Mar Biol (2008) 155:273–280 DOI 10.1007/s00227-008-1023-y ORIGINAL PAPER Macroalgal morphology mediates particle capture by the corallimorpharian Corynactis californica Kathleen M. Morrow · Robert C. Carpenter Received: 21 June 2007 / Accepted: 18 June 2008 / Published online: 10 July 2008 © Springer-Verlag 2008 Abstract The shallow kelp forest at Santa Catalina E. arborea canopy did not aVect particle capture. However, Island, California (33.45 N, ¡118.49 W) is distinguished D. undulata, found predominantly outside of the canopy, by several canopy guilds ranging from a Xoating canopy inhibited particle capture rates by 40% by redirecting parti- (Macrocystis pyrifera), to a stipitate, erect understory can- cles around C. californica polyps and causing contraction opy (Eisenia arborea), to a short prostrate canopy just of the feeding tentacles. These results suggest that the mor- above the substratum (Dictyopteris, Gelidium, Laminaria, phology of Xexible marine organisms may aVect the distri- Plocamium spp.), followed by algal turfs and encrusting bution and abundance of adjacent passive suspension coralline algae. The prostrate macroalgae found beneath feeders. 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 Introduction under E. arborea canopies than outside (approximately 1,200 versus 300 polyps m¡2 respectively). Morphological Benthic suspension feeders occupy the majority of avail- diVerences in macroalgae between these habitats may aVect able hard substratum in subtidal rocky reef environments, the rate of C. californica particle capture and serve as a which results in frequent competitive and facilitative inter- mechanism for determining polyp distribution and abun- actions among neighboring organisms for access to space, dance. Laboratory experiments in a unidirectional Xume light, and/or food particles (Jackson 1977; Knowlton and under low (9.5 cm s¡1) and high (21 cm s¡1) Xow speeds Jackson 2001). Community structure of sessile biota is examined the eVect of two morphologically distinct macro- mediated in part, by the strength of these interspeciWc inter- algae on the capture rate of Artemia sp. cysts by C. califor- actions. We examine here how diVerences between mor- nica polyps. These experiments (January–March 2006) phologically distinct macroalgal taxa can interact with tested the hypothesis that a foliose macroalga, D. undulata, water Xow to mediate the particle capture abilities of adja- would inhibit particle capture more than a branching alga, cent suspension feeders. G. robustum. G. robustum, found predominantly under the Although cnidarians co-exist with macroalgae on a wide variety of substrata in tropical and temperate marine habi- tats, few studies have demonstrated the competitive or Communicated by J.P. Grassle. facilitative mechanisms by which these two groups interact (Jompa and McCook 2002). Algal morphology and Xexibil- & K. M. Morrow ( ) ity has important implications for cohabitants through the Department of Biological Sciences, Auburn University, 101 Rouse Life Sciences Building, Auburn, AL 36849, USA alteration of water Xow, which can modify the rate of sedi- e-mail: [email protected] mentation, gas exchange, and food delivery (Eckman et al. 1989; Hurd and Stevens 1997; Irving and Connell 2006). R. C. Carpenter Previous mechanistic studies have focused primarily on Department of Biology, California State University Northridge, 18111 NordhoV St., Northridge, CA 91330-8303, USA symbiotic hermatypic corals and mechanisms of algal over- e-mail: [email protected] growth. In the present study, Corynactis californica does 123 274 Mar Biol (2008) 155:273–280 not have photosynthesizing symbionts and relies exclu- exposed habitats where E. arborea is found (Charters et al. sively on heterotrophic feeding. Therefore, experiments 1968). High wave energy constantly moves the fronds and allow us to test speciWcally the mechanisms by which reduces shading and sedimentation beneath the canopy, algae inXuence feeding success when uninhibited by algal while scouring the substratum and supplying inorganic shading. nutrients to understory algae and benthic invertebrates Water Xow determines the delivery rate of food particles (Kennelly 1989; Eckman and Duggins 1991; Connell to suspension feeders (Sebens et al. 1997). The potential 2003a; Connell 2003b). food available for capture is determined by the Xux of parti- Our previous studies suggested that the presence of mac- cles past the feeding surface, which is a function of particle roalgae has a positive eVect on the abundance of C. califor- concentration and the rate at which particles are encoun- nica, both underneath the E. arborea canopy and in open tered by the feeding surface (Okamura and Partridge 1999). habitats (Morrow and Carpenter 2008). However, a signiW- If all other variables are held constant, an increase in Xow cantly greater density of C. californica polyps (fourfold will increase the number of particles exposed to the feeding diVerence) and a lower density of macroalgae are found surface. However, the delivery of particles is disrupted by under E. arborea canopies. Predominant macroalgal taxa roughness elements such as the shape, size, Xexibility and under the E. arborea canopy were upright, branching red proximity of associated organisms and bottom topography species, while, foliose and decumbent brown species were (McFadden 1986; Koehl 1996; Sebens et al. 1997). The found outside the canopy (Morrow and Carpenter 2008). functional form and density of macroalgae can inXuence The present study investigated whether morphologically macro- and microhabitat water Xow in benthic marine envi- distinct macroalgae selected from under and outside the E. ronments, thus mediating food delivery to adjacent suspen- arborea canopy could inXuence the rate of particle capture sion feeders (Anderson and Charters 1982; Johnson 2001). by C. californica and thereby alter their distribution and In the present study C. californica (the strawberry anem- abundance between habitats. one) was used as a model suspension feeder to examine the We hypothesized that the branching red alga, Gelidium eVect of two distinct macroalgae on the rate of particle cap- robustum, would inhibit particle capture to a lesser extent ture. These anemones form large aggregations of small than the foliose brown alga, Dictyopteris undulata, due to polyps (<15 mm diameter) on rocky substrata in kelp forest diVerences in morphology and Xexibility. G. robustum and communities oV the northeastern PaciWc from British D. undulata were chosen because they represent the most Columbia to Baja, California (Chadwick 1987). They abundant species found under and outside the E. arborea reproduce asexually by longitudinal Wssion and sexually canopy, respectively. We also hypothesized that rates of C. with a seasonal cycle of gametogenesis, producing plank- californica particle capture would be lower at high Xow tonic larvae from November through January (Holts and (21 cm s¡1) than at low Xow speeds (9.5 cm s¡1), due to Beauchamp 1993). Compared to scleractinian and actiniar- deformation of the polyp tentacles and inXuence of Xexible ian Anthozoa, C. californica has a rapid rate of asexual macroalgal thalli as drag increased (Koehl 1977). reproduction, doubling clonal populations in <2 mo and Gelidium robustum has a single holdfast and thalli that reaching densities of t2,400 polyps m¡2 on horizontal sub- are pinnately branched with an irregular pattern of closely strata, interspersed with macroalgae, at Santa Catalina spaced cylindrical branches which are stiV and cartilagi- Island, California (Chadwick 1987; Chadwick and Adams nous (Graham and Wilcox 2000). Although G. robustum 1991; Morrow and Carpenter 2008). Large asexual aggre- fronds are closely spaced, the pattern is suYciently open to gations of unknown size may be distinguished by color allow the Xow of water through the thalli (Anderson and morph (pink, brown, orange, and white), although the basis Charters 1982). The mesh-like structure and narrow branch for this color variation is unknown (Holts and Beauchamp diameter of species similar to G. robustum slow Xow and 1993). The polyps are consistently spaced »8 mm apart on dampen turbulence at Xow speeds <12 cm s¡1, transitioning horizontal surfaces at Santa Catalina Island. The distance to turbulent Xow at speeds >14 cm s¡1 (Anderson and between nearest neighbors was measured using vernier cal- Charters 1982; Hurd and Stevens 1997). Turbulent Xow can ipers in haphazardly placed quadrats (Morrow 2006). create recirculating eddies behind algal thalli and may Shallow, wave-exposed environments at Santa Catalina lengthen the time that particles spend in the vicinity of Island are distinguished by stipitate kelps such as Eisenia polyp tentacles, thereby increasing capture rates. D. undu- arborea, as well as diverse assemblages of foliose and lata is a foliose brown alga with thalli that are Xattened and geniculate coralline macroalgae. E. arborea occurs highly branched in a repeatedly dichotomous pattern and throughout temperate regions of the PaciWc Ocean and can attached to the substratum by a small holdfast (Graham and strongly inXuence abiotic factors (e.g. light and water Xow) Wilcox 2000). Flexible algal thalli, as in D. undulata, can aVecting the community structure of understory species. be deformed passively into streamlined shapes at high Xow Maximum water