Our curators and researchers worked together with colleagues from NIWA and the Australian Museum to solve this century-old mystery. Rodrigo Salvador takes us through what they found.
If you ever went for a walk on a beach, chances are you’ve come across cuttlefish bones. Bird pet owners are also familiar with those because they are used as a calcium supplement.
Cuttlefish bones, or cuttlebones for short, can be found in all species belonging to the family Sepiidae. This family, known as cuttlefishes, belong to the animal group known as cephalopods, together with squids, octopuses and nautiluses.
But – spoiler alert – cuttlebones are not actually bones. They are nothing like the bones in our skeleton. In fact, they are completely different from real bones in all regards: chemical composition, microscopic structure, and the process that form them. Though they do have a similar function.
Cuttlebones are complex structures that can be divided into two parts: a mineralized chambered section that forms the bulk of the cuttlebone, and an organic dorsal shield.
The chambered portion is made up of calcium carbonate, like snail shells; it is hard but brittle. The dorsal shield is made up of chitin (the same substance found in insects’ exoskeleton) and proteins.
The cuttlebone act as support for the animal’s body, as you would expect from a bone. In addition, they also function as a buoyancy device that helps cuttlefish control their depth in the sea.
That’s great and all, but what about those cuttlebones on the beach?
Naturally, cuttlebones are the remains of dead cuttlefish. They tend to get washed up on beaches all around the world.
It is easier to find them in areas where the cuttlefish live, but sometimes cuttlebones can be carried around for quite long distances to places where no cuttlefishes dwell. Say, across the Atlantic from Europe to the Americas, or across the Tasman from Australia to New Zealand.
That’s right, there are no cuttlefish living in New Zealand, though we often find their cuttlebones on the shore. They have been carried all the way from Australia.
However, the species washing up here could never be identified with much certainty. That is because it’s not easy to identify a species based only on the features observed on the cuttlebones. It is much easier to identify the species when you can observe its whole body.
Besides, the cuttlebones that arrive in New Zealand are typically all damaged and broken, which makes things even more difficult.
In comes the DNA
But, we were very curious and just had to figure this out. So we decided to try a different approach to identify the species: DNA.
Typically, you’d need fresh soft tissue or fluids to extract DNA from. But mineralized tissues, like actual bones, also have small quantities of DNA. So we decided to try and figure out whether the cuttlebones had DNA.
We did this at the ancient-DNA lab at Te Papa and found out that yes, the dorsal shield has enough DNA to be of use to us. The carbonate portion of the cuttlebone, however, doesn’t seem to have enough DNA left.
Using a section of the DNA known as ‘barcode’, we could identify the most prevalent species found on New Zealand beaches: the Australian giant cuttlefish Sepia apama.
But that was not all those cuttlebones had to tell us.
You might have noticed from the cuttlebone photo above (specimen NMNZ M.018735 from our collection) that there are several weird marks on it. Punctures, cuts, scratches, you name it. What are they?
Well, we went full CSI on them. We did some detective work trying to match those marks to potential ‘culprits’. We used skulls of several animals that we have in Te Papa’s collections to produce artificial ‘bite marks’ in a mass of blu-tack. Then, we matched the marks on the cuttlebones with the ones on the blu-tack, leading us to the perpetrators.
The cuttlefish eaters
We determined there are 4 types of marks left on the cuttlebones by other animals, either predators or scavengers.
What we called Type 1 are triangular marks that look like a stylized letter A. They are caused by the beak of large seabirds like the white-capped albatross Thalassarche cauta.
The blade-like margins of their beaks also create what we called Type 2 marks: straight and narrow cuts. These marks are caused by birds feeding on dead cuttlefish that float on the surface of the sea.
Marks Type 3 and 4 are left by teeth. Type 3, an arc of long and narrow punctures, is caused by sharks. In particular, the teeth of blue sharks (Prionace glauca), bronze whalers (Carcharhinus brachyurus) and school sharks (Galeorhinus galeus) are very good matches to the marks we observed on our cuttlebones.
Type 4 is a line of dot-like punctures and is caused by dolphins such as Delphinus delphis. Sharks can either hunt live cuttlefish or feed on dead ones, but dolphins most likely feed on live cuttlefish.
This study was published in the New Zealand Journal of Marine and Freshwater Research, you can read it here: CSI – Cuttlefish Sepion Investigation: overview of cuttlebones found on Aotearoa New Zealand shores and analysis of predation and scavenging marks. It was a collaboration between three of our curators/researchers (Rodrigo Salvador, Lara Shepherd and Alan Tennyson), Amanda Reid from the Australian Museum, and Diana Macpherson from NIWA.
If you want to learn more about the cuttlefish in Australia, the book Cephalopods of Australia and Sub-Antarctic Territories, by our collaborator Amanda Reid, is your first stop. And if you want a good intro to all cephalopods of the world, cuttlefish or otherwise, your best bet is the book Octopus, Squid, and Cuttlefish from the University of Chicago Press.