Head to head
Nautilus vs Octopus
The nautilus and the octopus are both cephalopods, the group of shell-reduced molluscs, but they represent its oldest and one of its newest branches. Put side by side they show how far the octopus lineage travelled after it abandoned the shell.
| Nautilus | Octopus | |
|---|---|---|
| Shell | Full external coiled shell | None |
| Arms or tentacles | 60 to 90 tentacles, no suckers | Eight arms lined with suckers |
| Eyes | Simple pinhole eyes, no lens | Large, sharp, lens eyes |
| Brain and behaviour | Small brain; simple, instinct-driven | Largest invertebrate brain; learns and uses tools |
| Colour change | None | Instant colour and texture change |
| Lifestyle | Slow deep-water scavenger | Active hunter, mostly shallow |
| Lifespan | Often 15 to 20+ years | Usually 1 to 2 years |
| Age of the body plan | Roughly 500 million years, little changed | Shell fully lost within the last ~100 million years |
Nautilus vs Octopus: common questions
Is a nautilus a type of octopus?
No. Both are cephalopods, but they are on very different branches. The nautilus belongs to an ancient lineage, the nautiloids, that kept the external shell its ancestors had; its closest extinct relatives are the ammonites. The octopus belongs to the coleoids, the shell-less or internal-shell cephalopods, together with squid and cuttlefish. The nautilus branch and the octopus branch separated something like half a billion years ago, so a nautilus is far less closely related to an octopus than an octopus is to a squid.
Why is the octopus so much smarter than the nautilus?
Losing the shell is a big part of the answer. A shelled animal is protected but slow and limited; a soft-bodied one is vulnerable and has to survive by wits, speed, and flexibility. After the octopus lineage gave up the shell, selection favoured better eyes, faster information processing, quick learning, camouflage, and dexterous arms, and over time that produced the large, complex brain octopuses have today. The nautilus kept its armour and its slow, low-energy scavenging life, and never faced the same pressure, so it retained a small brain and simple senses. It is a case study in how a single change, dropping the shell, can open the door to a completely different evolutionary path.
Can you keep either in an aquarium?
Octopuses are kept by experienced marine aquarists, though they are challenging: they need pristine water, they are escape artists that must have a sealed tank, they are short-lived, and a bored octopus can be destructive. Even so, a well-kept octopus is one of the most engaging aquarium animals there is. Nautiluses are far harder and are generally discouraged for home tanks. They need deep, cold, high-quality water kept below about 25 degrees Celsius, they are slow feeders easily outcompeted, and wild populations are already under pressure from the shell trade, so the small number kept are almost all in public aquariums with specialised systems.