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Crabs - A Northwest Wildlife Population Wonder

John M. Regan

                 Depending on the source consulted, about forty species of crabs inhabit our shores and intertidal (exposed during low tide, submerged during high tide) zones.  This includes two invasive species, the Chinese Mitten Crab and the European Green Crab.  Even this large number of species, however, does not convey the staggering number of individual crabs that dance about our northwest shore.  Crabs are crustaceans grouped with the shrimp, isopods, and amphipods under the Class Malacostracea (the crustaceans), Order Decopoda.  In some sources crabs are further classified under the infraorder Anomura.  Unlike their Decopodian friends crabs do not have any freshwater representatives - all are inhabitants of salt water.  Our native crabby crustaceans are grouped into eight families:  Paguridae, Lithodidae, Porcellinidae, Brachyura, Magidae, Cheiragonidae, Cancidae, and the Galatheids. 

 Infraorder Anomura:

 Family Paguridae - These are the well known and immediately recognizable Hermit Crabs.

Family Lithodidae - This is the largest family of crabs in our area, although not necessarily the most well known.  The Lithodids include some of the oddest looking members of the crab family such as the Umbrella Crab and the Butterfly Crabs.  The largest crab in our area, the Puget Sound King Crab, is a member of this family.

Family Porcellanidae - Named for their legs that seem to break off like brittle pieces of porcelain.

Family Brachyura - The "True Crabs."  This is the species most often encountered along the shore; commonly referred to as Shore Crabs.  Referred to as "True Crabs" due their classic crab shape:  a large cephelothorax (front part of the body) and a small abdomen hidden under the shell.

Family Majidae - Another familiar group; this family contains the spider, kelp and decorator crabs. 

Family Cancidae -  Well known to any Northwest fan of crab meat, the Cancer Crabs boast the famous Dungeness and Red Rock Crabs as members of its group.  The Cancer crabs on the whole are larger than most

The Chreigonidae and Galatheid families are the smallest, with just one representative species each in our area.

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Appearance

                Crab species are distinguished, obviously, by size, color, and shape.  Not so obviously is their differing number of appendages.  Most species have four pairs of appendages, three for locomotion and the foremost pair terminating in the famous crab claws.  In the Lithodids, however, the fourth pair is often absent or quite small.   A number of Lithodids  have a hairy or fuzzy appearance.  The size and length of the claws in relation to the size of the animal is another indicator of species.  Some of the Porcelain crabs sport choppers that are truly colossal in relation to the rest of their body while others appear downright dainty.  The shape of the hermit crabs is immediately recognizable. 

                I am fascinated by the size of crabs as well, or perhaps I should say the lack of size.  Most of us are familiar with the big Dungeness or Red Rock Crabs, but the next time you examine a small tide pool look a little closer.  You might notice the Hairy Hermit Crab, a tiny orange fellow often less than three quarters of an inch long.  On the other end of the scale is our giant Puget Sound King Crab with a carapace a foot wide.  Most of our crabs are actually less than one inch in diameter when measured along the width of the carapace, the widest part of the crab shell.

From left to right above:  A Lined Shore Crab and a Purple Crab shore Crab display their beautiful colors.  Kelp Crabs gather by the dozens.  The Red Rock Crab in the photograph below is a comparative giant to the little guys above.

Food

                Consider the astounding numbers of crabs along our shore.  A single rock often hides five to twenty of the little beasts.  Care to estimate how many rocks there are along the shores of the Northwest?  Even a moment's contemplation yields an astounding number of crustaceans.  What are they all eating?  Quite simply - just about anything.  Crabs eat everything from barnacles to detritus and are quick to scavenge whatever they can get their claws on.

Life Cycle

                Females hold their eggs, numbering in the millions, underneath the carapace.  Once the eggs develop sufficiently they little ones are cast off to fend for themselves.  In this pre-larval stage their primary food is plankton.  Off they go by their millions into the ocean where the vast majority will become just another part of the food chain.  The larvae develop hard spines for protection.  If the young crab survives it begins development of the characteristic ten legs of crabs but still looks pretty much like a small shrimp.  Now, however, the crab begins its successive molts (instars).  With each molt the juvenile crab more and more resembles the adult.

                So the next time you're out strolling along the beach kick over a rock or two.  Count the number of little scuttling crabs you find.  Now look up and consider the number of rocks left to kick.  That's a lot of crabs!

Sources and links for further information:  FM 21-76 Army Survival Guide, National Audubon Society "Field Guide to the Pacific Northwest," Washington State University Website; "Seashore of the Pacific Northwest" by Ian Sheldon; "Whelks to Whales" by Rick Harbo; "Pacific Reef and Shore" by Rick Harbo; "Shellfish Guide" by J.D. Wade; Audubon society Guide to the Pacific Northwest; Oregon State University's Crab Identification Guide, 2005.