Roles for holes in sand dollars (Echinoidea): a review of lunule function and evolution

Abstract

The function of lunules in sand dollars is reviewed and it is argued that ambulacral lunules and the anal lunule evolved for quite different purposes. Whereas ambulacral lunules evolved to increase the perimeter of the test for food gathering, this was clearly not the original function of the anal lunule, although in some genera it has become secondarily adapted to this role. Many of the unique features of the anal lunule can be explained if it evolved as an outlet for ciliary feeding currents drawn centripetally into the mouth, without disrupting either oral or aboral currents. This allowed the entire margin of the test to be used for collecting the fine particles sieved through the aboral spine canopy. Anal lunules probably evolved only once, in the Miocene, and the Monophorasteridae and Mellitidae are probably sister groups.