Meeting Abstract

113.3  Saturday, Jan. 7  Flippin’ out: inverted sand dollars actively orient themselves in flow to maximize lift for righting HARDY, A.R.*; MERZ, R.A.; Swarthmore College, PA; Swarthmore College, PA

The fact that sand dollars are often dislodged and inverted is an inescapable consequence of living at or slightly below the sediment-water interface. Once inverted however, how do sand dollars effectively right themselves given their lack of sizable appendages? In the current study we considered the effects of the inherent asymmetrical morphology of Mellita quinquiesperforata and Dendraster excentricus on their ability to right in flow. Based on flow tank observations, the critical velocity required to flip an inverted sand dollar varies with orientation and increases with test size. In both species, inverted sand dollars were least likely to be flipped by the current when oriented so that their posterior edge faced directly upstream. In order to test whether inverted sand dollars would actively rotate into a more advantageous position, we exposed each inverted individual to the minimum flow expected to induce flipping and compared their response in three orientations – with their posterior edge upstream (the least favored position), downstream and perpendicular to flow. Time-lapse photography showed that within one hour, individuals of both species regardless of initial orientation remained stationary or rotated into positions that were not statistically different from the downstream orientation. These results for D. excentricus were further confirmed in the field at Argyle Lagoon, WA. Taken together these data suggest that inverted sand dollars are able to recognize flow direction, respond behaviorally and modify their orientation to maximize lift and drag for righting.