Tag Archives: Cephalopod

Marine Megafauna MOOC Assignment: Common Paper Nautilus

As I mentioned earlier, I am taking Marine Megafauna: An Introduction to Marine Science and Conservation, a free online course through the coursera platform.  This work for this course has included two writing assignments.  The first — a short-answer exercise involving spatial data — didn’t lend itself to a blog post,  The second, however, does.
Here’s a summary of the assignment, copied verbatim from the course website:
This is a writing assignment that focuses on developing a “species profile” for a marine megafauna species that is accessible and understandable by a general audience. This profile should cover key knowledge about the species, including information on its taxonomy, physical appearance, life history, foraging and feeding habits, and any other unique or special characteristics that may be present. The profile will also include a brief biography of an expert who studies that species, and it will provide a list of three references to direct readers of this profile to further information.
And here’s what I wrote (edited slightly, primarily to include more links to relevant information).  In case you’re wondering, the numbers preceding the paragraphs correspond to a peer-assessment rubric.

Female Argonauta argo with eggs, courtesy of Bernd Hoffman via Wikimedia Commons.
(1) Argonauta argo, the common paper nautilus, is not a nautilus at all but rather an odd octopod.  It shows tremendous sexual dimorphism in terms of both size and another interesting feature: females may grow brittle, foot-long (30 cm) shells, while males are generally about a centimeter in length and are shell-less.  Aside from this marked sexual dimorphism, the shell of the female is remarkable for two more reasons:  First, the beautiful shell allows females to practice “gas-mediated buoyancy [pdf],” allowing them to live easily at the water’s surface.  And second, recall that the common paper nautilus is an octopod — it is, in fact, one of only a very few to sport a shell.
(2) The taxonomy of Argonauta argo is as follows:  Along with clams and scallops and such, argo is a mollusk.  Within the taxon mollusca, argo is classified along with the true nautiloids as a cephalopod.  Within cephalopoda, argo joins squids and cuttlefishes in the taxon coleoidea.  From there, we zoom into octopodiformes, then to octopoda — all octopuses! — then incirrata (because argo doesn’t have fins).  Next argo finally separates from most other octopuses in the taxon argonautioidea, from which we zoom still further to argonautidea.  Within argonautidea we finally find our genus and species: Argonauta argo, or the common paper nautilus.  Within the genus Argonauta are three other recognized paper nautiluses or argonauts: Argonauta hians (the muddy argonaut); Argonauta boettgeri (Boettger’s argonaut); and Argonauta nodosa (the knobby argonaut).
(3) & (7) Again, when it comes to size, the common paper nautilus is marked by very significant sexual dimorphism:  The female is commonly measured by her shell diameter, which may reach approximately 30 cm.  The shell-less male is only about a single centimeter long.  Argonauta argo commonly appears to range in color from white to pale brown, but because of its chromatophores it also possesses — and exercises — the ability to change color.  Argonauta argo is generally epipelagic, or surface-dwelling, which is rare for an octopod.  It has been identified across a wide range of tropical and temperate waters.  These waters include [pdf] the coasts of the west Pacific (from Japan to New Zealand), the east Pacific (from California to Peru), the west Atlantic (from Massachusetts to Venezuela), the east Atlantic (from Spain  to South Africa), as well as the Mediterranean and Arabian seas.  It appears that there is no population estimate for Argonauta argo — at least, I could identify none — but according to a University of Michigan database it is “very common but is rarely spotted by humans.”  Neither is there any plan for management of this species.  Perhaps due to its apparently robust worldwide population, the common paper nautilus has not been evaluated for any conservation status by the International Union for Conservation of Nature, nor does it appear in the CITES (Convention on International Trade in Endangered Species) database.  Argonauta argo has relatively little interaction with humans and presently does not seem prone to any anthropogenic threats.  In fact, it has been suggested that due to rising ocean temperatures, the common paper nautilus’s range is expanding — it has recently been found in waters (northern Spain, for example [pdf]) where it previously had not been observed.  That said, it may not be a stretch to hypothesize that human-caused ocean acidification poses a long-term threat to the female Argonauta argo’s calcareous shell — and therefore to the species’s surface-dwelling way of life, its shell-reliant reproductive method, and perhaps its very existence. (Indeed, this threat has been studied with respect to Argonauta nodosa.)
(4) Like most octopods, Argonauta argo is thought to have a short lifespan of no more than a few years. Its method of reproduction is similarly typical of octopods.  The tiny male’s third arm is what’s called a hectocotylus — essentially a combination arm and penis.  The male places his hectocotylus into the female’s mantle to deposit the his sperm (via a packet called a spermatophore) and fertilize her eggs.  This process generally ends in the separation of the hectocotylus from the rest of the male.  The female  stores the fertilized eggs in her shell.  Eggs are very small — under a millimeter in diameter — and numerous.  It is thought that, unlike most other octopods, females are capable of reproducing more than once; males, on the other hand, probably are more typical among their taxonomic “cousins” and die after reproduction (at which point they may well be eaten by their partners).
(5) We don’t know much for certain about the feeding habits of Argonauta argo.  According to a paper from 1992, a 1932 paper described the species eating small fish and crustaceans; a 1977 paper instead suggested a diet of small invertebrates like sea snails and sea butterflies.  That more recent (but still twenty-year-old) 1992 paper described a predatory attack by Argonauta argo on a jellyfish:  The argonaut first grabbed the jellyfish with the suckers of two arms, then took two bites with its hard beak.  The argonaut seems to have used these bites in an effort to reach the “gastral cavity” or stomach of the jellyfish so it could feed not on the jellyfish itself but rather on the plankton the jellyfish had eaten.  This foraging method makes a lot of sense for a surface-dwelling ocean resident that is likely a predator.
(5) It should also be noted that, like most octopods, Argonauta argo possesses a rasp-like tongue called a radula and likely some form of venom as well, so it has the physical capacity to eat crabs (a favorite of most octopods) even if its surface-dwelling lifestyle rarely affords such an opportunity.
(8) Dr. Julian Finn is an expert on marine invertebrates who specializes in the paper nautilus.  Dr. Finn wrote his doctoral thesis on the genus Argonauta and recently was lead author on the paper discussing the female Argonauta argo‘s use of her shell for buoyancy.  He currently works as a Senior Curator at the Museum Victoria in Melbourne, Australia (where he also completed his PhD research).  Additionally, Dr. Finn has worked as a freelance cameraman and consultant on many documentaries.
(9) Further Reading:
Finn, J. K., and M. D. Norman. 2010. The argonaut shell: gas-mediated buoyancy control in a pelagic octopus. Proceedings of the Royal Society B 277: 2967-2971.  http://rspb.royalsocietypublishing.org/content/early/2010/05/18/rspb.2010.0155.full.pdf
Guerra, A., A.F. Gonzalez and F. Rocha. 2002. Appearance of the common paper nautilus Argonauta argo related to the increase of the sea surface temperature in the north-eastern Atlantic.  Journal of the Marine Biology Association of the United Kingdom 82: 855-858. http://digital.csic.es/bitstream/10261/25147/1/download.pdf
Heeger, T., U. Piatkowski, and H. Moller. 1992. Predation on jellyfish by the cephalopod Argonauta argo. Marine Ecology Progress series 88: 293-296.  http://www.int-res.com/articles/meps/88/m088p293.pdf

The original assignment did not require a robust reference system. I’ve done my best to restore some sourcing, but If I’m missing anything obvious that deserves credit, please let me know.
Perhaps another time I’ll focus in more on the shell of the female common paper nautilus — this is a feature that seems quite unique and (figuratively) colorful.  Even absent any further delving on my part, though, I hope you’ll agree based on the information I’ve already provided that this “odd octopod” is interesting indeed.

How Cephalopods Change Color

Here’s a nice overview from the Smithsonian Institute’s Ocean Portal about how octopus and squid are able to produce such an array of colors and patterns:

Many thousands of color-changing cells called chromatophores just below the surface of the skin are responsible for these remarkable transformations. The center of each chromatophore contains an elastic sac full of pigment, rather like a tiny balloon, which may be colored black, brown, orange, red or yellow. If you squeezed a dye-filled balloon, the color would be pushed to the top, stretching out the surface and making the color appear brighter—and this is the same way chromatophores work. A complex array of nerves and muscles controls whether the sac is expanded or contracted and, when the sac expands, the color is more visible. Besides chromatophores, some cephalopods also have iridophores and leucophores. Iridophores have stacks of reflecting plates that create iridescent greens, blues, silvers and golds, while leucophores mirror back the colors of the environment, making the animal less conspicuous.

Read the whole thing!