Macroalgal morphology mediates particle capture by the corallimorpharian <Emphasis Type="Italic">Corynactis californica</Emphasis>

The shallow kelp forest at Santa Catalina Island, California (33.45 N, −118.49 W) is distinguished by several canopy guilds ranging from a floating canopy (Macrocystis pyrifera), to a stipitate, erect understory canopy (Eisenia arborea), to a short prostrate canopy just above the substratum (Dictyopteris, Gelidium, Laminaria, Plocamium spp.), followed by algal turfs and encrusting coralline algae. The prostrate macroalgae found beneath E. arborea canopies are primarily branching red algae, while those in open habitats are foliose brown algae. Densities of Corynactis californica, are significantly greater under E. arborea canopies than outside (approximately 1,200 versus 300 polyps m⁻² respectively). Morphological differences in macroalgae between these habitats may affect the rate of C. californica particle capture and serve as a mechanism for determining polyp distribution and abundance. Laboratory experiments in a unidirectional flume under low (9.5 cm s⁻¹) and high (21 cm s⁻¹) flow speeds examined the effect of two morphologically distinct macroalgae on the capture rate of Artemia sp. cysts by C. californica polyps. These experiments (January–March 2006) tested the hypothesis that a foliose macroalga, D. undulata, would inhibit particle capture more than a branching alga, G. robustum. G. robustum, found predominantly under the E. arborea canopy did not affect particle capture. However, D. undulata, found predominantly outside of the canopy, inhibited particle capture rates by 40% by redirecting particles around C. californica polyps and causing contraction of the feeding tentacles. These results suggest that the morphology of flexible marine organisms may affect the distribution and abundance of adjacent passive suspension feeders.