Sand crabs, also called mole crabs, eat plankton and tiny bits of organic debris filtered from ocean waves. They are not scavengers or hunters. Instead, they use a pair of feathery antennae to sift microscopic food from the water as it rushes over them in the surf zone. Their diet is simple, but the way they capture it is surprisingly specialized.
What Sand Crabs Actually Eat
The primary food source for sand crabs is plankton, mostly single-celled organisms called dinoflagellates. These are microscopic algae that drift in seawater by the billions. Sand crabs also consume detritus, which is just a catch-all term for tiny fragments of decaying plant and animal matter suspended in the water column. That’s essentially the entire menu: microscopic algae and organic particles too small to see with the naked eye.
Sand crab larvae, which drift in open water before settling on a beach, are also filter feeders from the start. There’s no dramatic dietary shift as they grow. Both juveniles and adults feed on whatever plankton and particulate matter the ocean delivers to them.
How They Catch Their Food
Sand crabs bury themselves backward in wet sand right at the waterline, with only their eyes and antennae exposed. They have two sets of antennae, and the larger, feathery second pair does all the work. When a wave washes over them and begins to recede, they extend these plumose antennae into the retreating water like a net. The feathery structure selectively traps plankton and detritus particles while letting sand and larger debris pass through.
This is passive filter feeding. The crab doesn’t chase anything or manipulate its food. It simply holds its antennae in the right position and lets wave energy do the work. The entire feeding window lasts only a few seconds per wave, so sand crabs depend on the constant rhythm of the surf to eat. They position themselves in the swash zone, the narrow strip of beach where waves wash up and pull back, because that’s where water flow is strongest and most reliable.
Feeding Patterns and Tidal Cycles
Sand crabs don’t stay in one spot. They migrate up and down the beach face with the tide, always repositioning themselves in the active swash zone where feeding conditions are best. As the tide rises, they move higher on the beach. As it falls, they follow the waterline back down. This constant repositioning keeps them in the narrow band of wet sand where waves deliver food most efficiently.
Foraging time is constrained by the tidal cycle. On intertidal flats, crabs that live in food-rich patches adjust their behavior to feed longer in those areas rather than wandering widely. Seasonal patterns also play a role. In warmer months, some species forage over larger areas, though this appears to relate more to temperature and drying conditions than to food scarcity in the sand itself.
Sand Crabs vs. Ghost Crabs
If you’ve seen crabs scurrying sideways on the beach at night, those are ghost crabs, and they eat very differently. Ghost crabs are active predators and scavengers. Their diet consists primarily of mole crabs (sand crabs), small clams called coquina clams, and sand-hopping amphipods that live in piles of seaweed above the waterline. Ghost crabs will also eat marine carrion when they find it, though that makes up only a small percentage of their overall intake.
Sand crabs, by contrast, never hunt or scavenge. They are exclusively filter feeders with no ability to grab or tear food. The two species occupy the same beach but fill completely different roles: sand crabs convert microscopic plankton into body mass, and ghost crabs convert sand crabs (among other things) into energy higher up the food chain.
Where Sand Crabs Fit in the Food Web
Sand crabs are a critical link between the open ocean and the beach ecosystem. They pull plankton out of the water and concentrate that energy into a protein-rich package that larger animals can eat. Shorebirds pick them from the sand at low tide. Barred surfperch, one of the most common fish in the California surf zone, rely on sand crabs as their primary prey, with beach invertebrates making up over 89% of their diet by abundance and 97% by biomass. During large spring tides, surfperch diversify their diet somewhat, but sand crabs remain the staple.
This makes sand crabs one of the most important food sources on open coast sandy beaches. Without them, the energy stored in ocean plankton would largely bypass the beach environment entirely.
Why Their Diet Makes Them Pollution Indicators
Because sand crabs filter enormous volumes of seawater through their antennae and across their gills, they accumulate whatever is dissolved or suspended in that water. This includes not just food but also contaminants like petroleum hydrocarbons and heavy metals. Researchers have used sand crab populations as biological indicators of coastal pollution for decades, since the crabs concentrate chemicals from their environment at measurable levels.
Sand crabs likely absorb dissolved hydrocarbons across their gills while also ingesting contaminated particles during normal feeding. This dual exposure pathway means their body chemistry reflects the water quality of the swash zone with surprising accuracy. A sand crab’s tissues can reveal contamination that standard water testing might miss, precisely because the crab has been filtering that water continuously and concentrating pollutants over time.