Striped Shore Crabs Feeding on SF Bay

I came upon this large group of striped shore crabs (Pachygrapsus crassipes) , scuttling over shoreline rocks near the Golden Gate. They’re gorgeous little animals — a color wheel of hues. They were feeding on the plant matter covering the rocks, pinchers alternating left and right, gills bubbling to stay aerated out of water.

South of San Francisco, in Elkhorn Slough, large numbers shore crabs burrow into the banks and contribute to erosion. This burrowing “weakens the creekbank edges, so that whole chunks of marsh will sometimes calve off, and by lowering biomass they are reducing the ability of marsh plants to prevent erosion.” (Kathryn Beheshti, UC Santa Cruz).

Per this article in eos.org:

“In a new study in Nature, researchers found that sea otters have reduced bank erosion rates by 69% in Elkhorn Slough, a coastal wetland south of San Francisco, in the decades since their return to the estuary. Their big effect is due to their big appetites—the Elkhorn Slough salt marsh has been eroding, in part, because of root-munching shore crabs that burrow into the soil and destabilize the banks. A single otter can easily eat dozens of these destructive crustaceans each day. (The predator needs to chow down roughly a quarter of its body weight daily.) When otters move in, they clear out many of the crabs, and the salt marshes benefit.