Journal Entries from Sara Grady, Who is Studying Horseshoe Crabs
on Cape Cod
Sara Grady is pursuing her doctorate at the Boston University Marine Program in Woods Hole, MA. For the past two years, she has studied horseshoe crabs on Cape Cod. This summer, Sara will be conducting surveys of two estuaries in the Cape Cod region — Barnstable Harbor and Stage Harbor — as well as a fecundity study in Pleasant Bay and the aforementioned estuaries. She will try to update this journal every week.
BY SARA GRADY
Most of the horseshoe crabs one sees at the beach or in the water are adults. Of course, there are juvenile horseshoe crabs around, but they are harder to find. As part of my thesis — as well as a project for my summer intern, Chuck — we decided to find and measure juveniles and get an estimate of abundance and age structure, similar to the plan for the adult survey.
Because they don’t molt as often, adults have the time to get scuffed up and collect epibionts — anything that makes the crab’s shell its home. These include algae, slipper shells (with the excellent Latin name Crepidula fornicata referring to the orgiastic stacks it forms… if it’s possible to imagine anything so sessile coming close to being orgiastic), barnacles, and the Limulus leech (Bdelloura), which isn’t a leech at all but a flatworm. Juvenile horseshoe crabs lack these living accessories and thus are more aesthetically pleasing, as seems to be the case with most young animals anyway. Of course, they aren’t adorable or cute, really, except that they are miniature versions of the adult, and that’s somehow endearing. Not quite like a puppy or kitten, but that also makes them not nearly as cloying as small fuzzy creatures. They have smooth shells and produce something cryptically called “exudate,” which prevents things from sticking to them and gives them the same sort of lubricated feeling as a supermarket cucumber. They feel slippery, but nothing gets on your hands. Their spines are a lot sharper than an adult’s because they molt before they have a chance to wear down. They are difficult to see because their shells are the same shade of tan as the surrounding sand, and they are usually feeding and thus partially buried. As they plow along eating worms, amphipods, and small particles of organic matter from the sediment, they leave haphazard winding trails in the sand, and it is easiest to find them if you first find the trails and follow them to the end where the crab should be sitting, behind a pressure front of sand.
It all sounds so simple. In fact, the students who did horseshoe crab research in our lab a few years prior told me that once you find one, you’ll find many. They must have been working in an area of very high abundance, though, because Chuck and I were not nearly as lucky. We went out for two straight days, and found only five juveniles.
On the first day, we were on a sand and mud flat in Barnstable Harbor, dutifully brushing our hands over the surface of the ground to feel for telltale spikes. It takes surprisingly long to carefully comb through one quadrant — a two-meter by two-meter square. When we saw winding trails, we would drop to our knees, only to find a mud snail or isopod grazing instead of our quarry. We would stand up, a bit defeated, mud dripping off our reddened knees, and walk further along the shore. While we were walking, searching for sand that looked more fruitful, an older woman hailed us. “Hey kids? You do know that you can only clam on Mondays, Wednesdays, and Saturdays, and even then you have to have a permit?” I was put off by how she addressed us because it was a bit condescending. Chuck is 21 and I’m 24, so we’re not exactly kids. However, it’s best to befriend every beachfront property-owning citizen we can, so I forgave the infraction.
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