Researched and written by Timothy Foley
The chambered nautilus is a large mollusc related to the octopus, snail and clam. It is a cephalopod (meaning “head-foot”) as its tentacles are attached directly to its head. The nautilus is the only cephalopod which has a full shell for protection. When threatened, it retreats into its shell, sealing the gap with a piece of flesh called a hood. Unlike their relatives, they have poor vision with no lenses on their primitive eyes. Nautiluses have more than 90 tentacles. Instead of suckers, they use grooves and ridges to grip prey and bring it to their parrot-like beak.
The nautilus is an invertebrate, weighing up to 1.3kg when fully grown. Its shell can grow up to 25 cm in diameter, and is coloured white with brown stripes for camouflage. They live in oceans in the Indo-Pacific region, and have a lifespan of between 15 and 20 years. They spend most of the day at depths of about 700m, but at night slowly migrate up coral reef slopes to forage for food at a depth of 80m. They are carnivorous scavengers, eating dead crustaceans, fish and other organisms. However, they also prey on hermit crabs, and dig sediment from the sea floor in search of small prey.
Their shell contains over 30 chambers, which form a logarithmic spiral shape as they grow. The largest outermost chamber is where the soft body of the nautilus is located. The remaining other chambers act as ballast tanks, helping the nautilus remain buoyant. These chambers fill with gas as the nautilus moves closer to the surface. It can then use a duct called the siphuncle (which connects the chambers) to allow water in, flooding the chambers and causing the nautilus to sink. By using this mechanism, the nautilus is able to control the height at which it feeds in the ocean.
Nautiluses move by a form of jet propulsion. Water enters the mantle cavity and is forced out of the siphon (the small gap at the end of the mantle cavity). This propels it in a specific direction, allowing it to reach food. Nautiluses take from 10 to 15 years to become sexually mature. As they take a long time to become sexually mature, and only live for 15-20 years, they have a relatively short time in which to mate. They only mate in tropical waters, so have to travel to find a female. They mate by the male transferring his sperm packet to the female using a kind of modified tentacle called a spadix. The female produces eggs – between 10 and 20 per year – which are then fertilised, laying them one at a time. This process can last throughout the whole year, and it may take up to a year before the fertilised eggs hatch.
There are several other species in the family Nautilidae, with five species in the genus Nautilus and two in the genus Allonautilus. The largest is Nautilus repertus (“The Emperor Nautilus”), which is between 20 and 25 cm in diameter. The smallest – Nautilus macromphalus – is only between 15 and 18 cm in diameter.
Before the dinosaurs, there were giant cephalopods that lived in the sea. Prehistoric nautiloids had shells that spanned up to 3m in size. Their most likely food source was a type of arthropod called a trilobite. Nautiloids dominated the sea, as fish hadn’t yet evolved to compete with them for food. Their shells were originally straight, but over time they evolved into the coiled shape we see today. Animals similar to Nautilus have been alive for the last 500 million years, apparently unchanged, so it is often referred to as a ‘living fossil’.
A video explaining more about the chambered nautilus can be found here.