— Written by Maria Rivera
Collecting hundreds of kilograms of sediment samples is challenging but finding microscopic shark scales in the midst of hundreds of sand grains is another thing entirely. In the process of extracting denticles from large sediment samples, several steps such as lab processing, many gallons of acetic acid, and hundreds of hours of microscope time are required. First of all, we need to collect the samples (bulk of sediments), from a region with high and low shark abundance to be able to make comparisons. For example, with my project on a small island in the Caribbean called Curaçao, there are samples from two sites. One site has high human impacts while the other has less human impacts. A potential hypothesis might be that the more impacted site might have less sharks and therefore less denticles.
Once the samples are collected, we then weigh each sample in the lab. These samples are then separated into different sized fragments using sieves with various mesh sizes (2 mm, 500µm (micrometers), 250µm, 106µm, and 63µm). The denticles, which are the size of several strands of hair, are found in the 106, 250, and 500 µm fractions. The bulkiness of these samples would make it nearly impossible to pick through, like finding a needle in a hay stack, thus, we add 10% acetic acid to reduce the size of the mass significantly. This gets rid of all the carbonate (coral and shell) material, which compromises over 90% of the samples’ mass. We also have to rinse the sample with hydrogen peroxide to get rid of any excess organic material. All that is left is the non-organic and non-carbonate residue that the denticles are found in. Now it is feasible to pick through the different sized grains and identify our denticles under a microscope. Once we identify our denticles in a sample, we measure them and look at the ridges and how they are shaped. This helps us determine the type of shark form which it was shed. When we identify the denticles, we can plot our data and analyze/make inferences about how shark communities vary across different human interference sites. Going back to Curaçao, once we have all the data, we would be able to analyze what types of sharks roam each site and their relative abundance, as well as how that has changed over time. This will shed a light to how humans have impacted the coral reef shark communities in both areas.
It is important, however, to realize that a number of factors can impact the number of denticles we find in a sample. For example, denticle abundance can be affected by denticle shedding rates, transportation, and coral reef growth and sedimentation rates. In regards to shedding rates, we do not know how often sharks shed denticles or whether shedding rates differ between pelagic and demersal species, and this can impact how many denticles we find in a sample. When it comes to transportation, denticles can be transported horizontally by currents as they sink. However, our collection sites are located on very sheltered, low energy reefs, and modeling exercises have shown that sinking denticles do not travel very far. With coral growth, if a coral reef is growing at a fast rate, denticles would be deposited and then buried at a quicker rate. As a result, denticle densities per amount sediment would be lower at faster accumulating reefs. To correct for this, we date corals to determine how much time is encapsulated in each sediment sample. These are just some of the few things that we need to consider when we are analyzing denticle abundance and trying to relate that to shark abundance.