by Dr. Sarah Treanor Bois, PhD
Director of Research & Conservation at the Linda Loring Nature Foundation
If you’ve spent any time on the south shore of the island (i.e. the wavey beaches) you have probably seen one of our most resilient residents. They may be a bit secretive, a little camouflaged, but these shoreline residents are a fun find and great to explore with kids. We’re talking about the Atlantic mole crab.
Mole crabs are crustaceans in the family Hippidae, found in nearly every coastal region of the world except the Arctic and Antarctica. Twelve species in the genus Emerita are abundant on the Atlantic and Pacific coasts of the United States, and other parts of the world. The mole crab we find on Nantucket is the Atlantic mole crab (Emerita talpoida). The mole crab (also commonly known as sand crab or sand flea) is a small (0.5-2 inches) but mighty sea creature about the size of your thumb that lives just below the surface of the sand. They dig into the sand at the shore break and move up and down with the tide in order to stay in wet sand but not get turned over by wave action.
Mole crabs are uniquely adapted to live in this relentless environment of constant movement and changing tides of the turbulent, wave-swept shoreline (the swash zone). On Nantucket and nearby Massachusetts beaches, you can find them buried in the damp sand during the spring and summer months. Keep an eye out for tiny, V-shaped air bubbles or shifting sand just as a wave recedes. They migrate up and down the beach with the changing tides, always staying right at the water’s edge. Chances are you have most likely encountered mole crabs without even knowing it. Perfectly camouflaged, if one is 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 noticed it’s there.
The egg-shaped body of the mole crab is hydrodynamically sculptured for living in wave-pounded and shifting sands. They are called mole crabs, of course, because they burrow, but only in wet sand. They cannot dig in sand that is too dry or firmly packed. It has to have enough water to separate the grains and make the sand penetrable.
Mole crabs are filter-feeders. They bury themselves backwards into the sand and use their feathery antennae to filter plankton and algae directly out of the crashing waves. Its head is pointed upward facing the surf, its stalked eyes and its small first antennae, which it uses as kind of a snorkel for respiration, projecting from the sand. When a wave goes out, it curls its large, feathery second antennae backward and uses them as nets to capture the phytoplankton. The food is scraped from the antennae by other specialized, brush-like appendages and delivered to the mouth.
When washed back out to sea, most mole crabs become food for other animals. They serve as a crucial food source for shorebirds, marine mammals, and near-shore fish like striped bass.
To find some yourself as you walk along a beach, examine the wrack line for clues. Among the surf clam fragments, seaweed, and other flotsam and jetsam, you may find mole crab shell fragments. The wrack line will be full of these fragments in an area with a healthy mole crab population – they definitely congregate in numbers. Then look to the shoreline where the waves action meets the wet sand. When you see the tiny holes in the sand at the water’s edge, dig there and chances are you will find some buried mole crabs.
Like other crabs, these little one-inch creatures have ten legs, but theirs are adapted for swimming and digging, not for walking or defense—they have no claws. This makes them great candidates for checking out with small children. They even may try to burrow into your hand making for a tickly encounter. My son and I used to dig them up and have “mole crab races” to see whose chosen crab would reach the water first.
At places like Cisco or beaches west and east of there, you usually only have to dig a shallow trench with the heel of your foot parallel to the water to see clusters of them. They try to burrow back down into the sand but are easily scooped up and put into a bucket.
They also make good bait for beach fishing. Mole crabs are an important food source for our stripers at certain times of the year. Because they serve as a crucial food source for shorebirds and fish and act as an indicator of coastal health, recent declines in some areas have raised ecological concerns. On-island, some people, including beach anglers, have noticed mole crabs aren’t as abundant on certain beaches as they once were.
While Atlantic mole crabs are generally abundant, scientists and environmentalists have observed potentially concerning regional population declines. As key indicator species for coastal health, their numbers fluctuate based on human activity and environmental shifts.
Research indicates that mole crab populations are highly susceptible to anthropogenic (human-caused) and environmental changes. One possible threat is habitat alteration. Activities such as dredging and beach nourishment can have a direct impact on crab populations. These actions can bury the crabs too deep or change the sand composition, impacting their swash zone habitat. Heavy human foot traffic, mechanized beach cleaning, and off-road vehicles compact the sand and directly trample mole crabs. And while the beaches on the south shore of Nantucket haven’t been manually altered, a significant amount of sand has shifted through the system.
Another threat to mole crabs that has been studied on the west coast is microplastics. Since mole crabs eat small plankton and other invertebrates in the water, they can ingest microplastics, which can lead to higher mortality rates. Climate-related factors such as ocean acidification, harmful algal blooms, and compounding problems like sewage runoff degrade the delicate water column that larvae depend on during development causing further harm.
Most of the data about mole crab threats and declines comes from the West Coast, where populations have crashed on some beaches. Hopefully, we can learn from those areas, and any observed east coast decline is just a blip. Mole crabs are pretty adaptable, and their toughness may help them be more resilient.
How can you help? Right now, we’d love to get a baseline of where our mole crab populations are and if they seem healthy. Having a baseline will allow us to track any declines. To do that, we are documenting mole crab locations using iNaturalist. This can add to the biodiversity information for the island and provides data on the health of the mole crab population. There is information about how to make an observation on help.inaturalist.org.
Feel free to send any of your mole crab observations to me at stbois@llnf.org.
