After a long series of flights over the course of an entire day, I found myself in Washington, D.C. It’s almost ironic how working abroad has actually brought me directly to the heart of American government itself and to a place where the institution I work for rings more bells in people’s minds. After spending the last five months in Panama, returning to the States and seeing the Stars and Stripes again sparked a resurgence of patriotism within me. Having never visited D.C. before, I wasn’t really sure what to expect, and frankly, now that I’m here I wish I had given myself more time to explore the city. Rather than meandering the streets and ogling historic buildings as a tourist, I’ve plunged right in to city life, joining the workers as they briskly walk their daily routes to and from the office, headphones in and consumed in their thoughts and the fast-paced lives they live. I love this routine and seem to fit right in. I’ve even been asked for directions. This city is a breath of fresh air. In some respects, I’d refer to it as the government’s college campus. Its layout, general air, momentous architecture, and work-oriented yet hip culture bring me back to my time at Stanford. For that alone, my experience here has been incredible.
But now, why am I here? I’ve come to the nation’s capital to sample the Smithsonian Institution National Museum of Natural History’s ichthyology collections and excise pieces of shark skin for inclusion in my dermal denticle reference collection. I’ve encountered a great diversity of denticle morphologies in my sediment samples. While the number of denticles I find per kilogram sediment provides insight into the relative abundance of sharks in the past vs. today, examining and categorizing denticle morphology will hopefully reveal whether shark communities have shifted over time. I’ve introduced the previously-documented denticle functional morphotypes in my last two blog posts, but now I intend to not only refine my denticle classifications but to take them one step further. My next challenge is to identify the denticles to taxonomic family. This is easier said than done, as denticles are known to vary across species and families as well as across the body of an individual shark. The key is to find common themes and patterns in denticle morphology amidst all this variation, which is where the reference collection comes into play.
Most studies of denticle morphology examine entire pieces of skin with the denticles still intact and overlapping. However, I encounter single, isolated denticles in my sediment samples. In order to compare apples to apples so to speak, I am constructing a reference collection of individual denticles from standardized locations across the body of sharks from families known to inhabit my study areas. By capturing this variation in the form of individual denticles, I can get closer to producing a set of criteria by which to group the denticles taxonomically. This secondary set of classifications will accompany and reinforce the picture painted by the functional morphotype data.
But first, this involves a little fishing, although not in the traditional sense. Isolating the denticles requires collecting skin samples, and to collect these skin samples, I need to locate the proper preserved sharks. These are stewing in massive tanks of ethanol in a pod within the Smithsonian’s Museum Support Center in Maryland. For those of you who read Dan Brown’s The Lost Symbol, this is the real Pod 5. Each metal tank contains between 10 and 40 sharks, ranging from around 30cm to over a meter in length, submerged in a semi-murky bath of 75% ethanol. This is where you must be prepared to “fish.” Donning thick, black rubber gloves that slid up to my armpits and a heavy apron, I would reach down into each tank to find the species or specific catalog number of shark that I needed for my reference collection. In most cases, I wanted to find the largest shark of each species on my list since denticles are also known to vary with ontogeny. This broadened my search since I wasn’t targeting specific catalog numbers. However, some sharks were more elusive than others. Sometimes the one I wanted would be right on top, but most of the time I ended up removing most of the sharks from a given tank before finding the one I needed. This literally left me with a pile of dead, preserved sharks balanced on a large plastic tray, ready to be re-submerged in ethanol, and my prized shark of interest sitting alone on a separate tray, ready to be sampled. While this was a time-consuming activity, I luckily had help. It was also fun to look at all of the different sharks in each tank as I pulled them out one by one… or maybe that was just the ethanol fumes getting to me.
Tank Room, Pod 5, Smithsonian Museum Support Center
Preserved sharks soaking in a bath of ethanol in a tank
Upon finding the shark that I wanted to sample, I transferred it to another large plastic tray, measured its total length, took a picture, and then covered it with ethanol-soaked cheesecloth to prevent it from drying out. Then, scalpel in hand, I would cut small squares of skin from the body and fins. The layer of skin was so thin on the fins that I found it easier to just “shave” off a small piece. I guess it was a bit like peeling a vegetable to an extent. Each piece of skin was placed with a corresponding label into a separate plastic bag called a Whirl-pak to be brought back to Panama for processing, analysis, and maintenance. After sampling each shark, I would return it to the tank of ethanol, making sure that it and all of its compatriots were fully submerged. This became a three-dimensional shark jigsaw puzzle when the ethanol level was low and there were a lot of specimens in the tank, as bits of the fins or head of the sharks would occasionally refuse to fit completely beneath the surface. In some cases, a lot of rearranging was required.
My sampling station
Carcharhinus limbatus, ready to be sampled
Over the course of four days at the facility, I sampled 40 individual sharks, representing 16 different families, for a grand total of nearly 160 skin samples. Most of the actual dissections went pretty quickly once the shark was located, with the exception of one family: the nurse sharks (Ginglymostomatidae).
Nurse sharks are bottom-dwelling reef sharks and are one of the most common sharks remaining in the Caribbean, as they are of low commercial and nutritional value, making them one of the last sharks to decline in the face of anthropogenic stressors. Due to their demersal nature, they have large, thick “abrasion strength-type” denticles that resemble little stones. This I had read in the scientific literature. What I wasn’t aware of was that sampling their skin would be like attempting to slice through armor attached to beef jerky. Good thing I brought extra scalpel blades with me because I only two I broke during my visit where while sampling this family. My surgery-level scissors also met their match and could hardly cut through these sharks’ sturdy exterior. In comparison, they were rarely needed to sample the other families. This striking contrast certainly highlighted the difference between denticles in demersal vs. faster predatory sharks in my mind.
Ginglymostoma cirratum denticles, 80x magnification
All in all, the trip ran really smoothly, and I was able to sample all of the sharks on my list with a little time to spare. Everyone I interacted with at the museum was incredibly welcoming and organized, making sampling a pleasant and efficient endeavor. While I successfully sampled all of the material I need for the time being, I hope to get the opportunity to work more with the Smithsonian in D.C. in the future. However, now I have my work cut out for me in terms of processing and photographing all of the skin samples I excised to actually build and publish the reference collection. But first, I think I’ve earned a day of rapid-fire sightseeing in D.C. before I return to Panama.