Along for the ride -- Part 2

It's been an interesting week learning about Pelagic Red Crabs (Pleuroncodes planipes).  It's made me think about how much fun it is to watch and wonder about an animal that you've read about or seen pictures of but have never encountered before.

Pelagic Red Crabs are very rare in northern California.  According to the records we can find, this is only the second time they've made it this far north.  The last time was over 30 years ago (in 1985, following the 1982-1983 El Niño)!

 

On 24 January 2017, we collected some of the live Pelagic Red Crabs that washed up on the beach and put them into an aquarium.

Observation #1 — The crabs didn't float at the surface, but instead settled to the bottom.  They often sat in a "tip-toe" position, elevated off the bottom, raised up on their long legs.  (We learned that even though they're often called Pelagic Red Crabs, much of their life is spent on the bottom.)

Observation #2 — They're excellent swimmers and adopt a very streamlined profile when jetting through the water using "tail-flips."  After swimming to the surface, sometimes they passively drift down through the water column with their legs spread wide (some descriptions liken this to a parachute).

Observation #3 — The crabs use a variety of feeding methods, but they spend a lot of time suspension-feeding, i.e., filtering plankton from the water.  They use small mouthparts to generate a water current (drawing water towards their mouth) and then sweep noticeably setose appendages  through the water (like fine nets) to capture food particles (in photo below, see center appendages with tufts of fine bristles):

 

And then when we put one of the crabs in a bowl of seawater to take a look through the microscope, Eric noticed something else:

Observation #4 — They have "hitchhikers!"

Remember the mystery photo from last night?  (see below)

 

The claws are covered with dense setae (hair-like bristles).  These long bristles increase surface area and slow sinking rates (an adaptation for spending time near the ocean surface and in the water column).  But there was another animal there, too, growing among the setae:

 

 

This is the hydroid Obelia, possibly Obelia dichotoma.  Obelia has a life cycle that includes a sessile stage (a colony of feeding polyps that lives attached to an object) and a free-swimming medusa stage (similar to a jellyfish). 

 

Modified from The Light & Smith Manual (2007)

Eric was recording some video footage of a crab when a little medusa swam by!  The tiny medusa had just been released from one of the hydroid colonies.

So you can see for yourself, here's a collection of video clips showing some of the above.  Watch for (1) sweeping, brush-like mouthparts, (2) large compound eyes, (3) long claw with hydroids growing on it, (4) close-ups of the hydroids, (5) close-up of the setae (long, hair-like bristles), (6) swimming medusa (in the background starting at about 51 seconds).

[If you're receiving this via e-mail and can't see the video clip, click on the title of the post above to go directly to the web page.]

It's fun to think about what it would be like to be a hydroid living on a Pelagic Red Crab and all of the things you'd encounter while drifting along!