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
Once again, it was time to go into the field to find females. This week we expected the crabs to have finally moved ashore, so we were going to check all three bays: Barnstable Harbor, Stage Harbor, and Pleasant Bay. It was a gorgeous spring day, so Alison and I were in good spirits. You may be thinking that it’s impossible to be in three places at once at high tide, but thanks to different bathymetries and locations, the high tides in the three estuaries are spaced about an hour apart. I always thought it was pretty consistent everywhere when I was growing up, but that’s because I was at a relatively straight, flat beach that opened out to the ocean. The more complex the path is that the water has to follow, the greater the discrepancy.
As Alison and I walked to the waters edge, we were accosted by two women in their seventies who had been staring at the bay with some degree of confusion. Apparently they thought the tide was especially high, and I explained that when the moon is full, the tides are higher. This full moon tide was going to be especially high because the moon was directly lined up with the earth. This was also the cause of that night’s eclipse. They seemed quite astonished. Then one said, “Ah. I thought it was because we’ve been having so much rain.”
Barnstable Harbor’s water temperature was still in the low 50s, so we were both wearing our waders. I don’t mind wearing them in the water, although there is a strange moon-walking sensation that comes along with the flotation of the neoprene on your legs combined with the weight of the attached boots. I hate walking on land in them, because the legs seem to be a few inches shorter than mine. This causes a peculiar clipped step that makes my legs ache after a while, especially when walking across unstable marsh. The water was beautiful. We didn’t see any horseshoe crabs, but we saw some herring, a few ctenophores (comb jellies), and Alison found a squid that had gone the wrong way and ended up stranded in the marsh grass.
Stage Harbor, while not as clear as Barnstable, was much warmer. We did manage to find a single pair of crabs, partially hidden under a dock, their paired tails giving away their position. Alison kept an eye on them while I climbed down a set of very slippery stairs to the water. Unfortunately, they were impossible to get because they were in water just below chest height, and if I bent down to get them, my waders would flood. We decided to leave them, since they were the only ones around anyway.
When we reached Pleasant Bay, the tide was so high that the marsh was almost entirely flooded. Before we even reached the water’s edge, we could see the grass undulating with movement that couldn’t be attributed to wind or waves. The horseshoe crabs were everywhere. It was nothing short of an orgy, as female crabs were attended by chains and piles of males. Usually one male manages to clasp on, while other satellite males linger to try and break in on the action. In some cases, there’s no female at all, just a collection of males grabbing at each other’s carapaces because they’re so eager to mate. They even try to mate with your boots. We managed to collect small, medium, and large females for our study, measured, weighed, and labeled them, and brought them back to the lab. There we put them in seawater with a high concentration of clove oil and left them until the next morning.
Much to our relief, we discovered that clove oil does “permanently anesthetize” the crabs. We began dissecting them together, but soon realized that if we split up the egg removal and sorting, things would go much faster. I wanted to finish as soon as possible because it was also my birthday, and I wanted to spend more of it doing birthday-appropriate things. Alison took charge of the removal because she is better with the saw. She would remove the eggs along with some other gut material to which the eggs have a tendency to stick, place it all in a large yogurt container, and bring it to me in the lab, at which point I would sort the eggs from the rest of it.
When the container arrived at the lab bench, it was filled with all sorts of tissues along with the eggs, and there was a layer of cloudy water on the surface. First, I manually sorted out the large pieces of tissue, massaging them between gloved fingertips to work out any clinging eggs and break the tissue up into smaller pieces. This formed a slurry, which I put into a large beaker along with about half a liter of seawater. A magnetic stir rod spun it rapidly, and when the stirring stopped, the dense eggs settled to the bottom and the tissue bits floated to the top. Separating the grain from the chaff, literally and figuratively, I poured the tissue off into a waste strainer, and repeated the process. After a few iterations, very little tissue remained and I had a beakerful of eggs, which look like glistening, green couscous. Although they don’t smell particularly good, and have a tendency to stick to everything so I occasionally heard a subtle pop as an egg stuck to me, my tools, or the table ruptured, seeing them cleaned up in a beaker like that is very pleasant. I can’t say I’d choose it over other birthday activities, but it is certainly enjoyable in its own way.
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