Spine reorientation influences drift particle capture efficiency in sea urchins

Many marine organisms use behavior to navigate hydrodynamic landscapes. The subtidal sea urchin Strongylocentrotus franciscanus reorients its spines as water velocity increases to reduce drag and remain attached to the substratum. Streamlining may be advantageous in this regard, but it is unclear how this change in drag profile will affect particle capture, a feeding strategy employed by these organisms. Streamlining in urchins results in a “spines down” posture while particle capture benefits from spines remaining erect, a difference that could potentially lead to decreased feeding rates in high water velocities. To investigate this we ran flow tank experiments with three species of urchin (Strongylocentrotus droebachiensis, Strongylocentrotus franciscanus, and Strongylocentrotus purpuratus) which differ in size and spine length. All urchins studied displayed some degree of streamlining, although the threshold water velocity at which the behavior occurred varied among species. Particle capture was highly dependent on urchin size, with S. franciscanus, the largest of the three species studied, capturing the highest total mass across water velocities. However, taking into account urchin size and the changes in particle flux at each water velocity, S. purpuratus was significantly more efficient at capturing particles with “spines up” - an advantage which disappeared once spines were lowered. These results show that size and spine orientation affect how particles interact with urchins in flow and imply that spine morphology plays a role in whether or not an individual adopts a streamlined posture.