A model of particle capture by bryozoans in turbulent flow: significance of colony form.

A hydrodynamic model was developed to examine particle capture by lophophores of encrusting bryozoans. Particle capture rate is predicted to increase with increasing speed of the feeding current. There should be a large feeding advantage when lophophores are tightly packed and excurrents are vented through chimneys. This prediction contradicts conclusions of an earlier model study and suggests that selection for colony integration has a basis in the acquisition of food. If lophophores are not tightly packed, particle-capture patterns depend on two key ratios: the advection ratio (feeding current velocity to shear velocity) and the separation ratio (lophophore spacing to lophophore diameter). At high separation ratios, particle capture rates should be fairly uniform among zooids. At high advection ratios, lophophores located near the upstream colony edge should experience higher rates of particle capture. Rates of particle capture in turbulent flows should greatly exceed those in laminar flows (of identical speed) when excurrent waters are locally remixed into the flow above lophophores. However, when lophophores are tightly packed and excurrents are vented through chimneys, feeding rates should be identical in turbulent and laminar flows. Thus, colonies that vent excurrents through chimneys may be uniquely able to exploit weak laminar flows.