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
During the full and new moons, high tides are higher than usual, and low tides are lower than usual. These spring tides, so called not because of the season but derived from the German word springen, “to leap up,” are the most favorable for mating horseshoe crabs. The females lumber ashore, males desperately clasped to their backs with specially modified claws, buoyed by the extreme high tide. The females dig a nest and lay their eggs, the males do their part, and the crabs go their separate ways as the tide recedes.
Since the beginning of March, I have been going out into the estuaries with a fellow graduate student, Alison. For a few hours around midnight, every two weeks during the full and new moons, we are ecological voyeurs, collecting females for a fecundity study that will be a minor part of my doctoral dissertation and a major part of Alison’s masters thesis. The water has been cold this winter, so everything has been delayed. Each time we went out, we were so hopeful, telling each other, “They’ll be there this time” as we slogged through waist-deep water in waders, carrying flashlights. Any dark shadow of approximately a foot in diameter slowed us down for a closer look, but all we found were rocks, patches of seaweed, and deceptive bathymetric contours.
On the day before the May 1st full moon, the sky was crisply blue and the temperature had finally climbed above 60 degrees, and it seemed ludicrous to waste a good field day waiting for midnight to come. I suggested this to Alison, and she agreed, so we approached our advisor. He smirked, and asked, “What does the literature say? Are they around during the day?” Well, the literature says that they are somewhat more copious at night, but I reasoned that during the day our visibility is greatly increased, so although there might not be more total crabs present, we would see more of the ones that came out. Despite the fact that “horseshoe crabs breed during the nighttime, full moon, high tide” has become a mantra, it isn’t necessarily fixed. I’ve seen them during the day, during other moon phases. Everyone seemed to agree that we might as well try.
There is one place within one of the estuaries that is highly conducive to mating. The shore gently slopes and is surrounded by protective salt marsh grasses. If the crabs were out at all, they would certainly be along this kilometer of shoreline. Alison and I pulled on our waders next to the car, trying out best to minimize the transfer of thick marsh mud into our boots. We collected our equipment from the back of my jeep, and headed toward the water across the wet spongy ground. If the fields that get the most attention, like genetics and molecular biology, are the James Bonds of biology, then ecology is certainly MacGyver. We may not have million-dollar sequencers, but we can get an entire season of study done with only a laundry basket, a ruler, a thermometer, and a transect tape.
Once we reached the waters edge we walked in until the water was up to mid-thigh. We saw the first crab after five minutes of slowly moving up the edge of the marsh, scanning for dark objects. I reached down into the water, which was surprisingly warm (61 degrees Fahrenheit… there’s that handy thermometer), and tucked my fingers inside the rounded front edge of its shell. Flipping it over, we noted it was a male, with rounded “boxer” claws. They really do look like tiny boxing gloves. We returned him to the water and he hurried away.
We continued to find a few males here and there, but no females, until I moved up into a sandy depression on the marsh edge that teems with crabs during the peak of spawning. She was slowly making her way back to the deeper water, a male still clasped onto her. I lifted them both, and in her struggle she shrugged off her suitor, whose genital pores continued to ooze. He was returned to the water, and she was placed in the laundry basket, claws scrabbling against the plastic, tail poking out of one of the holes in the side. Despite an hour of searching, she was the only female we found. It makes you feel worse when you don’t get to see the ones that live. The only female of the day, and she wasn’t coming back home.
Back at the lab, in a small room that tends to be too warm, and smells like burning grass because it also houses a drying oven, we arranged our supplies. The first step was to take a blood sample for future genetic analysis. I wiped the membrane between the two parts of her carapace with alcohol, and inserted a needle, which popped through the gristly material and yielded a syringe filled with pale blue blood. Horseshoe crab blood contains copper-based hemocyanin, not iron-based hemoglobin like ours, so just as pennies turn blue-green and old nails turn rusty red when oxidized, so do our respective bloods. Once this was complete, it was time to kill her. Horseshoe crabs don’t have a distinct nerve center that can be disconnected, like a frog in a high school biology lab. You can’t pith them. Alison and I concluded that the best option would be to drain its blood, despite the fact that these crabs can have 30% of their blood volume removed to minor ill effect. We cut across that same tough membrane and the blood poured out. Pumping the crab by folding it in the same way it does naturally to protect itself caused more blood to squirt into the sink in pulses. After some time, the blue blood stopped, and reddish-brown bits of tissue started to ooze out. I started to get dizzy, which embarrassed me because I like to think that I have a strong stomach. I excused myself to go get some tools in the main lab.
When I returned, the crab was in a shallow plastic tub, and was walking around as if little had occurred. It seemed to slowly weaken, but both Alison and I were unnerved. It finally slowed to what seemed to be simply twitching, like when an insect is crushed and its legs keep walking. We looked at each other, shrugged and nodded, and started up the Dremel cutter. Alison started at one corner of the carapace, cutting an arc along the edge. The cutting disk was not quite sharp enough for the tough chitin, and the motor strained until the disk shattered. Eggs began to leak out of the cut we had managed to make. Suddenly, the crab lifted itself up onto its legs and walked. “Oh, God … it’s alive,” I said.
“Are you sure?”
“Well… I think so.”
“Let’s put it in the freezer and maybe it will die. We’ll deal with it tomorrow.”
The next day we retrieved our crab from the freezer. It thawed out slightly and started to move. Alison consulted with some people at the Marine Resources Center and was told to use clove oil, which can act as a temporary or permanent anesthetic depending on dosage. We put the crab in a tub filled with salt water and 5 times the concentration of clove oil required to kill a fish. It was still moving after an hour. So, it had survived 24 hours of being out of the water, total blood loss, being partially sawed open, freezing, and an overdose of anesthetic.
It eventually stopped moving a few hours later, and we finished sawing it open with a much better tool. We scooped out, rinsed, and measured the eggs. Such effort for a single crab’s eggs. Hopefully we can perfect our methods soon, because I can’t handle another extended death.
SUGGESTED READSJournal Entries from Sara Grady, Who is Studying Horseshoe Crabs on Cape Cod: November 5
by Sara Grady (11/5/2003)
Journal Entries from Sara Grady, Who is Studying Horseshoe Crabs on Cape Cod: May 20
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Journal Entries from Sara Grady, Who is Studying Horseshoe Crabs on Cape Cod: May 28
by Sara Grady (5/28/2003)
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