Refuge utilization and preferences between competing intertidal crab species Evie A. Wieters ⁎, Emilie Salles, Stella M. Januario, Sergio A. Navarrete Estación Costera de Investigaciones Marinas & Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Católica de Chile, Casilla 114-D, Santiago, Chile abstract article info Article history: Received 16 July 2008 Received in revised form 26 January 2009 Accepted 7 April 2009 Keywords: Chile Habitat modification Habitat preference Interference competition Refuges Turf algae Many invertebrates avoid predation risk by seeking and defending refuges that can be in limited supply, producing strong intra- and inter-specific interference competition. Previous experimental studies in central Chile demonstrated that interference competition for refuges is the primary factor driving habitat segregation between the predatory crabs Acanthocyclus gayi and A. hassleri, with the latter species monopolizing galleries inside mussel beds in the mid intertidal zone and limiting A. gayi to rock crevices. Yet, habitat partitioning between rival species can result from differences in habitat preferences and not solely from interference interactions. Moreover, since A. gayi is also known to shelter in turf-forming algae (predominantly Gelidium), which dominates extensive areas in the low intertidal zone, among-sites variation in the turf morphology and abundance could modify habitat preferences and the pattern of inter-specific interactions. We experimentally evaluated refuge habitat preferences of individual, similarly-sized adult A. gayi and A. hassleri in the laboratory, comparing choice patterns across multiple trials with paired combinations of the main refuge types commonly used by crabs in the field: a) mussel galleries, b) rock crevices, c) short algal turf, and d) tall algal turf. Our results showed that both species display a strong ranking of preferences for some refuge habitats over others. In general, mussel galleries were the preferred refuge type for both crab species, but their preference rankings changed depending on turf morphology. When turf was short, A. hassleri and A. gayi made identical refuge choices, strongly preferring mussel galleries over crevices and these over the short turf. In contrast, when the turf was tall A. gayi selected equally the tall turf or mussel galleries, and these were strongly preferred over crevices. A. hassleri, on the other hand, largely ignored tall turf and kept the highest preference for mussel galleries. A field experiment in which crabs were offered to foraging birds demonstrated that A. hassleri is more susceptible than A. gayi to predation by kelp gulls when outside refuges. Differences in patterns of coloration between crabs may underlie between-species differences in predation susceptibility and their habitat choices. These results suggest that the among-site differences in turf morphology (height and shape of fronds), which is largely driven by varying intensity of upwelling, could affect crab preferences for refuge habitats and the relative importance of inter-specific interference competition. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The partitioning of limited local resources by different species is an effective way of reducing inter-specific competition and promoting coexistence at a local scale (e.g. MacArthur and Levins, 1967; Amarasekare, 2003). Thus, patterns of segregation of either prey and/or habitat in space and time by similar species have been amply documented on marine rocky shores. While under many circum- stances predation can act as a selective source of mortality, facilitating coexistence among competitors (Paine, 1966; Connell, 1978; Menge et al., 1994), in many cases predation can increase competition between prey species (Chase et al., 2002). Such is the case when predation risk forces mobile invertebrates to seek refuges and these refuges are in limited supply. Agonistic interactions and direct interference competi- tion is then common, and defending refuges can involve a high fraction of the energy and time of individuals (Kneib, 1987; Steger, 1987; Navarrete and Castilla, 1990; Moksnes, 2002, 2004). Differential patterns of habitat use between species can also result from interspecific differences in habitat preferences, which might be completely unrelated to competition in the past (Wiens, 1977; Connell, 1980). Provided that the preferred habitat or refuge-type offers a fitness advantage for each species, different habitat prefer- ences could be selected for, and inter-specific competition would not be necessary to maintain differences in habitat use among species. In this case, individual refuges can still be aggressively defended against con- and hetero-specifics in the rare occasion that individuals encounter each other. Therefore, results of short-term behavioral experiments confronting species in a given habitat or refuge-type can be misleading without further information. Indeed, habitat Journal of Experimental Marine Biology and Ecology 374 (2009) 37–44 ⁎ Corresponding author. Tel./fax: +56 35 431670. E-mail address: ewieters@bio.puc.cl (E.A. Wieters). 0022-0981/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2009.04.006 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe