Tiny beach-dwelling crustaceans are big-time sand movers, shaping the California coast

On a California beach, one can often find themselves standing atop an active earthworks site, due to the significant sand movement performed by tiny beach-dwelling crustaceans known as sand hoppers (Megalorchestia spp.). These shrimp-like creatures, commonly found eating kelp on the shore, have been discovered to excavate up to 110 pounds of sand per meter of shoreline per day, placing them among the most prolific bioturbators on Earth.

According to UC Santa Barbara marine scientist David Hubbard, sand hoppers possess one of the highest rates of sediment displacement by living organisms globally. Their findings, published in the Journal of Geophysical Research: Earth Surface, suggest that these amphipods play a crucial role in shaping the California coast, comparable to coastal longshore processes and even some major Southern California rivers.

Though their burrowing activity may appear stationary, sandy beaches function like rivers of sediment, constantly shifting sand transported by waves, winds, and currents along the coast. This dynamic environment, referred to as a littoral cell, cycles sediment from sources like cliffs, dunes, inland rivers and streams, and onshore processes that move sand from nearshore to the beach.

Physical geographer Tim Baxter, who recently completed postdoctoral work at UCSB, collaborated with marine scientists David Hubbard, Kyle Emery, and Jenny Dugan to investigate the impact of sand hoppers on the sediment system and coastal transport. They conducted a field experiment at Isla Vista Beach, a bluff-backed stretch of shore adjacent to the UCSB campus.

By placing metal frames in the sand hoppers' preferred burrowing zone, the researchers observed the crustaceans digging up to 10-30 cm into the sand or hiding under seaweed during the day, emerging at night to feed on kelp.

The researchers found that sand hoppers move up to 50 kg of sand overnight, highlighting their significant contribution to the coastal ecosystem. These burrowing activities not only cycle nutrients into the deeper layers of sand but also aerate the sediments, providing oxygen for aerobic microbial processes. Furthermore, the burrows loosen the sand, making it more available for erosional processes and potentially increasing the roughness of the beach surface, which has implications for coastal dune formation.

The sheer volume of sand displaced by sand hoppers, when extrapolated across 25 kilometers of Southern California coastline, is comparable to the daily sediment loads of some major Southern California rivers and longshore drift processes. This discovery adds a new dimension to the understanding of the roles of sand-dwelling creatures in the physical processes shaping the California coast.