The first big experience I had as a marine biology student was being an intern as part of a scientific investigation at the Smithsonian Tropical Research Institute in Panama. I had the opportunity to help Erin Dillon with her project by processing sediment samples for dermal denticles. During the past three months, I learned a lot about shark dermal denticles’ morphology and physiology. I also practiced the methods to process marine sediment (washing, drying, digestion, peroxide, and picking) in order to collect the denticles from each sample. The skills I developed through this internship helped me comprehend what lab work is like, how long the procedure can be, and how patient a scientist has to be in order to collect data and get results. In the end, I gained priceless knowledge, and helping Erin was another experience that confirmed my passion for marine life.

-Yamilla Samara

What’s next for Yamilla?: Yamilla is now headed to Florida International University as a transfer student to continue pursuing her studies in marine biology. She’s very excited to take upper division classes, explore the multitude of topics within the field of marine science, and further refine her interests.

How do we move from the residue remaining after a round of acid digestions to an isolated denticle ready for measurement and identification? It’s not as easy as it sounds.

As I peered down through the ocular of my microscope at the diverse array of particles illuminated by the bright light above, I spotted countless sponge spicules (minuscule glassy spears that compose their porous skeletons), fish teeth, bone fragments, otoliths (fish ear bones), and the occasional strand of organic material or remnant chunk of calcium carbonate. These stood out against the black gridded background of my metal picking dish, recounting the history of the menagerie of creatures that left their mark on this particular patch of sediment before it was collected from the reef. Paintbrush in hand, I was ready to begin. However, I would be painting no masterpieces today.

I spread a small scoop – maybe equivalent to the size of a pinch of salt – of my sediment sample out over the area of the picking dish, trying to evenly distribute it and produce a single layer of particles. Now came the fun yet tedious part. I began to manually brush through the particles, visually scanning them with care in hopes that a denticle would catch my eye. Each time I found one, a little flurry of excitement would well up within me and I would suddenly feel like I had the endurance to pick for five more hours (as well as the inspiration to write a blog post). It’s a seemingly endless treasure hunt, but my tiny yet precious haul of denticles validates every minute of the search.

After working hard to collect and process the sediment, the first denticle has emerged! This is an ‘abrasion strength’ denticle extracted from the fossil reef at Cañón de Buho.

While sieving the bulk bags of sediment allows us to constrain the particle sizes that we eventually pick for denticles, we are still left with around 260kg of sediment to work with. This is an enormous task, perhaps impossible. However, with acetic acid, we can make it feasible. Our samples are largely composed of coral and shell (calcium carbonate). These react with acetic acid to produce water, CO₂, and calcium acetate, therefore eliminating the carbonate. This reaction can remove up to 99% of the sample weight, leaving behind just pieces of rock and resistant microfossils such as bone, teeth, denticles, and sponge spicules that we are interested in studying.

Our samples are so large that we’ve built a processing ‘factory’ outside the lab, where the sediment can react with acid and bubble off CO₂ without ventilation issues or space limitations. Each size fraction is placed in a five gallon bucket, and acid rinses (10% acetic acid) are added daily over the course of about a week and a half. A little over 300L of 100% acetic acid is required to complete the digestion of the 65 bulk bags we collected in the Dominican Republic. That’s a lot of acid! We can also process 39 size fractions, or 13 bulk bags, simultaneously, which is 11 bulk bags more than what I was able to digest in the past using just the fume hood in the lab. This has cut the total digestion time down by months.

While this step requires a large quantity of acid to complete, it is crucial, as it turns the task of finding a needle in a haystack into the task of finding a needle in a small sprinkling of hay. Denticles are sparse in the sediment samples, but digesting away the calcium carbonate makes it possible for us to find them and reveal their secrets.

Like a gold miner crouched on the bank of a stream, shoveling sediment into his pan and hoping for a golden glint to catch his eye, we stood at the edge of a washing table, placing scoops of reef sediment onto our stack of sieves. However, we’re interested in something far more minuscule. Our collection of five sieves, with mesh sizes ranging from 63µm to 2mm, helps narrow the search by partitioning the sediment by particle size. Large pieces of dead coral and shell remain on the top mesh screen, while the smaller particles fall through and are caught by the finer mesh. Denticles are approximately 100µm to a little over 1mm across, so we process the 106µm-2mm size fractions to find them, saving both time and acetic acid. The very largest (>2mm) and smallest (63-106µm) fractions are stored in the lab for future use.

With two people working simultaneously, each 8-10kg sediment sample takes around two hours to sieve. After sieving, each size fraction is placed on a separate tray and deposited in our ‘drying bubble’ for a day or two until completely dry. It is then either digested with acid or stored. With one to two samples sieved per day, our mountain of bulk bags is gradually dwindling.

After several days’ wait, our precious and much-anticipated delivery of calcium carbonate cargo arrived at the lab upon clearing customs in the Panama City airport. Months of work went into preparing the agreements for these samples to pass into the country, and they were completed just in the nick of time. Now we were off and running with phase two. We also welcomed two new lab members to the team: Yamilla Samara and Henbelk Hernandez.

First, we had to unpack all of the samples from the crates and remove the plastic bags that they had previously been packaged in, allowing them to dry completely. In particular, the sediments collected underwater were still a bit soggy. Left outside or in the lab, these large, damp bulk bags could take a couple of months to dry completely without extra help. To expedite the drying process, some were moved to the ‘drying bubble’ that we had constructed in advance before leaving for the field. The drying oven that we had previously used was not nearly big enough for all of these new samples, so we decided to make our own! The ‘drying bubble’ consists of two metal bookcases, two dehumidifiers, and a fan completely enclosed by plastic, creating a hot, dry, confined space to dry samples simultaneously in large batches. It’s essentially a walk-in closet for sediment, complete with a Velcro sealed door flap. And trust me, it gets hot working in there. Now, this first batch of wet bulk bags would be dry in a week.

After each bulk bag of sediment is completely dry, it is weighed and stored in preparation for the next step: sieving. Fortunately, most of our fossil sediments were already dry, so no time was lost.