Meeting Abstract

80.2  Sunday, Jan. 6  Do Wave Impact Forces Limit the Size of Intertidal Organisms? JENSEN, M.M.*; DENNY, M.W.; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University mmjensen@stanford.edu

Although intertidal organisms are generally much smaller than their terrestrial and subtidal counterparts, the biological and physical mechanisms that limit size have not been determined. While hydrodynamic forces due to breaking waves are theorized to limit size, models based on drag and accelerational forces are poor predictors of maximum sizes in the field. However, these models may be incomplete: the transient force occurring at wave impact — the impingement force — is neither well-characterized nor included in current size-prediction models. Impingement may limit organism size through differing scaling exponents: tenacity is generally proportional to attachment area (the square of characteristic length). If the largest force experienced by the organism also scales with area, there is no theoretical size limit as the organism grows isometrically. If maximum force is proportional to organism volume, force increases with the cube of characteristic length as the organism grows. In this case, force increases at a faster rate than attachment strength, potentially limiting the organism’s size. While scaling behavior of other hydrodynamic forces is known, the scaling exponent of impingement has remained unstudied. To test the scaling behavior of impact forces, rectangular prisms of various sizes were exposed to impingement forces using simulated waves from a gravity-driven water cannon. Data show that impingement scales with both area and drag coefficient of shapes tested. Analysis of the water cannon jet shows a spike in jet velocity concommittent with measured impingement forces, further suggesting that transient wave impacts are caused by brief increases in drag. Thus, impingement is not likely to limit organism size — leaving intertidal size limitation a mystery.