Colony growth rate of encrusting marine bryozoans ( Electra pilosa and Celleporella hyalina)

Abstract

Growth experiments were performed in a strong current environment (Menai Strait, UK) with cloned, transplanted colonies of encrusting Electra pilosa and Celleporella hyalina growing on microscope slides mounted on a raft. Colony growth was also recorded for the same species, both appearing naturally on macroalgae and in controlled laboratory experiments. For the laboratory experiments, the lowest algal concentration that still resulted in maximum growth rate (about 0.07 day −1) for E. pilosa was at most 1500 Rhodomonas cells ml −1, corresponding to 1.9 μg chl a l −1, close to but greater than the mean value measured in the Menai Strait. For the raft experiments, colonies were exposed to naturally occurring fast flow (about 40 cm s −1) and to reduced flow (about 30 cm s −1). The mean phytoplankton concentration was 1.1±0.6 μg chl a l −1, and the specific growth rate was 0.10–0.12 day −1, regardless of species and flow regime. The somewhat slower growth of natural bryozoan colonies on macroalgae (0.08 day −1) may be due to their larger initial size. Based on estimates of the thickness of the viscous sublayer it is concluded that suspension feeding by encrusting bryozoans may be restricted to the viscous sublayer, and that increasing current velocities do not reduce the growth (and thus feeding).