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What Is the Difference Between a Sand Flea & a Mole Crab?

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For humans, a stroll along the seashore is often a relaxing pleasure. Whether you're wading in the ocean or simply walking in the sand, it’s hard to imagine all that's living beneath your feet. Certain creatures in the sand can make your beach walk unpleasant with their bites or stings. Others, though, quietly help save our lives. Among them are some crab groups commonly called sand fleas.

Fleas in Name Only

Sand fleas are commonly referred to as beach flies or sand flies. But by no means are they insects; they’re tiny crustaceans from taxonomic family Talitridae in the Amphipoda order. About 60 genera of them exist. They’re common on North American beaches, and are usually less than an inch long. They jump around in the sand, giving an impression of fleas -- but they look more like shrimps. According the University of Florida, they’re also known as lawn shrimp. They feed on organic and rotting plants when they can’t make a meal out of their favorite organic substance, human blood.

These Fleas Bite

Sand fleas often cluster around seaweed that has washed ashore, and a beach goer’s legs make an opportune meal. Female sand fleas use the protein from the blood as nutrition for egg-laying. Their saliva has an anticoagulant that makes it easier for them to drink, and their saliva is what causes the human body to react with welts and itching.

Mole Crabs Are Bigger

Mole crabs, indeed crabs, are often called sand crabs, and they're sometimes thought to be sand fleas. But they're members of the Emerita genus of Decapoda, a different order of crustaceans than the sand fleas. They’re slightly larger, averaging 1 to 2 inches long. Like regular crabs, they have hard outer shells. However, like sand fleas, they do not have claws. They cannot pinch, nor do they feed on humans. These crabs are filter feeders: They bury themselves in the sand in the area of breaking waves, leaving their eyes and antennae above the ground. When a wave recedes, they uncoil their antennae feeding on the plankton as they flow across.

The Beneficial Mole Crab

Mole crabs are a popular food for seabirds, shorebirds and fish. For certain fish, these crabs make up the majority of their diet. Because of this, fishermen and fisheries often use them as bait. Since they are filter feeders, they play an important ecological role, and scientists use them to indicate levels of toxins in the water, such as Domoic acid. Domoic acid is neurotoxin caused by algae. It is a source of shellfish poisoning and is dangerous to humans.