Weekly “What is it?”: Sand fleas (mole crabs)
For a lot of people who grew up on the Gulf Coast, sand fleas elicit a specific memory of childhood days on the beach. These beach trips were spent with a purpose, though; no throwing frisbees or lounging on the sand. These days entailed hours of searching for sand fleas (also known as mole crabs–Emerita portoricensis), which are considered by most fishermen to be the perfect bait for catching pompano. Using a tool that is part rake, part shovel, and part sieve, the children of local fishermen have collectively spent hundreds of hours of their young lives scouring the wet beachfront for these elusive crabs.
On a good day, you can see dozens of sand fleas washing up in the waves and then digging back into the sand as the waves wash back. To catch them, you take the sand flea rake and drag it in the sand between where the waves break and the highest point they wash up on shore. As the waves roll in, you pull the rake through the sand and bring it out. Once full of sand, you dip it in the water to rinse sand out of the rake and the sand fleas are left behind. Most folks collect them in 5-gallon buckets. These beach days (along with the Vienna sausages and honey buns his dad brought for snacks) made such an impression on my husband that he more clearly remembers finding sand fleas than catching actual pompano.
Biologically, these 10-legged crustaceans are important building blocks of the nearshore coastal ecosystem. Occupying the high-energy swash zone of the beach where waves constantly crash in, they are quick burrowers who hide in the wet sand. They burrow and swim backwards, using a body part located on their undersides, called a telson, to dig. The crabs are about a half inch to an inch long, with rounded white/gray carapaces and small antennae. The antennae are used to filter food from the water. Unlike most crabs, they are clawless, and cannot bite or pinch, making them harmless to humans. They are a favorite food for many larger animals, including pompano, redfish, and shorebirds.
A similar sand crab of another species is found less often, but in the same habitat. We had a recent question about this one, captured in a beach visitor’s dip net. Unlike the short antennae of the common mole crab, Lepidopa benedicti has long antennae, often three times as long as their 2” bodies. These sand crabs, along with the mole crab’s Pacific cousin, also stick their long antennae out of the sand to filter feed for detritus in the water.
When I worked the emergency operations center during the 2010 oil spill, many callers had questions about the state of the local beaches. The wondered if they were ruined forever, how the wildlife would recover, and shared memories of times spent along our shores. One man, in tears, recounted the same childhood memory of hunting sand fleas while his dad fished for pompano. He was heartbroken by the idea of their populations being wiped out. Because the crabs feed on detritus and microorganisms in the sand, they do make an excellent sentinel species for scientific studies. Any pollutants that affect these crustaceans at the bottom of the food chain can quickly make their way to us at the top, reminding us even the smallest organisms have an important role in the ecosystem.