Wednesday, September 7, 2016

Pelagic Red Crabs at Asilomar State Beach, Pacific Grove

In August, while wandering along the rocky coast in the Pacific Grove area (a favorite spot), we came across a small stretch of sandy beach nestled between rocky shoreline.  Unlike similar stretches of beach a few hundred yards away, this one was dotted with bright pelagic red crabs (Pleuroncodes planipes).
Pelagic Red Crab (Pleuroncodes planipes), Asilomar State Beach, Pacific Grove
Typically, this species occurs at more southern latitudes, in the area of Baja California.  But during warm El Nino weather, currents carry them farther north along the coast of California.  Pelagic red crabs have a habit of swarming near the surface of the water, possibly to spawn.  These masses of crabs are carried by currents and wash up on sandy beaches, causing mass mortality.  (A Google Image search will turn up some pretty incredible photos of beaches covered with pelagic red crabs.) 
Washed up on the beach, this crab is a soon-to-be meal for an opportunistic gull.
The word "pelagic" in the common name refers to the open ocean.  (As opposed to benthic, referring to the lowest level of a body of water and the substrate, or littoral, referring to shallow water near shore.)  Though called "pelagic," these crustaceans split their time between swimming freely in the water column and settling on the bottom to lead a more benthic lifestyle; they are not  typically associated with the shore.
Pelagic crab, in the hand
Pelagic red crabs are a valuable part of the marine ecosystem, filter-feeding on phyto- and zooplankton in the water column and detritus on the sea floor.  In the food web, they play the role of converting microscopic plankton to a valuable source of energy for larger predators, such as tuna, sea turtles, some whales and a number of sea birds.  
I'm sure the gulls at Asilomar Beach were thrilled to find that the sea had provided them with an easy meal that day!

3 comments:

  1. Bethany and I saw a swarm of these swimming at the Monterey Warf. We couldn't tell if they were shrimp or crabs at first. Thanks for the info :)

    ReplyDelete
  2. This comment has been removed by a blog administrator.

    ReplyDelete