A model of feeding currents in encrusting bryozoans shows interference between zooids within a colony

A potentially important consequence of colony morphology in colonial suspension feeders such as bryozoan colonies is the hydrodynamic interaction between modules (zooids). In this paper, a detailed numerical model of water flow through an encrusting bryozoan colony is developed, using the theory of flow around slender bodies in slow flow (low Reynolds number), which calculates flow properties everywhere within a colony. Ciliated tentacles are represented by an extension of previous slender body theory to include cylinders with a “traction velocity” at their surfaces. The traction velocity, which is tangent to the cylinder surface, represents the macroscoic flow induced by ciliary action. This modeling approach makes it possible to calculate the self-induced flow through a complex geometry—in this case a colony of up to 121 zooids—which would be computationally difficult using mesh-based methods. Calculations of flow through model colonies indicate that there are strong, negative hydrodynamic interactions between zooids in a colony of this type. Zooids interfere with one another because areas of high shear beneath the lophophores cause viscous resistance to excurrent flow, resulting in high pressure in the interior of colonies. The ability of interior zooids to pump water through the lophophores is thereby greatly reduced. In contrast, a linear arrangement of zooids, as found in less-integrated and stoloniferous colonies, greatly reduces interference. Strong hydrodynamic interference between zooids suggests that there must exist other functional factors which enable highly-integrated colonies to compete successfully with arborescent and stoloniferous forms.