Spring cleaning

If you've looked closely at a porcelain crab, you've probably noticed that it has a large pair of claws and three pairs of striped walking legs.  The photo below shows the Chocolate Porcelain Crab (Petrolisthes manimaculis).  They're found under cobbles and boulders in the rocky intertidal zone. 

  

It's easy to overlook that porcelain crabs also have a 5^th pair of limbs that are greatly reduced in size relative to the walking legs.  Porcelain crabs along with some other types of crabs (such as hermit crabs, mole crabs, and pelagic red crabs) belong to a taxonomic group called the Anomura.  Unlike Cancer crabs, kelp crabs, and other “true crabs,” anomuran crabs never use their last (or 5^th) pair of limbs for walking.  In the drawing below, the 5^th pair of limbs are marked with red arrows:    

 

Modified from Fleischer, J., M. Grell, J.T. Hoeg, and J. Olesen. 1992. Morphology of grooming limbs in species of Petrolisthes and Pachycheles (Crustacea: Decapoda: Anomura: Porcellanidae): a scanning electron microscopy study. Marine Biology 113: 425-435.

These appendages are small enough that they are easy to miss.  The close-up photo below shows this 5^th pair of limbs marked with black arrows:

 

Given the small size of these appendages, one might wonder if they are vestigial structures that no longer serve a useful function?

Well, it turns out that in porcelain crabs, these appendages are actually important for grooming.  The slender grooming limbs can be folded up like a jackknife when not in use, but they are remarkably long and flexible when extended.  As you can see in Siena’s drawing below, the grooming limbs end in a claw.  They are also covered with several types of bristles (or setae).

 

Drawing by Siena Watson.  Modified from Fleischer et al. 1992.

The grooming limbs are used both to clean the gills within internal chambers and to clean the external surface of the crab of detritus, debris, parasites, and other attached organisms.  The bristles act as cleaning brushes, whereas the toothed claw is well equipped to remove objects that are more firmly attached.  Some of the bristles on the claw are sensory structures that appear to allow the crab to both feel and taste the objects that are encountered during grooming.

Eric captured some nice video of the grooming limb of a porcelain crab in action.  Porcelain crabs are suspension feeders.  Near the beginning of the video you will see the crab sweeping its specialized fans through the water to capture food.  Then the crab starts using one of its grooming limbs (white arrow at ~15 seconds) to vigorously scrub around its back, eyestalks, and long red antenna!  It is amazing to see the flexibility and dexterity of the limb as it diligently cleans and grooms.

Porcelain crab grooming from Jackie Sones on Vimeo.

[Apologies for not posting more this week.  I'm recovering from surgery on Monday, and my ability to stay upright is more limited than I had hoped.  I'm feeling better now though, and I hope to resume more frequent postings in the near future.  For now, Eric offered to help put together the information for a really fun post.  Many thanks to Eric and Siena!]