Long before humans existed, before dinosaurs roamed the Earth, even before vetebrate life made the move to land, there was the nautilus. Out of the 10,000 nautilus species that once existed, only six are still around – including the Chambered Nautilus, which first appeared about 2.6 million years ago during the early Pleistocene, and is physiologically nearly identical to much older species. Nautiluses are a deep-branching lineage within the cephalopod family tree, meaning that they split off early on from other lineages. The nautilus's shell is just one mark of this ancient evolutionary divergence. This post will explore the amazing attributes of the Chambered Nautilus through the lens (actually, nautilus eyes don't have lenses, so maybe "pinhole" is more appropriate) of their differences from and similarities to the rest of the Cephalopod class.
The nautilus is one of the few cephalopod species that engages in polycyclic reproduction (individuals reproduce multiple times rather than dying after reproduction). Males have four modified tentacles that form a reproductive complex called a spadix, which is used to transfer sperm to the female's mantle. When the male is ready to mate, his shell becomes larger than the females (for reasons not yet known to science, most nautilus populations are found to have significantly more males than females). Nautiluses mate facing each other, and stay in that position for hours. Once the female is fertilized, she lays her eggs (only a dozen or so, a very small clutch compared to other cephalopods) on a rocky surface in warm, shallow water. The eggs take several months to develop before they hatch. The infant nautiluses have a shell diameter of 1.2 inches, and it takes them 5-10 years to reach sexual maturity. The average lifespan of a Chambered Nautilus is about 20 years. This is much longer than the lifespan of a typical squid or octopus, which usually only live 2-5 years.
The nautilus brain is much simpler than the brains of other cephalopods. There was most likely never the pressure to evolve a complex brain because they have a less active lifestyle and do not actively hunt prey like coleoids. Dr. James Wood (who runs one of my go-to research sources for this blog, The Cephalopod Page), describes nautiluses as little more than "swimming snails", writing that they "are not as active and...intelligent and responsive as other cephalopods". While nautiluses may not have the complex intelligence of an octopus, research has shown that they do possess short and long-term memories.
The nautilus's shell is an example of a logarithmic or equiangular spiral, often seen in nature. A logarithmic spiral "follows the rule that, for a given rotation angle (such as one revolution), the distance from the pole (spiral origin) is mulitiplied by a fixed amount". Discovered in 1638 by Descartes, the logarithmic spiral differs from the Archimedean spiral in that the distance between the arms increases in geometric progression rather than remaining constant.
The nautilus is a fascinating animal. Here is the final stanza of a poem by Oliver Wendell Holmes, one of 19th-century New England's Fireside Poets and also a celebrated physician, that uses the nautilus's shell as a metaphor for his spiritual growth:
"Build thee more stately mansions, O my soul,
As the swift seasons roll!
Leave thy low-vaulted past!
Let each new temple, nobler than the last,
Shut thee from heaven with a dome more vast,
Till thou at length art free,
Leaving thine outgrown shell by life's unresting sea!"