Feeding with their feet

I’m excited to share some photos and video of crinoids (also known as feather stars).  These intriguing animals are echinoderms (related to sea stars and sea urchins).  However, we don’t get to see crinoids often along the California coast because they are restricted to deeper water.  When Kate brought back some of her offshore oceanographic equipment from Cordell Bank, she found a few of these feather stars (Florometra serratissima) attached to her gear.   

 

All echinoderms share the unique feature of tube feet — hydraulically powered structures that are usually arranged in rows along the arms.   Most echinoderms (like sea stars, sea urchins, and sea cucumbers) use their tube feet both to attach to rocky surfaces and to crawl along the bottom.  For example, here's a close-up of the tube feet of a Sunflower Star (Pycnopodia helianthoides):

 

However, it is believed that the tube feet of ancient echinoderms were not used for locomotion, but rather were used to capture suspended food from the water.  The tube feet of crinoids have retained this ancestral function of suspending feeding.   

Crinoids hold their long, feathery arms aloft in the water.  Their arms have many fine branches (called pinnules).  A groove runs down the middle of each branch, and the tube feet are arranged in sets of three along the groove.   

 

The primary tube feet are long and are used to capture food particles from the water (see next photo).  They are covered with mucus and fine projections (papillae) to help snare food.  The primary tube feet can also bend over rapidly to flick particles into the food groove.

 

The shorter (secondary and tertiary) tube feet help transfer captured particles from the primary tube feet into the food groove.  Once particles are in the food groove, they are transported by cilia down the groove to the mouth, which lies at the base of the arms.  The extended arms, branches, and tube feet of the crinoid all face upward into the water, creating a cone-like structure with a lot of surface area for capturing food.     

In the video below, you will see two feather stars perched on a rock, followed by close-ups of their tube feet.  As you watch the video of these crinoids feeding, think about how different their tube feet are from those of more familiar echinoderms like a sea star.  For example, notice how the crinoid’s primary tube feet are long, slender, and papillate, and extend up into the water.  In contrast, a sea star’s relatively smooth, muscular tube feet end in suckers and face downward for crawling on the bottom.  Crinoids are so different in their appearance and behavior that they somehow seem more like prehistoric fossils than living animals!

crinoids_sanford from Jackie Sones on Vimeo.

P.S. Many thanks to Kate for sharing these amazing animals with us.