Under the wet sand along beautiful shorelines, just beneath our feet during beach strolls, there lies an entire ecosystem. The mole crab (also known as a sand crab or sand flea) is a small but mighty sea creature that lives just below the surface of the sand.

When thinking of a sand flea, the first thought that comes to mind is probably not pleasant. After all, fleas are not the most welcomed guests in any room. Despite this unpleasant first impression, these creatures can be endearing with their quirky habits and harmless nature. 

Dive deeper into the small world of the mole crab.

Habitat

Less than one inch long, mole crabs are small yet durable ocean creatures. These animals live in colonies within the swash area (the area on the beach where the waves wash over the sand) of the intertidal zone on tropical and subtropical beaches. The intertidal zone is no picnic for its inhabitants. Constant waves crash over this zone and can move around things in its path. Tides also change as the day progresses, so mole crabs are tasked with following the tides to stay in the wet swash zone of the sandy beaches. This means that entire mole crab colonies move up and down the swash zone as the tide shifts every day—talk about hard work and perseverance. 

Appearance

These sand fleas are barrel-shaped creatures equipped with tough, brown-speckled armor that blends into their sandy surroundings. Their exoskeleton acts as protection from oncoming waves. Underneath they have five leg-like appendages and a short tail-like appendage known as a telson (the perfect tool for digging) that they hold into their body when bracing for impact. These features allow them to float around in the tidal waves and land safely somewhere new. Mole crabs also typically have eyes on short stalks and feathery antennae that are used for filter feeding.

Feeding

Mole crabs use their appendages to their advantage in the swash zone, the beach area where waves wash up, employing their legs to back up and burrow their bodies just beneath the surface of the sand. Some species leave their protruding eye stalks above the surface so they can watch for oncoming food (although they’re so tiny it would be quite difficult to spot them from above). Mole crabs are filter feeders, meaning they take advantage of the water moving around them to feed. As the tides roll in, mole crabs release their antennae and use these feathery tools to filter food like kelp, plankton and detritus. The crabs then pass their antennae over their mouths to release the gathered mixture of sand and nutrients. 

Reproduction

These tiny creatures have a relatively short lifespan of about two to three years. When mating, males will sometimes attach themselves to females, becoming semi-parasitic until they eventually outgrow the area. Females are typically larger than males and carry orange eggs ranging as many as 45,000 on their undersides. That’s a lot of mole crabs.

Predators

For these small sand fleas, the crashing tidal waves aren’t their biggest threat. Many seabirds love to walk along the shore and search for mole crabs hiding just beneath the surface of the sand. Other species of crab and some fish also feed on these tiny sand fleas. Mole crabs face their hungry predators constantly which means their ability to hurriedly burrow into the dense wet sand is highly necessary for their survival. Against a predator, these tiny crabs’ best defense is a swift tide rolling in which allows the crab to quickly disappear into the sand.

Despite looking like a creepy crawly, mole crabs are harmless to humans and don’t bite or sting. So, unless you are part plankton, you have nothing to worry about. Their filter feeding techniques and tough exteriors render them amazing tenants of the intertidal zone and fascinating ocean animals that make the word “flea” seem less daunting. These creatures have their own little ecosystem right below our feet, unbothered as we walk along the sand above them. Perhaps on your next walk on the beach, you can shift your attention to the sand where you just might see a tiny sand hole fit for a mole crab.

Want to learn more about the weird and wonderful animals of the sea? Check out our Ocean Conservancy Wildlife Library to explore more fascinating creatures beneath the surface of our ocean.