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.KEY WORDS: Branching · Colony form · Density · Foraging behaviour · Intraspecific competition plasticity · Bryozoa Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 415: [83][84][85][86][87][88][89][90] 2010 the same fixed resource -is widely believed to determine the structure of many benthic communities (Jackson 1977, Vermeij & Sandin 2008.To date, research on phenotypic responses to density in such systems has emphasised changes in overall growth and reproductive output, especially in encrusting bryozoans and ascidians (e.g. Sebens 1982, Buss 1990, Stocker & Underwood 1991, Dias et al. 2008. As expected, individuals at lower densities with access to more space (and hence to more energy and nutrient sources like sunlight, suspended food particles or dissolved chemical compounds; Sebens 1982) often grow faster, larger and more fecund than crowded individuals. In other cases, however, this expectation is not met (Stocker & Underwood 1991, Abdo et al. 2008, possibly because adverse effects of competition on performance are ameliorated by responses in other (e.g. morphological) traits. For example, barnacles at high conspecific densities may express a tall 'hummocked' morphology that improves access to suspended food particles (Bertness et al. 1998). Similarly, encrusting bryozoans that are encroached upon may start to produce stolons that slow the ...