May 14, 2021

Morro Bay Wildlife Spotlight: Striped Shore Crab (Pachygrapsus crassipes)

Striped shore crab identification and habitat

What striped shore crabs look like

The striped shore crab is a type of crustacean, about one to two inches (or 3 to 5 centimeters) wide.  Their carapace (i.e. hard, upper shell) is a very dark purple, red, or even green, and lined with bright yellow-green stripes. Though this color combination makes striped shore crabs eye catching when you see them out in the open, it helps them disappear into dark, rocky crevices where they hide amongst sea lettuce, rock weed, and bits of kelp. Its pincers, also known as chelae, are often a deep red. Male striped shore crabs are slightly larger than the females.

Two striped shore crabs hide under a rock surrounded by barnacles and yellow, green materials on the rocks.
Two striped shore crabs hide in a crevice under a rock. Their dark carapaces help them blend into the shadows, while the color of their stripes echoes the yellow-green of the growths on the rock surrounding them. Photo by Heather Paul, via Flickr shared under Creative Commons License.

Where to find striped shore crabs

Striped shore crabs live all along the West Coast of North America, from Baja California, Mexico, in the south, to Vancouver Island, Canada, in the north. If you look carefully, you can find striped shore crabs in estuaries, tidepools, mussel beds, in the burrows they sometimes dig into sandy banks, or scuttling along shoreline rocks.

The variety of habitats they exist in mirrors the variety of foods they’ll eat. Though they feed mostly on green and red algae and diatoms (a kind of phytoplankton) growing on the water or rocks around them, they are opportunistic and will also eat animals including dead fish, limpets, snails, isopods, worms, and mussels.

See a striped shore crab eat

Striped shore crabs like the one in this video, filmed in the Morro Bay estuary, often tear and pick up algae or other food substances with both chelae. They bring the food to their mouths rhythmically, eating alternately from the left and right pincers.

Interesting facts about striped shore crabs

  1. Though most crabs and many shore crabs spend the majority of of their time underwater, the striped shore crab is an exception. They spend 50% or more of their time on land, though they typically stay close to the water’s edge.
  2. Lined shore crabs mate between April and September every year. A single female may produce as many as 50,000 eggs,  and, once they are fertilized, she will carry them under her abdomen as they develop.
  3. Like many crabs, striped shore crabs molt, meaning that they shed their hard shells as they grow too large for them. When they shed, they do so through a small slit at the back of the shell, leaving the molted shell pretty much intact. After they molt, the new exposed shell is soft and hardens over time. One striped shore crab may cannibalize a fellow striped shore crab while the second crab’s shell is still soft after molting.
  4. Though this may look like a very pale or dead striped shore crab, it is actually a molted shell.
    Though this may look like a very pale or dead striped shore crab, it is actually a molted shell. Photograph by Don Ehlen, shared via Flickr under Creative Commons License.

How to protect striped shore crabs

Wherever you find striped shore crabs and other wildlife, please observe from a distance. This lets the crabs stay safe, find and eat the food they need to survive, and keep away from predators. In short, leaving the crabs to their own devices helps them stay alive and wild.

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