A fundamental morphological and ecological division among sessile colonial invertebrates and sponges is that between encrusting and erect growth habits. The distributions of such organisms results both from their patterns of larval recruitment onto the substratum and their subsequent growth and interactions (Buss 1979b; Jackson 1979a). Sessile encrusting organisms [runners, sheets, and mounds of Jackson (1979a)] grow primarily out over the substratum. Such growth sets no special mechanical constraints for colony support and growth is potentially infinite, although limited by extrinsic factors such as the extent of the substratum, interactions with other organisms, or physical environmental factors. The alternative pattern is one of growth primarily up or away from the substratum surface [plates, vines, and trees of Jackson (1979a)]. This may set overall size constraints relative to mechanical support and attachment (Cheetham 1971; Cheetham et al. 1981; Cheetham and Thomsen 1981; Schopf et al. 1980), often with approximately determinate growth as for solitary animals (Jackson 1977a, 1979a).