What's in a Shell? Strange Shells of Cephalopods
A shell's shape
Cephalopods with shells on the outside—like some extinct species and the living nautilus—might resemble snails. But, while snail shells have just one continuous space inside, the interior of cephalopod shells is divided by walls, or septa. The walls separate the newest part of the shell that the animals actually live in from the older parts that contain gas to provide buoyancy. The walls aren’t complete; they are connected by a tube-like thing called a siphuncle.
We think that as the cephalopod grows, it adds more shell to the front edge and moves forward into the newest part of the shell as it builds a wall, mostly closing off the older part of the shell behind it. The walled-off shell then fills with gas through the siphuncle, giving the animal lift or buoyancy. Can you imagine how much easier this makes life for the swimming animal? Instead of working really hard to tread water, the cephalopod can hang weightless in the water column.
Cuttlefish shells—or bones
Although the shell in most fossil cephalopods was coiled and (we think) outside the animal, the shell in cuttlefish is inside and mostly flat. We figure that, despite these differences, the similarities are so big that these structures must reflect the animal’s evolution.
Cuttlefish (mostly in the genus Sepia) have tentacles like squids do, which shoot out and grab prey. Cuttlefish, though, are different from squids: their bodies are flat rather than mostly circular, and inside there is a hard “cuttlebone” instead of the flexible gladius that squids have.
Cuttlebones, which might be related to shells, are amazing. Pretty much every day, another thin layer is added to the cuttlebone, and the fluid in the previous layer is replaced with gas. The air in the mineral-rich structure gives the cuttlefish a lift (buoyancy) at night; around sunrise, the addition of a new layer helps the animal sink to the bottom where it hides during the day. It sounds like magic, but we think this is an ancient adaptation of cephalopods to make sure life went on swimmingly. It is also one that lets us identify fossil cephalopod shells.
A special squid
There is one “squid” called Spirula spirula, or the ram’s horn squid, which also has an internal, loosely coiled shell with septa; hardly anyone knows about it because it lives in the deep sea. Sometimes after a ram’s horn squid dies, its shell floats and washes up on beaches. We don’t know how this squid fits into the cephalopod tree of life, but it’s fun to imagine it as an animal unchanged over time patrolling modern oceans!