Settlement behavior of cyphonautes larvae of the bryozoan Membranipora membranacea in response to two algal substrata Paul G. Matson, 1,a Benjamin T. Steffen, 2 and Richard M. Allen 3 1 Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, California 93106, USA 2 Integrative Biology Department, University of California Berkeley, Berkeley, California 94720, USA 3 School of Biological Sciences, The University of Queensland, St. Lucia, Queensland 4072, Australia Abstract. Settlement is an important process in the biphasic life histories of many marine invertebrates. Little is known regarding the fine-scale behavioral mechanisms for finding and attaching to a suitable substratum, particularly under conditions that may impose a potential challenge, such as flow. In this study, we examined the settlement behavior of cyphonautes larvae of the bryozoan, Membranipora membranacea, in response to two different algal sub- strata. Larvae showed a strong preference for settling on the kelp Nereocystis luetkeana over the red alga Mazzaella splendens. We then tested whether the behavioral mechanisms used by larvae to attach to these algae differed when presented with the challenge of flowing water during settlement. We found that larvae exhibited different behaviors on the two species of algae in flowing water. Larvae were more often observed in direct contact with the preferred alga (N. luetkeana) exhibiting fine-scale active search behaviors. On the less preferred alga (M. splendens), larvae were less frequently observed in direct contact with the alga, and ap- peared to be exhibiting broad-scale passive search behaviors along the surface of the blade. Our results suggest that cyphonautes larvae alter their behavior in response to their preferred settlement habitat. Additional key words: flow, Mazzaella splendens, metamorphosis, Nereocystis luetkeana The transition from a planktonic larva to a benthic adult is an important event for marine invertebrates with biphasic life histories. Successful settlement al- lows for recruitment into marine populations, and therefore may affect the abundance and distribution of adults (Keough & Downes 1982). As settlement on a suitable substratum is essential for the sur- vival of an individual, a variety of settlement strategies have evolved (Keough & Downes 1982; Pawlik et al. 1991). Competent larvae may passively drift over the benthos relying on chance to find a suitable settlement site (Keough & Downes 1982), or actively seek out particular substrata to settle upon (Abelson 1997). Many larvae respond to specific environmental or chemical cues in order to settle and metamorphose (e.g., Zimmer & Butman 2000; Kingsford et al. 2002; Steinberg et al. 2002; Koehl 2007). Settlement choices have been shown to affect competition and predation, as well as reproductive success, resource acquisition, and survival (Keough & Downes 1982; Keough 1984; McKinney & McKinney 1993; Rodriguez et al. 1993; Babcock & Mundy 1996; Delany et al. 2003). While these previous studies have demonstrated potentially adaptive significance to settlement choice, less infor- mation is available regarding the behavioral mecha- nisms that larvae use to locate suitable sites for settlement at a fine scale. Once a larva contacts a preferred substratum, metamorphosis typically takes place within a rela- tively short time period (Hadfield 2000; Hadfield et al. 2001). However, such a rapid response may be costly if the settlement cue is unreliable. Because there are potentially huge costs from poor choices, natural selection may favor larvae that sample and search the available substrata before making the de- cision to settle (Raimondi & Keough 1990). Indeed, larvae exhibit a variety of settlement behaviors both before contacting a substratum and once a poten- tially suitable substratum is found (Raimondi & Keo- Invertebrate Biology 129(3): 277–283. r 2010, The American Microscopical Society, Inc. DOI: 10.1111/j.1744-7410.2010.00203.x a Author for correspondence. E-mail: pmatson@gmail.com