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Tuesday, February 27, 2018

Under the umbrella

Perhaps it's because there's rain in the forecast (finally!) — recently I was thinking about Umbrella Crabs (Cryptolithodes sitchensis).

This is an unusual crab.  The edges of the carapace are expanded outward (like a shield).  So although the legs are visible in the picture above, when they're pulled in the legs are completely concealed (when the crab is viewed from above).

Here's a look at the crab's underside.  Note how all of the legs fit entirely inside the edges of the carapace (including the first pair with claws, even though one is flexed in the photo).

Also note that the abdomen (triangular piece between the legs) has an interesting pattern, somewhat similar to scutes on a turtle shell.  [This species is also known as a Turtle Crab, probably because of the way the shell covers all of the body parts.  But I also wondered if that name could have been derived (in part) from the abdominal pattern?]

Umbrella Crabs are highly variable in color.  Here's another example (below).  [And for a wonderful overview of the many color patterns in this species, check out these photos on iNaturalist.]

The genus, Cryptolithodes, means "hidden like a rock."  When the crab is viewed from above with its legs tucked underneath, it can look very much like the surrounding rock!  (I hear they're eaten by fish and octopus, so perhaps there's an advantage to being well camouflaged.)

These crabs are usually whitish underneath; pictured below is the same individual as shown above.  (Can you find the short, red-and-white striped antennae?)

I haven't observed it myself, but I read that Umbrella Crabs eat coralline algae (!).

One more viewa close-up of the eyes peaking out from the sides of the shelf-like rostrum:

Let's hope that thinking about Umbrella Crabs leads to a need to carry an umbrella!


John W. Wall said...

Very cool crab! Never saw one before.

Jackie Sones said...

In the Bodega Bay area, I'd say this crab is rare to uncommon in the intertidal zone. It's around, so we're not surprised to see one, but it's not encountered that often. I was wondering about what your chances would be if you actually went out to look for it. (We usually just come across them while doing other things.) Perhaps I need more experience with this species (and it's true that I've never really tried to find one on purpose), but at least around here, I think that your chances of encountering an Umbrella Crab are pretty low. (I think I've only seen a handful during the past 10+ years.) So it's a good idea to appreciate this crab when you do find one!

:) Jackie