One of the more unusual cephalopods of my acquaintance (and I do not say this lightly) is the ram's horn squid, Spirula spirula. The species is named for its beautifully coiled internal shell, which is all most people (including me) have ever seen of it.

by Fritz Geller-Grimm

I wrote about these strange little fellows for Nautilus Night 2009. The occasion for revisiting them now is an article by Danielle Wright in the New Zealand Herald that mentions the prevalence of these shells on Whatipu Beach:

The iron-rich sand is black against the white of the spirula spirula, or ram's horn, shells we're collecting from Whatipu Beach - except they're not shells, rather the backbone of a small deepsea squid.

Cephalopods are invertebrates and do not have backbones. Spirula is no exception. However, there is a legitimate comparison to be made between the internal shell of most squid, called a pen, and a backbone. Both pen and backbone give the body shape and give the muscles something to push and work against.


Pen, or gladius, of a Humboldt squid.

However, it's obvious just from the shape of the spirula shell that it can't play the same role as a pen. In fact, what it looks the most similar to is the external shell of a nautilus.


Chris 73 / Wikimedia Commons

Indeed, it is functionally very similar. That beautiful, delicate little "ram's horn" contains chambers full of gas, just like a nautilus shell, and both animals can control the concentration of gas to alter their buoyancy. Also like the nautilus, spirula lives in the deep sea during the day, and rises to the surface at night.

Back in the Herald, Wright continues,

When the squid dies, the buoyant "shell" floats to the surface and is washed up on beaches after storms. It shows the cruel nature of the water here that there are so many of these rare squid remains washed up.

It's nice and poetic and all, but the connection's a little iffy. Wikipedia reminds us,

Most sources cite this species [Spirula] as tropical, and they are observed to be plentiful in the seas around the Canary Islands. However, significant quantities of shells from dead Spirula are washed ashore even in temperate regions, such as the western coasts of South Africa and New Zealand. Because of the great buoyancy of the shells, these may possibly have been carried long distances by ocean currents.

I don't think anyone knows if spirula really lives near New Zealand, or if the shells are swept there from more tropical waters. The abundance of shells on Whatipu may illustrate more the "connectivity of ocean currents" than the "cruel nature" of the local water.

Wikipedia also gave me a huge smile with this line:

Much of the organism's life history has not been observed; for instance, they are thought to spawn in winter in deeper water, yet no spawnlings have been directly seen.

SPAWNLINGS!

Part of me wants to edit the article to say "hatchlings," which is actually a word, but another part of me thinks "spawnlings" is awesome and should stay. Thought, o gentle readers?