Though well-camouflaged by a mottled gray shell, sand crabs spend most of their time submerged in the shifting sands. Five pairs of legs are used in swimming, crawling and burrowing — with one caveat: all movements are accomplished solely in a backwards fashion. To feed, sand crabs burrow into the sand, exposing only their periscopic eyestalks (to see) and first antennae pair (to breathe). As a receding wave travels overhead, sand crabs uncoil a second pair of feather-like antennae and comb the water to filter tiny plankton, mostly diatoms and dinoflagellates. The crabs then coil the hairy limbs back into the body and consume the tidbits. This feeding sequence cycles so fast that the crabs have time to trawl for food several times per ebbing wave. I have good luck spotting a swarm of crabs feeding particularly during low tides. Amidst the otherwise smooth sandscape, look for islands of rippled surfaces, which hint at a large aggregation of crabs subsurfacing.
Speaking of aggregations, sand crabs are not distributed uniformly across a beach. Scientists have proposed various biological (like mating) and physical (like wave shock) reasons, but a mix of influences likely cause the behavior. For sure, a sand crab is no match for the ocean’s power, even though it has a heavily-armored, curved body and pointy legs that help maintain a toehold in the perpetually moving sand. When a wave returns to sea, some sand crabs invariably get sucked out, during which they valiantly swim or tread water. Some must be lost to the sea, but others bodysurf in with the next breaking wave where they dig in at a new swash zone location. At any rate, on any given beach, a sand crab population can vary drastically and from year to year, depending on environmental conditions.
Mating occurs mostly in the spring and summer seasons, when females cart around a clutch of bright orange eggs on the body’s underside. Once hatched, larvae swirl in the water column, where they grow and molt, repeating the process until large enough to survive shoreline existence. They then migrate to the surfzone and settle onto the beach. New recruits can reproduce during their first year of life if fish and coastal bird predators don’t beat them to it. Humans make a dent in their numbers as well — surf fishers use sand crabs for bait and scientists use them to determine the health of the beach ecosystem. Since sand crabs live in sand — ground zero for toxin contamination — they are like canaries in a coal mine. For example, when sand crabs eat diatoms that produce the neurotoxin domoic acid, the crabs themselves become toxic to whatever eats them. When that animal is eaten, the toxin is passed along, progressing up the food chain and becoming more and more concentrated. By the time it reaches big fish that are consumed by marine mammals and humans, serious or fatal amnesic poisoning can result. Consequently, domoic acid levels in the crabs’ flesh correlate to toxin levels in the surrounding water.
A sandy beach environment is not an easy place to live. For one, constantly crashing waves and changing tides make it impossible to put down roots, literally or figuratively. And ever-present water and weather changes, along with predators ready to pounce from ocean, land or air, all vie for crab mortality. As a species that is constantly confronting potential bodily harm, there is plenty to learn from sand crabs — such as that security is an illusion. Instead of worrying about and wasting energy on trying to gain a foothold in the shifting sand, stay focused on the Golden Rule, with eyes open and facing the next wave, which may be as unpredictable as the future.
— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org.