P3.133 Friday, Jan. 6 Falling faster: Size and folding behavior decrease descent time in a brittle star (Ophiocoma aethiops) BADGER, M.A.*; JONES, M.A.; University of California, Berkeley; Florida State University, Tallahassee firstname.lastname@example.org
Fluid living organisms often undergo shape changes in order to move more effectively through the environment. Brittle stars (Ophiuroidea) living in high-energy intertidal and shallow subtidal zones may occasionally be dislodged and carried into the water column by wave energy. This open environment likely increases predation pressure and puts the individual at risk for drifting into an unsuitable habitat. In response, a brittle star folds its arms, which decreases its descent time to the relative safety of the bottom substrate. We measured falling speeds in Ophiocoma aethiops for both open and folded arm configurations. Body mass ranged by more than a factor of 100, and large individuals were geometrically similar to small individuals. Small brittle stars may have a greater need to avoid gape-limited predation by fishes. Therefore, we hypothesized that (i) large brittle stars would be less likely to fold during a descent than small individuals. Because larger bodies generally have higher terminal velocities, we hypothesized that (ii) large brittle stars would fall faster than small individuals regardless of the folding behavior. We also hypothesized that (iii) folding behavior would increase falling speed within individuals. Folding frequency was not dependent on disk size. Folded brittle stars fell faster on average than they did when in the open position. Falling speed increased significantly with size for stars in the open configuration. For stars in the folded configuration, however, falling speed was not related to size. These results indicate that the folding behavior may be more beneficial for small brittle stars than for large brittle stars. A potential source of this difference is the smaller stars’ greater need to avoid gape-limited predators.