancy.
The shell grows continuously and every so often the owner builds a new septa and moves into the outer new and larger chamber. Adults can have over 30 septae (? more like 100) and live for over 20 years. The laying down of the new septa co-incides with the lunar cycle of 29 days and indicated by the lines across the ceiling in each chamber. Fossil nautilus have fewer ceiling lines and this has been correllated with changes in the lunar-cycle in geological time (Kahn & Pompea 1978), but this is contraversial and not widely accepted.
PAPER NAUTILUS genus Argonauta
The paper nautilus is so named because the females have a thin shell which serves as an egg case. The female manufactures the egg-case using the first pair of her tentacles, which have membraneous extensions (Richards & Thorpe 1988). The males are tiny and do not have a shell.
World-wide there are at least four species in the genus Argonauta:
argo(1758) ..... hians(1786) ..... nodosa(1786) and nouryi(1852)
while boettgeri, cornutaandpacificamay be synonyms of hians or nouryi and
americana, argus , compressa, cygnus, maximaandoryzatasynonyms of argo
(Hall 1997, Mangold et al.(2010) adjusted to accord with Museum Victoria, Australia).
Argonauts are pelagic and they “drift along the upper waters of tropical and subtropical oceans” (Richards & Thorpe 1988). This drifting in a circulating gyre in mid-ocean may not be random but governened by environmental factors such as ocean currents and the weather. The gyre is a cyclonic circulation feature up to 200 km in diameter which also has vertical component as there is mixing of currents and up-welling. Within the gyre are symbiotic associations of a great diversity plankton and gelatinous invertebrates (Banas et al. 1982). Thus gyres contain oases of locally high primary productivity which attract apex predators to aggregations of enhanced food supply (Biggs et al.1997). Argonauts can adjust their buoyancy to remain in a favourable position with the gyre (Finn & Norman 2010).
No argonauts live along the eastern shores of South Africa. The commonest paper nautilus found here is A. argo, while a smaller species A. boettgeriwith more widely spaced tubercles is rarely found at Bonza Bay. There is a specimen of A. hiansin the East London Museum whose collection locality is given as “from the East London coast”.
The paper nautilus is something that many persons would love to find. It is sometimes washed up on our beaches in the winter. Of the 51 specimens where the month of collection is recorded : there is one record each for March, May, September and October, 4 in June, 6 in July, 9 in April and the majority, 28 in August.
Besides people looking for the shells, there is competion from gulls who eat the animal and damage the shell. This winter of 2012 there was an unusally large number of paper nautilus washing up and besides those at Bonza Bay there were reports from between Morgan Bay and Hamburg.
The paper nautilus above was found at Bonza Bay by Karen Breetzke.
Mass strandings of paper nautilus, where the animals come ashore dead or stressed, are known in America, Australia (Hall 1997) and Japan (Okutani & Kawaguchi 1983). “There appears to be no set cycle to these mass strandings with currents, winds, krill schools are being speculated as the causes” (Norman 2007), In 2004 along beaches in Uruguay an unprecedented mass stranding of A. nodosa occurred which was ascribed a typical oceanographic and meteorological conditions which advected oceanic water onto the coast (Demicheli et al 2006). The local opinion is that paper nautilus only wash up here after a certain weather episode and no other reason has been postulated.
In the winter months, April to September, the weather of South Africa is dominated by a series of cold fronts which pass along the southern oceans at about weekly intervals. At Bonza Bay the weather follows a route: after a cold front has passed there follow a few days of fine weather with easterly winds. Then as the inland high pressure cell re-asserts itself there is an off-shore flow and berg-wind conditions. This followed by southerly winds as the next cold front approaches. The passing of the cold front brings cold weather which may bring clear skies or rain according to the path and intensity of the cold front.
The synoptic chart for the 7th August 2012
The day before, the 6th August, paper nautilus were beached at Bonza Bay
If the cold front is deep-seated and intense, in other words: it has its origins way down in the “roaring forties” of Antarctica and the pressure gradient is steep, then the south-flowing Agulhas current is forced on-shore, displacing the coastal north-flowing counter-current.
During episodes of such severe weather, the paper nautilus and other marine life drifting on the Agulhas current are washed ashore. Because of the southern origin of these cold fronts in the “roaring forties” the compaction of the Agulhas current is more frequent and more intense at Port Afred to the south of Bonza Bay. Accordingly, paper nautilus are more commonly found at Port Alfred.
These are subjectiveand generalised conclusions based on incidental observations.
“Nor did we orselves discover this number, but rather nature teaches it to us” (Ovid).
The oceanic and meteorological phenomena and interactions are more complex and beyond the understanding of a lay-person. Further there is no data about the relative occurrence of paper nautilus at different because beach-combers don't keep such records.
Beautiful paper-nautilus – but no record of date and place
MEASUREMENTS OF PAPER NAUTILUS SPECIMENS
The length, height and width was measured of all paper nautilus examined, and number of pairs of dorsal tubercles counted. A correlation coefficient between length and height was calculated and compared the known 0.618 based on a logarthmic spiral.
The egg cases measured range in length from 32 to 272 mm (n=75). The largest egg case found this year, 2012 with a length of 234mm was found at Cintsa by Sean Price. The world-wide record length is 300mm (Pisor 2005).
The height of 67 egg-case specimens ranged between 21 and 180 mm.
The maximum width of the apex, between the pairs of tubercles of 63 egg-cases varied between 3 and 9 mm. As expected the apex width increased in proportion to egg-case length. However this was only a “broad” relationship as the apex width of small egg-cases,
length 50mm varied between 3 and 6 mm while in large egg-cases, over 200 mm in length the apex width varied between 6 and 9 mm.
The number of pairs of tubercles of 28 egg-cases varied between 17 and 104. In a similar
manner to the variation in the width of the apex of the egg-cases, there was no clear correllation between the number of pair of tubercles and egg-case length.
These observations suggest that either the paper nautilus were polymorphic or the specimens originated from different populations. The answer to one question simply raises a host of others – enough to keep a naturalist entranced.
In theory, the shape both the chambered nautilus shell and the paper nautilus egg-case conform to a logarithmic spiral known as spira mirabilis.
The spiral can be defined by φ phi - a mathematical constant where the proportions of height and width have a ratio of 1 : 0.618 This is the golden ratio that was an ispiration to Leonardo da Vinci (Askew & Ebbutt 2011).
The length: height ratio of the paper nautilus specimens measured approximated to 100:64 but that relationship was more precise for egg-cases whose length was less than 125mm than those larger (Fig. ). The observed ratio of 100:64 is a close approximation of the expected ratio of 100:62 if the paper nautilus conformed precisely to the mathematical laws of the spra mirabilis. This suggests that the female argonauts in constructing their egg-cases inately conform to those laws, and that ideosyncratic environmental factors cause deviations from them which become more pronounced with increasing egg-case size.
Justin Lindsay, naturalist and serious fisherman kindly loaned me his collection of paper nautilus. Karen Breetzke, Roger Ellis, John Goldsmit, Pat Johnstone, Andrew Patterson, Kelvin Rivett and Cheryl Wright, all gave me details of their collections.
Mary Bursey Cole malacologist at the East London Museum allowed me to measure argonauts in their collection and also provided observations and comments.
My friends Greg Brett, marine biologist of the East London Museum and Neil van Rensberg aided my understanding of matters meteorological and oceanic.
My friend and mentor Larry King of Newstead, Tarkastad made the mathematical of sprirals more comprehensible.
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The spiral can be drawn by placing squares of increasing size together, and running a curved line from one corner of a square â�¡to the corner opposite. Start with a square â�¡1with sides of 1 unit in length and draw the curve from the top right corner to the bottom left corner. Now draw a second square â�¡2of the same size, with one side in common with â�¡1, immediately adjacent and to the left of the first â�¡2.â�¡1. Then run the curved line through the second square â�¡2from the bottom right corner to the top left corner.
Draw a new â�¡3immediately above, with a common boundary along the top of the first two â�¡2.â�¡1and with sides of 2 units - the length of â�¡1+â�¡2. Continue the curve from the bottom left corner of square â�¡3 to the top right corner. The next square â�¡4 is drawn with a common boundary with the east side, and length of 3 units ie the sum of squares one and three â�¡1+â�¡3 .
Continue the curve from the top left corner to the lower right corner of square four â�¡4.
This can be re-iterated adding more squares â�¡5 â�¡6 â�¡7 …. etc and the spiral pattern of the curved soon becomes apparent. Square â�¡5 has side length of 5 units, that of square 6â�¡is 8 units, while square â�¡7 is 13 units and square â�¡8 is 21 units.