Mechanical Organization of Cantileverlike Sessile Organisms: Sea Anemones

Engineering beam theory has been used to analyse the ways in which body shape and elastic modulus of two species of sea anemones affect their mechanical responses to flow.

1. 1.

   Anthopleura xanthogrammica is exposed to wave action, but because it is short, wide, and thick-walled, maximum tensile stresses in its body walls due to flow forces are an order of magnitude lower than those in the tall, slim, thin-walled, calm-water sea anemone Metridium senile.

2. 2.

   The elastic modulus of M. senile body wall is more dependent on extension rate than is that of A. xanthogrammica. Because the extension rate of M. senile body wall in tidal currents is higher than that of A. xanthogrammica in wave surge, the moduli of walls from these species when exposed to such flow conditions are similar, between 0.1 and 0.3 MN.m⁻².

3. 3.

   The flexural stiffness of M. senile is lowest in the upper column where the anemones bend in currents: this orients their filter-feeding oral discs normal to the currents. The flexural stiffness of A. xanthogrammica is one to two orders of magnitude higher than that of M. senile; A. xanthogrammica remain upright in wave surge and feed on mussels that fall on their oral discs.

4. 4.

   The deflexions of these anemones predicted using beam theory are consistent with those observed in nature.

5. 5.

   The critical stress to produce local buckling is an order of magnitude lower for M. senile than for A. xanthogrammica.

6. 6.

   Several general principles of the organization of cantilever-like sessile organisms are revealed by this study of sea anemones.