Beach Houdinis: Mole Crabs
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
I spent the past week introducing a group of high school students from Worcester, Massachusetts to some of Nantucket's flora and fauna as we explored various habitats around the island. One of their favorite activities was a morning spent at Codfish Park in 'Sconset. We took samples of the sand to determine average grain sizes using a stack of sieves back in the lab and measured the change in elevation from the back dunes to the rapidly growing midsection of the beach down into the surf zone. We also talked about how creatures adapt to the pounding waves, saltwater immersion, intermittent dessication, and constant pressure from inquisitive predators like sanderlings. We found an excellent example of a critter that exhibits all these characteristics and is one of Nantucket's literally hidden treasures when we came upon thousands of tiny tunneling creatures called “mole crabs.“
The intertidal area of our beaches can seem to be relatively lifeless to the casual observer. Between the beach grass anchoring the upper dunes and the swash zone where surf pounds the sand, there is a sea of life hidden under the sand that is the basis for an intricate food web. If you scoop up some sand in the swash zone you'll see a flurry of little bodies burrowing down, scattering in all directions. You are watching the frantic stampede of the mole crab (Emerita talpoida, Say 1817) also known as the Atlantic sand crab to distinguish it from its Pacific brethren. An Italian naturalist, medical doctor, and expert on mercury poisoning named Giovanni Antonio Scopoli first discovered the Emerita (Latin feminine form for retired professor, bishop, or professional) genus which includes five other sand crab species in 1777 and the American naturalist Thomas Say officially laid claim to the discovery of the crustacean and subsequent taxonomic flag planting by selecting the descriptive species name “talpoida” which is derived from the Latin root “talpus” and Classical Latin word “talpa “ for “mole.” It is pretty obvious how they got their name if you watch them quickly bury themselves into the sand when exposed. Mole crabs have several other common names such as sand crabs, sand fleas (which is a bit confusing because true sand fleas are normally tiny amphipods), sand fiddlers, and beach hoppers. 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 you even know it's there.
This crustacean has evolved to have a body uniquely suited to thrive in the surf zone. They range in size from 1/4 of an inch to over an inch. Their lifespan is two to three years. They are the color of the rippled sand at the water's edge and live mostly buried in the sand in the intertidal zone. These little one-inch creatures have ten legs, like other crabs, but these are adapted for swimming and digging, not walking or defense. Mole crabs are found along the Atlantic coast from Massachusetts southward. The mole crab is an egg-shaped crustacean with a smooth, convex carapace. Not exhibiting a typical crab shape, this crab’s abdomen is broad in the front and tapers to its tail, which has a pair of forked, leaflike appendages. A long, spear-like tail piece folds under the body and is used for anchoring in the sand and protecting the eggs as we'll see later.. A pair of dark eyes sit atop long, thin eyestalks. Unlike other crabs who tend to move side to side, mole crabs dig and swim backwards, using the flattened legs that are tucked into the concave carapace equally well as either oars or shovels. Nearly everything about its anatomy has evolved to facilitate getting under the sand as quickly as possible. They are known as “swash riders” and from now on I may call them “wave cowboys” because they move up and down with the tide in order to stay in wet sand but not get turned over by wave action.
This species has two sets of antennae. The first pair are hairy and used as sensors to pick up vibrations from approaching waves. They also form siphons to draw in water to the gills when the animal is buried. The larger feeding antennae 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. They collect the phytoplankton from the water as the waves recede. Mole crabs bury themselves in the sand, anchoring their bodies in place with their tail plates and extending their antennae. The breathing antennae are closed together to form a small funnel that takes in oxygenated water and filters sand grains away from its gills. Sometimes you can see the “V” shaped ridges they leave in the wet sand as they filter the water or you may even see tiny bubbles marking the spots where they are hiding.
Between tides, they dig into the sand to hide from the shorebirds who would eat them (larger crabs and fish also make meals of them). Only their eyes and antennae peep out above the sand. The feathery feeding antennae are also used for cleaning themselves, truly an “all-purpose” appendage suitable for late night infomercial fame. There are a lot of fascinating You Tube videos showing the feeding behavior and “now you see them, now you don't” disappearing act of the mole crabs. The effectiveness of the feathery antennae for filter planktonic bits from the water column is especially obvious in some of the videos.
Females grow to about one inch, while males grow to about half an inch. Males reach sexual maturity when they are only 0.125 inches in length. Both sexes move into deeper water in the winter to avoid temperature extremes. In summer, orange egg masses will be noticeable on the abdomens of the females. While it is not easy to know the first time which end of a mole crab is the front, you can find out by holding one in your hand and watching which direction the crab moves: the end that moves first is the rear end. When placed back into the sand, they can bury themselves in seconds. They don't have pincers or claws, so they can't hurt you, which is why children enjoy playing with them. Do be kind and put them back where you found them, preferably on their bellies so they can escape.
Like most crabs, Emerita is a brooder, which means they carry the eggs on their body and protect them until they are ready to be dispersed. The female 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 by the tide. The females times each of the egg releases to coincide with the outgoing waves so that the larvae aren't stuck “high and dry.” 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. Because the planktonic young may move well offshore before settling to the bottom, they must swim a long distance to find their way to the nearest surf zone. Very few of them survive this trek.
This species is commonly used for bait for flounders, drum, and other sport and commercial fish and they are collected from the intertidal area by fishermen using wire mesh rakes. Some people even eat them, although it takes several pounds to make a meal. Mole crabs are an important, some might say even critical food source for shorebirds who forage in the surf zone. They have been used recently to evaluate pollution effects and other stressors on beach habitats. Mole crabs have also been used around the country as indicator species, or “canaries in the coal mine” to evaluate beach nourishment effects on intertidal areas and to measure the concentrations of toxins from harmful algal blooms. Man-made freshwater discharge outfall sites have been shown to drastically reduce mole crab populations. Students on the West Coast are monitoring mole crab populations to determine if they have changed over time. Young people in the LiMPETS program (Long-term Monitoring and Experiential Training for Students) have been monitoring Pacific mole crabs on San Francisco Bay Area beaches since 2002, and their recent findings give cause for concern. Students from 30 schools looked at 15 different sites, including Stinson and Ocean beaches. Data collected at Stinson reveal a 90 percent decrease in abundance between 2002 and 2007, while numbers at Ocean Beach also decreased dramatically, from 100 to 300 crabs per square meter in 2002 to only 6 in 2007 (www.farallones.org/education/student_monitoring.php). Other scientists look at the distribution of different age mole crabs in the intertidal area to see how quickly these creatures can move and to figure out how they manage to stay in the surf zone, essentially they are studying mole crab dance patterns. The adaptability of these creatures and their toughness may win out over threats to their existence and insure that they can support the food chain.
Next time you are at any of our beaches, dig in the upper sections of the surf zone and see if you can find this hidden treasure.
