How have shark communities shifted?

We’ve seen a wide diversity of denticle forms, both in the modern sediment and in the 7,000-year-old sediment. However, it does look like there’s been a shift in the forms that we’re seeing. Different types of sharks have different forms of denticles that have various functions. For instance, fast-swimming sharks have thin denticles with many narrowly spaced ridges, whereas bottom-dwelling sharks possess thicker, smoother denticles that form a protective armour. I’ve been working a lot on our reference collection right now to verify the existing descriptions of denticle types using measurable traits and to better understand the variation of denticles across different families of sharks. I’m also measuring the denticles to explore what other ecological information we may be able to glean from them. I then apply what I’ve learnt about these relationships between denticle form and shark ecology to the isolated denticles I find preserved in reef sediments. We’ve observed an increase in the relative abundance of thicker ‘abrasion strength’ denticles that are often found on nurse sharks and a decrease in thinner, ridged denticles that are often found on requiem and hammerhead sharks. This suggests that there has been a shift in the community of sharks: a decrease in the faster, predatory sharks and an increase (or no change) in nurse sharks. This makes sense, given that nurse sharks aren’t of very high commercial value. They are nocturnal bottom-dwellers, so they may also avoid being caught as by-catch by fishermen. It’s promising that we’re seeing plausible trends with this novel approach.

Can dermal denticles help us to understand questions about abundance or only relative abundance?

Right now, we’re looking at relative abundance; that is, whether there was relatively more of one group of sharks than another in the past as compared to the present. We can work this out by looking at denticle accumulation in the sediment. Sharks continually shed their denticles. These denticles are transported a little bit through the water column and they eventually settle on the substrate surface. At the same time, there is sedimentation and reef growth, so we look at the number of denticles per a given amount of sediment. We take bulk samples that are approximately 10 centimetres (four inches) deep from the substrate surface. Then we can use some of our existing sediment core data to understand how fast the reefs were growing at that time and approximately how much time that 10 centimetres represents. This can give you a relative measure of how many sharks there were based on the composition of denticles. We still need to investigate denticle shedding rates and how well the different denticle types are preserved, as well as compare the denticles we find in the sediments to modern surveys of the living shark communities in order to understand absolute abundance.

Do you have plans to expand the study to other sites?

In Dominican Republic there’s another really neat suite of fossil reefs that are between 7,000 and 9,000 years old. Visiting this site can show us whether the composition of pre-human communities of sharks varied over space as well. We hope to see the same trend at the two sites, but even if we don’t, that’s also going to paint a really interesting picture of how different locations even within the Caribbean had very different pristine shark communities. And we’re hoping to do something similar at Palmyra Atoll, in the Northern Line Islands. A collaborator of ours has collected sediment from Palmyra, which is considered to be one of the gold standards of ‘pristine’ in the Pacific.

What kind of interest have you had in your research?

I’ve talked to several people in Panama and the United States. Other researchers seem very excited about the potential of the tool and the prospect of developing it further, because the unique insight that it can give us into shark baselines is not really accessible by traditional survey methods. This information is locked in the past. Scientists have tried to use fishing logs, interviews and other anecdotal accounts. They have even looked at pottery, photos and historical illustrations to reconstruct the past, but these don’t always give you an empirical answer. So in that sense, fossilised denticles are really one of the few ways to understand quantitatively what healthy shark communities looked like in the past.