What’s in a sawfish tooth?

I would guess that the average person might say, ‘there is nothing inside a sawfish rostral tooth’, after seeing and handling one. They are hard, often very sharp, and relatively heavy. They look solid and don’t appear to be holding any secrets. Still, my students and I spent lots of time this summer looking at what is inside these teeth. Why?

What might be inside this massive sawfish rostral tooth from Venezuela? Photo © Jens Hegg

Sawfish teeth certainly hide immense amounts of information in their chemistry, as you might have read in our last blog post. But, if that were the answer to the question “Is there anything inside a sawfish rostral tooth?” that would feel a bit like a trick question. Sawfish rostral teeth actually have quite a bit going on inside them. They are full of little tubes, whose structure might tell us something about their size and age.

Thin sections of rostral teeth appear to have a tree-like patterns of small “branch” tubules coming from a larger central tube, as shown in this tracing of a microscope image. Image © Jens Hegg

One important question in sawfish research is how to determine the age of a sawfish from it’s size. If this sounds pretty basic you are right, size-at-age models are one of the most basic tools of a fish biologist. But, the models of largetooth sawfish size-at-age for populations around the world are not that precise, especially for larger fish. This is because many of the populations crashed before detailed studies had been done. The most precise way to know the age of a sawfish is to look at the rings in it’s vertebra…but that is obviously not a solution since it would mean killing the fish. We need all the sawfish we can get! That’s where the inside of rostral teeth come in. They might hold information about the age and size of sawfish.

The tubes inside a sawfish rostral tooth are organized in a shape that looks a little like a Christmas tree. They grow from the bottom, so you can think of these tubes like a stack of cups, with new cups added at the bottom as the fish grows. This pattern is clear under a microscope, but the curve of the tooth and the 3-dimensional branching pattern make it hard to quantify in a thin section of tooth. So, we set out to look inside each tooth in 3D to see what we could find.

We spent ten days at Friday Harbor Labs on San Juan Island, WA scanning rostral teeth with a micro-CT scanner. This reconstructs the teeth in 3 dimensions, with super high resolution so we can see the true structure of the tubes inside. The results were surprising and the images were quite beautiful in their highly organized repeating pattern of tubules.

The tubules in a micro-CT cross-section of a largetooth sawfish rostral tooth are highly organized. Tubules moving out from the large, central tube the tubules are arranged in an alternating pattern that appears to form rings. These rings increase along the tooth, fewer when the fish is young, more as it gets older. Image © Jens Hegg

Each rostral tooth has a single, large, central tube that goes from tip to base. From that, smaller tubules branch out like we see under the microscope. But, if you cut this 3D image from the side you see something interesting. While the smaller “branch” tubules look sort of unorganized under the microscope, they are highly organized in micro-CT cross-section. As the tooth grows it appears to add new rings of “branch” tubules in very strict order.

We think that there is a relationship between the age of the sawfish and the number of these rings we can see in the tooth. Knowing how fast sawfish add a new ring of “branch” tubules might let us better calculate its age. And, since there are lots of preserved rostrum in the world, we might be able to improve the size-at-age models for sawfish without ever having to touch a living sawfish.

Running the microCT required lots of late nights. The tank of juvenile chimaera was always willing to greet us regardless of the hour. Video © Jens Hegg.