When most people see these creatures, they’re not quite sure what to think. Are they ressurected dinosaur fossils, searching to ruin our beach days with zombified claws? Are they sent from another planet to spy on us while we swim? And most importantly…Do they bite? Those have always been my thoughts, at least.

Please Don’t Eat Me, Mrs. Crab.

Here at the field station, one of our jobs is to track the amount of horseshoe crabs that we have on the beach during their mating season. Our numbers get tallied up with those from a whole bunch of other field stations on a whole bunch of other shores throughout the country to help formulate a rough estimate of the total population.

Horseshoe crabs have been around in a similar form for about 400 million years. Their eggs provide a food source for many shorebirds, and each female can lay up to 80,000 of these life-sustaining meals. The birds’ populations fluctuate in relation to the crabs’, as less females around to lay eggs means less food to go around.

Humans present one of the biggest dangers to the horseshoe crab population. Fisheries use them as bait, and so without any sort of regulation, we’d be likely to lower the crab count enough to impact negatively the shorebird population as well. In an effort to combat the damage we cause, the Atlantic States Marine Fisheries Commission has set up harvest quotas for the animal which fishermen are not allowed to exceed. But how does the ASMFC know how large these quotas should be, or whether or not they’re having any effect?

That’s where we at the Field Station come in. We get sent out twice a day to walk the shore and count how many crabs we can see within a 5 meter range from the beach.

Horseshoe crabs come to the shallows in order to mate. It’s important for us to record whether we’ve found a crab of a certain gender on its own, or whether we’ve caught two of them in the act.
When we see a crab on its own, there are a few ways to discover its gender:

• Flip it over, and see if the crab has “boxing-glove” covered feet, or thin feet. Males possess the boxing gloves, while females have more dainty legs.
• Judge by size. Males are generally smaller than females.
• Judge by behavior. Males will usually try to crawl towards you, while you often find females burrowing themselves into the ground.

Whenever two crabs are seen right next to each other, or rather, on top of one another, we can assume that one male and one female are taking part and add a tally to each column.

Most of the crabs I’ve seen have been in pairs, which I was at first quite thankful for on account of their appearance. Despite assurances that they were harmless as long as you stay away from the tail, I couldn’t help the slight suspicion that flipping them over would end up with a trip to the hospital, with one of the lamest injury stories of all time. When it came to be that time, however, and I had to check the legs, I returned pleasantly unscathed. I’m now certainly more comfortable with the idea that they’re probably not out for blood, but I still think they look weird.

Jake Daniels

Read more about Horseshoe Crabs: http://www.yesterdaysisland.com/2011/science/5.php