Of all the marine environments organisms can live in, one of the harshest is the "wash zone" of a sandy beach, at least those beaches that are not protected inside of bays and harbors. Millions of tiny grains constantly shift as each wave rolls in, preventing algae from taking root and animals from gaining any kind of foothold.
There are some species that have adapted quite well to this harsh environment. Sandpipers and plovers run up and down the beach, following the ebb and flow of the sea and feeding on tiny worms and shrimp carried in by the waves. Their tiny prey usually exist here either desperately clinging to uprooted seaweed tossed onto the shore, or constantly digging back down into the moving sand as each wave passes.
Few fish are found in this habitat, except for the occasional school of bait fish chased into shore by predatory bass or bluefish. But these are temporary visitors and leave the wave zone as soon as the attack is over.
Surf clams are one of the few mollusks that thrive in this harsh place, thanks to the hard shells and the nearly unique ability among bivalves to dig as fast as the sand can shift.
But one crustacean has mastered the art of surviving here. The mole crab (Emerita talpoida), which resembles an egg more than a decapod. These little one-inch creatures have ten legs, like other crabs, but these are adapted for swimming and digging, not walking. And dig they can. If you’ve ever visited a beach on the East Coast, you most likely have encountered mole crabs without even knowing it. Perfectly camouflaged, if exposed by a wave, a person’s foot or a child digging at the surf’s edge, it will dive back under the sand before your eyes can even register that it was there.
The claw-less Mole crabs dig and swim backwards, using the flattened legs that are tucked into the concave carapace equally well as either oars or shovels. The carapace, that top shield-like part of the exoskeleton found on all crabs, is smooth and comes to a point on both ends. Nearly everything about its anatomy has evolved to facilitate getting under the sand as quickly as possible.
Here’s one caught mid-dig. The legs tucked under the carapace first move backwards and are then extended and pushed forward, displacing sand and pulling the crab straight down. The part you see sticking out of the sand, with the little antennae, is the front of the body.
Which brings us to the next problem these animals face. Lacking claws, and not having much potential food around them anyway, how do they eat? Well, they’ve solved this problem by becoming filter feeders, capturing the minute floating plankton that is carried with each wave that passes over their bodies.
This species has two sets of antennae. The first, the ones you see above, are sensing structures. They also form siphons to draw in water to the gills when the animal is buried. The larger feeding antennae kind of resemble an old-fashioned feather pen. The hundreds of bristles on the antennae are extended out into the water as a wave passes over them. These trap the plankton and then the little front legs are used to wipe off the food and transfer it into the mouth. The crabs always bury themselves facing the ocean, with their backs to the beach. Feeding is only done as the wave comes in. When it is receding the crab flattens the anntenae against the sand.
Click here to watch some mole crabs feeding (courtesy of the Monterey Bay Aquarium). These are behaving a little differently than I described above because they are in an aquarium with no flowing waves. It’s still pretty good footage. About half-way into this you'll see them start licking the food off the antennae.
Like most crabs, Emerita is a brooder. The females, which are about twice the size of the males, holds up to 50,000 eggs in the swimmerets beneath her tail. They are protected here until they hatch and swarms of larvae are carried away. To prevent the larvae from washing up on shore immediately after hatching, the mother will tuck the tail against her body as a wave rolls in, then extend the tail, releasing the young as the wave recedes. This is done multiple times until all the larvae are released.
Here’s a photo of the zoea stage of the young. This is the stage just before the animal’s last juvenile molt, after which it will swim out of the water column and into the sand. Because the planktonic young may move well offshore before settling to the bottom, they must travel underground to find their way to the nearest surf zone. Very few of them survive this trek.
Fun Fact: Mole crabs are popular bait items for fishermen searching for drums.
Other diaries in this series can be found here.