RESOURCE PARTITIONING BY SAND DOLLARS IN CARBONATE AND SILICEOUS SEDIMENTS: EVIDENCE FROM PODIAL AND PARTICLE DIMENSIONS

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Abstract
The sand dollars, Leodia sexiesperforata (Leske) and Encope michelini L. Agassiz, have overlapping geographical ranges and may co-occur in mixed flocks. Leodia is restricted entirely to biogenic carbonate sediments. Mellita quinquiesperforata (Leske), which has a similar geographical range to Leodia, occurs only on siliceous terrigenous substrates and the two species never co-exist. Encope michelini L. Agassiz occurs on both types of substrate. All three species are podial particle pickers, and use barrel-tipped podia, especially the long type surrounding the geniculate spine fields of the oral surface, for food collection. A typical mellitid of 100 mm diameter can have up to one million barrel-tipped podia. These podia have the same mean diameters in Leodia (71.6 .+-. 5.62 .mu.m) and Mellita (71.8 .+-. 3.59 .mu.m). The diversity of sizes is significantly greater in Leodia. The barrel-tipped podia of E. michelini are very much larger (104.4 .+-. 11.1 .mu.m). The substrates inhabitated by the three species have approximately 90% of their particles in the 100-400 .mu.m range. Whereas Mellita is nonselective in collecting food particles, Leodia clearly selects small particles (50-200 .mu.m) and shuns those above 200 .mu.m. Encope michelini inlcues 26% of particles over 200 .mu.m in its food grooves, but does not take those below 100 .mu.m. Differences in feeding behavior thus provide a basis for resource partitioning between these sympatric species. They are discussed in reltaion to podial dimensions and spination, and compared with feeding behavior in Mellita quinquiesperforata.