Sessile colonial invertebrates that must tolerate environmental variation without the luxury of mobility may be expected to cope with such variation through phenotypic plasticity. While plastic responses to a range of biotic factors (e.g. predation) are increasingly documented, the details of responses to competition (mediated by the densities of conspecifics or heterospecifics vying for the same resources) remain unclear, despite a rich literature for terrestrial plants on which to draw. We examined phenotypic responses to conspecific density in a colonial invertebrate (the arborescent bryozoan, Bugula neritina) under field conditions. We found that colonies at higher densities were generally less fecund, lower in biomass and more elongate than those growing virtually alone. We also found such responses to vary only subtly with the life history stage of neighbours, with colonies exposed to higher densities of contemporary recruits achieving elongation via increased budding between bifurcation points and the elongation of individual zooids, while colonies exposed to established adults achieved elongation via increased budding only. These responses are broadly consistent with those of terrestrial plants competing for light and are probably due to the combined effects of resource limitation and altered flow in high density stands. Future studies are needed to disentangle these effects and the extent to which such responses to density in branching colonial invertebrates are a passive result of food limitation, or reflect active (and potentially adaptive) plasticity to other aspects of the local density environment.