Suspension Feeding Adaptations to Extreme Flow Environments in a Marine Bryozoan

We describe the effects of extreme flow on the growth and morphology of a bryozoan, Membranipora membranacea, encrusting laminarian fronds in the Rapids of Lough Hyne (=Ine), County Cork, Ireland. An ultrasonic current meter was used to characterize ambient flow regimes at the level of the algal canopy over a complete tidal cycle at three sites within the Rapids. Colonies collected from sites exposed to different flows showed a trend towards miniaturization with increased flow: the zooids were less elongate, the lophophores were smaller in diameter and had fewer tentacles, and the distances between excurrent jets were shorter. These morphological changes probably place feeding surfaces into slower flow regimes of the boundary layer. Similar growth rates of colonies at sites differing in flow provide evidence that this miniaturization is adaptive and that bryozoans are capable of adopting appropriate morphological responses to varying environmental regimes. Such plasticity should be considered when assessing feeding from different flow regimes because particular colonies may be adapted to a limited and specific range of flow conditions.