Sand Dollars

Mellita isometra is the common sand dollar of the east coast found on siliceous sands from Chesapeake Bay to central Florida.

 
     
 

With spines removed the five slots, called lunules, are clearly visible. Flying them in a wind tunnel shows that the lunules act as lift-spoilers and stabilize the animal in moving water. In other words, they are a hydrodynamic "contrivance."

     

Leodia sexiesperforata, with six lunules, occurs on carbonate sands from south Florida and the Caribbean.

 
     
 

This species, Encope emarginata, has open lunules or notches and is found on mixed sediments throughout the Gulf of Mexico.

     

The highly distinctive Indo-Pacific Echinodiscus bisperforatus has only two lunules.

 
     
 

The northern sand dollar, Echinarachnius parma, has no lunules. It can be found from New England to Newfoundland. Without lift-spoilers it is restricted to less hydrodynmically active environments.

     

The remarkable Dendraster excentricus of the west coast, Mexico to British Columbia, stands upright to capture suspended food particles.

 
All pictures came from Rich Mooi, California Academy of Sciences.