The marine bryozoan Electra pilosa typically inhabits ephemeral substrata in the intertidal and shallow subtidal, and is probably the ecologically most successful bryozoan species in British waters. Modular organisms like E. pilosa frequently evolve pronounced phenotypic plasticity to cope with the ecological challenges resulting from passive larval dispersal into unpredictable habitats, and temporal variability of the environment colonized by the immobile adult stage. E. pilosa colonies on wave-exposed shores differ morphologically from those found on sheltered shores in possessing numerous long-spined zooids. The present study demonstrates that spine formation in E. pilosa is environmentally inducible by wave-related abrasion by macroalgae; additionally, the spines also have a fortuitous anti-predator effect in discouraging predation by the nudibranchs Adalaria proxima and Polycera quadrilineata. It is suggested that the inducible spines of E. pilosa constitute an adaptation for the protection of feeding polypides in high-energy environments, and that plasticity for the trait is of adaptive value in this organism which exploits a diverse range of habitats. Although a number of traits in this species clearly are subject to considerable phenotypic plasticity, other attributes apparently are highly deterministic, heritable and genotype-specific. Electra pilosa displays pronounced among-genotype variation in colony growth rate, and the present study shows that this variation is due to proximate factors which affect growth rate and covary with genotype. This study also presents the first evidence of senescence at the zooid level in E. pilosa: Zooids deteriorate systematically over time, as indicated by decreasing polypide life spans and increasing polypide regeneration times, but in contrast to this, whole-organism senescence does not appear to occur in this species.