422. Horseshoe Crab Tracking

February 3, 2015

In June, 2003, I became a tracker of horseshoe crabs in Taunton Bay, where they are at the northern limit of their global range. Friends of Taunton Bay had a grant from the State Planning Office to do a one-year pilot project in bay management. The tracking effort was part of an assessment to provide background for that study.

I am a lifetime member of Friends of Taunton Bay, a nonprofit group keeping an eye on the bay through a variety of monitoring programs, starting in 1990. We partnered with Maine’s Department of Marine Resources in attaching sonar transmitters to thirteen crabs in each of two sub-embayments.

We were trying to figure out the horseshoe crabs’ annual patterns of movements, and whether or not they left the bay in the winter months for warmer waters in the Gulf of Maine, which it was generally believed they did.

 

Sonar Transmitter

Attaching Sonar Transmitter to Horseshoe Crab, June, 2003

My job was to track those twenty-six crabs with a sonar receiver carried about in a small boat. When the signal in my earphones from a particular crab was as loud as I could make it by fine-tuning my maneuvers, I marked my GPS (geographic positioning system) coordinates on a chart, figuring I was directly over that crab so my position was also its position as viewed from overhead.

Horseshoe crabs come ashore only during a two-week breeding season in the spring, so it’s no surprise that I saw only one during the two-and-a-half years I was tracking crabs from late April through late November. I judged that one to be directly under the boat when it was lodged against shoreline rocks; I backed off a few feet—and there it was with its mate, blue sonar transmitter epoxied to its prosoma (the forward part of its shell).

 

Horseshoe crabs

Gravel shore lined with mating horseshoe crabs.

As the tracking effort turned out, horseshoe crabs in Taunton Bay stay in the bay year-round, burying themselves in the mud for half the year during colder months. They rouse in late April, and immediately take off upslope from their over-wintering sites.

Not one of the crabs we were studying left its native embayment; there was no evident bridge between the two distinct populations that were separated by a distance of only about two miles. The channel bearing cold water into the bay from Frenchman Bay passes by a particular point of land that leaves no room for a warmer passage between the two shallower habitat sites.

 

Female horseshoe crab with male attached

Female horseshoe crab digs a nest for her eggs.

The movements of the crabs appeared almost random, but when females began giving off pheromones during the breeding weeks, males and females got together on their traditional breeding shores, males clasping females with a foremost pair of legs suited to that task, females navigating for both of them, making trials at digging suitable nest sites in sandy bottom soils, moving on if it didn’t work out, typically laying eggs in several sites in a row once it did.

 

Mating horseshoe crabs.

Mating horseshoe crabs.

I will write more specifically about my engagement with the crabs in the following post.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: