What a difference a month makes. Back in April I brought my marine biology class to the Outer Banks of North Carolina for our end of semester field trip. We saw lots of surf clam shells on the beach then, and some arks, but not a lot of other diversity in shells. And the month before on a spring break trip the beaches were covered in purse crabs. But this week it’s clear that mole crabs are in season, as the beach is littered in little Emerita molts.
Adult mole crabs start mating in early spring and go through to fall. Like other arthropods, males copulate with females and fertilize eggs internally. The females then hold the developing embryos under their abdomens for 2-3 weeks (see image below), after which the larvae leave the females and live on their own in near shore areas. The larvae then leave the water column to settle back into the surf zone in June/July and September/October, where the juveniles and adults feed on small plankton and detritus in the swash zone of the beach. The molts I have been finding on the beach this week are all about 2 cm in length, and may be from actively mating crabs. We did find some crabs in the swash zone back in April, but not this large number of molts. Are the females molting prior to mating, like in blue crabs?
A very interesting study examined whether female mole crabs time the release of their larvae. They found that the larvae do in fact leave the females in quick 5-15 minute bursts just after it becomes dark. What is really interesting is that the larvae themselves control this timing, as the rhythmicity is also seen in egg masses that have been removed from the females. And this rhythm continues in constant darkness, showing that it is due to some internal clock in the embryos, not simply a response to darkness.
These little ubiquitous beach crabs can pull off some impressive tricks.