Macroalgal morphology mediates particle capture by the corallimorpharian Corynactis californica

Abstract

The shallow kelp forest at Santa Catalina Island, California (33.45 N, i118.49 W) is distinguished by several canopy guilds ranging from a Xoating canopy (Macrocystis pyrifera), to a stipitate, erect understory can- opy (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. Densi- ties of Corynactis californica, are signiWcantly greater under E. arborea canopies than outside (approximately 1,200 versus 300 polyps m i2 respectively). Morphological diVerences in macroalgae between these habitats may aVect the rate of C. californica particle capture and serve as a mechanism for determining polyp distribution and abun- dance. Laboratory experiments in a unidirectional Xume under low (9.5 cm s i1 ) and high (21 cm s i1 ) Xow speeds examined the eVect of two morphologically distinct macro- algae on the capture rate of Artemia sp. cysts by C. califor- nica 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 aVect particle capture. However, D. undulata, found predominantly outside of the canopy, inhibited particle capture rates by 40% by redirecting parti- cles around C. californica polyps and causing contraction of the feeding tentacles. These results suggest that the mor- phology of Xexible marine organisms may aVect the distri- bution and abundance of adjacent passive suspension feeders.