Date of Award

Spring 2012

Document Type

Restricted Thesis

Terms of Use

© 2012 Katherine C. Cushman. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts



First Advisor

Rachel Merz


Phenotypic plasticity is one mechanism by which intertidal organisms can thrive in a variety of local flow environments. The barnacle Balanus glandula exhibits plasticity in both its feeding behavior and the morphology of its feeding cirri. In general, barnacles feed actively at lower flow speeds, but transition to feeding passively at higher speeds, thus taking advantage of ambient currents instead of using metabolic energy to create flow. Additionally, individuals in habitually low flow environments have longer and thinner cirri than barnacles in high flow regimes. The slender cirri of barnacles from low flow areas deform in fast flow speeds, while barnacles from high flow areas can still feed with their shorter cirri. I hypothesized that long thin cirri are advantageous for feeding in slow velocities, where effects of viscosity limit how well suspension feeders can capture food. Compared to shorter cirri, I expected that longer cirri allow barnacles to feed more passively in slower flows and to alter the flow regime at the tips of their feeding appendages to decrease effects of viscosity. I compared barnacles from two areas of Argyle Lagoon, WA - a tidal channel (flows ranging from 0-79 cm/sec) and an adjacent bay (flows ranging from 0-6 cm/sec). These areas share the same water supply, so the two groups of barnacles came from the same larval pool and experience the same temperature and food source. In a flow tank, barnacles were exposed to water velocities varying from 0-10 cm/s. Long cirri barnacles fed most passively at flow speeds of 4-9 cm/sec, with a decreasing ability to feed in high flows already apparent at the fastest speed of 10 cm/sec. In contrast, short cirri barnacles fed most passively at the fastest flow speeds of 7-1O cm/sec. If passive feeding requires less energy than active feeding, then barnacles benefit from the ability to feed passively in the flows they regularly encounter. Having long thin cirri reduces the total range of velocities at which a barnacle can feed, however this cirral form allows barnacles to feed using less energy in slower flows. Additionally, my calculations show that cirral elongation allows barnacles to increase the velocity at the tips of their feeding appendages relative to flow. This increase in speed can help barnacles overcome limitations to feeding caused by viscous forces.