— Written by Maria Rivera

How can we know how many sharks there are in a specific area? We cannot track or observe them as easily as other animals, especially land animals. This is a major issue because we do not have sufficient data that tells us about shark communities in the past nor present. Without this knowledge, we cannot analyze how humans have impacted shark communities near reefs. Fortunately, there are new techniques being developed that help us measure the relative abundance of sharks near coral reefs. One of these new techniques involves analyzing dermal denticles. Dermal denticles are the microscopic tooth-like scales that cover sharks’ bodies. Sharks shed these denticles, and they are deposited and preserved in coral reef sediments. Hundreds of denticles can be preserved in the sand, recording a timeline of what shark communities where like in the past and how they have changed.

Fig. 1 shows different types of denticles that belong to different kinds of sharks. Denticle A has high ridges for swimming, while denticle B is smooth and thick for protection. Photographs courtesy of Natalie Minouei taken on July 6, 2018.

How do denticles help us estimate/determine the relative abundance of sharks? By comparing the denticles we find in sediments from different regions and time periods, we can qualitatively estimate the abundance of sharks. Not only do the number of denticles help us interpret the abundance of sharks near reefs, but we can also use the wide array of denticle shapes and sizes to learn more about the ecology of sharks. Variation in denticle morphology serves different purposes. For example, thin denticles with high ridges are for drag reduction and allow the shark to swim faster (Fig. 1). These denticle characteristics are common with pelagic sharks that spend a majority of the time in open waters and are fast swimmers, like great whites for example. In contrast, there are denticles that can be smooth and thick for abrasion strength, this provides protection to the shark from the ocean floor (Fig. 1). Denticles like these would be found on sharks that live near reefs, such as nurse sharks. Variations of denticles for different sharks gives us an insight into the structure and composition of the shark communities roaming that reef.

We can also look at denticle distributions in areas across gradients of human impacts to investigate how shark communities are affected by human activities. For example, if there is a lot of fishing in an area, there might be lower numbers of sharks and thus lower number of denticles. If we compare that to a place that is less impacted by humans, we would expect to see more sharks and more denticles. This can help us understand how humans have impacted the marine ecosystem through time and in a context of local baselines. This is a great way to analyze the relative abundance of sharks near coral reefs when there are limited tools and scientists cannot go physically count sharks one by one in the ocean.