If you asked me as a kid what my favorite animal was, there's a good chance I'd respond "chambered nautilus", though I probably would mispronounce it. I don't know if it's still my favorite but it's definitely up there in the pantheon of weird critters. For this Wet Beast Wednesday, I'll discuss my childhood favorite.
The nautilus is a cephalopod that lives in a curved shell and looks similar to (but is not closely related to) the extinct ammonites. There are 6 living species in two genera, but 90% of the time when someone is discussing nautiluses they are referring to the most well-known species: Nautilus pompilius or the chambered nautilus. Nautiloids are ancient, going back to at least the late triassic with their more primitive ancestors going back as far as the ordovician period, a time when only invertebrates and primitive plants occupied the land and true fish had not yet appeared. Because of their ancient history, nautiluses are sometimes considered living fossils. I have ranted before on how misleading the term "living fossil" is so I'll spare you that for now. Nautiloids are considered a sister group to the celoids, which contains all the squid, octopus, cuttlefish, and everything else we thinks of as cephalopods. Nautiluses should not be confused with paper nautiluses. Also called argonauts, paper nautiluses are a group of octopi that make an egg case which looks like a shell.
The most noticeable feature of a nautilus is its shell. The shell is smooth and finely curving, naturally growing in the shape of a logarithmic spiral (though not, as is commonly stated, a golden ratio spiral). The shell has a stripy outer layer and an inner layer coated with nacre. Internally, the shell is divided into camarae (chambers) separated from each other by walls called septa. Each septum has a small hole in it through which a strand of tissue called the siphuncle passes. Most of the nautilus's body is in the foremost and largest chamber. The shell grows new septa as the animal grows, with the nautilus's body moving to a new chamber as it becomes too large for previous ones. Juveniles are typically born with 4 septa, with adults having as many as 30. In addition to providing protection from predators, the shell is also key for regulating buoyancy. The septa can contain pressurized gas or water and the siphuncle regulates their contents by either adding or removing water to increase or decrease buoyancy. Because of its pressurized contents, the shell can only withstand pressure at depths up to 800 M (2,400 ft) before imploding. Oddly enough, nautiluses can be safely brought up from deep waters where most animals would be killed by the pressure changes. To move, the nautilus pulls water into the first chamber of the shell using its hyponome (siphon) and shoots it back out. The chambered nautilus is the largest species, with a maximum shell diameter of 25 cm (10 in), though most get no larger than 20 cm (8 in).
(image: a diagram of nautilus anatomy. source)
Where celoid cephalopods have tentacles, nautiluses instead have numerous cirri. Unlike tentacles, cirri are less muscular, are not elastic, and have no suckers. They are used to grab objects using their ridged surfaces and can hold in so hard that trying to take an object away from a nautilus can rip off its cirri, which will remain firmly attached. In addition, the nautilus has modified cirri that serve as olfactory receptors and a pair that serve to open and close the shell when the nautilus needs to retract into it or emerge. Nestled within the cirri is the beak, which is used to consume the nautilus's primary prey of invertebrates, though they have also been seen scavenging fish. Their eyes are less developed than most cephalopods, lacking a lens and consisting of a small pinhole that only allows the nautilus to see simple imagery. Their brains are differently structured than most cephalopods and studies have found them to have considerably shorter long-term memories.
(image: a chambered nautilus (upper left) next to a rare Allonautilus scrobiculatus. source)
Cephalopod reproduction is quite different than that of other cephalopods. While most cephalopods are short-lived and semelparous (reproducing only once), nautiluses can live over 20 years and reproduce multiple times (iteroparity). They do not reach sexual maturity until around 15 years old, with females laying eggs once per year. Eggs are attached to rocks and take 8 to 12 months to hatch. Males have a structure called the spadix composed of 4 fused cirri that they use to transfer sperm to females. Females lose their gonads after laying their eggs and will regenerate them for the next year's mating season. Interestingly, male nautiluses seem to vastly outnumber the females.
(image: nautiluses mating)
Nautiluses are found in the Indo-Pacific reagion of the ocean and can be found on the steep slopes of coral reefs. They prefer to inhabit waters several hundred meters down. It was once believed that they would rise to shallow waters at night to feed, lay eggs, and mate, but their vertical migration behavior has since been shown to be more complex than that. They have noon been fished by humans for their shells, which have become popular subjects in art and can be made into a number of decorative pieces. The nacre of the shell can be polished into osmeña pearl, which can be quite valuable. Demand for the shells combined with the late sexual maturity and low fecundity is threatening all the species. As of 2016, nautiluses have been added to the CITES Appendix II, making them protected by limiting international trade of their shells. Despite this, they are still threatened and require further protection
(image: a carved and painted nautilus shell from the Poldi Pezzoli Museum, Milan)
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