Title Seasonal variation in utilization of biogenic microhabitats bylittorinid snails on tropical rocky shores

Mobile species may actively seek refuge from stressful conditions in biogenic habitats on rocky shores. In Hong Kong, the upper intertidal zone is extremely stressful, especially in summer when organisms are emersed for long periods in hot desiccating conditions. As a result, many species migrate downshore between winter and summer to reduce these stressful conditions. The littorinids Echinolittorina malaccana and E. vidua, for example, are found on open rock surfaces high on the shore in winter but the majority migrate downshore in summer to the same tidal height as a common barnacle, Tetraclita japonica. In the laboratory, where environmental conditions could be controlled to approximate those occurring on the shore, we tested whether the downshore migration allowed littorinids to select barnacles as biogenic habitats to reduce stress and if this behaviour varied between seasons. In summer, littorinids demonstrated a strong active preference for the barnacles, which was not observed in the cool winter conditions, when animals were found on open rock surfaces even when barnacles were present. Littorinids, therefore, only actively select biogenic habitats during the summer in Hong Kong when they migrate downshore, suggesting that such habitats may play an important, temporal, role in mitigating environmental stress on tropical shores. Introduction The rocky intertidal is a dynamic environmental gradient defined by variation in the duration that organisms spend submersed in seawater or emersed in air at low tide, with associated thermal and desiccation stresses (reviewed in Little et al. 2009). As a result, species are distributed along this gradient according to their ability to withstand these environmental changes (Wolcott 1973; Garrity 1984; Helmuth and Hofmann 2001). Species inhabiting thermally stressful environments utilize a variety of physiological or behavioural responses to minimize their exposure to harmful temperatures. Whilst physiological responses determine the tolerance limits of an organism (Somero 2002; Pörtner and Farrell 2008), behavioural responses such as utilizing refuges, forming aggregations or adopting postures which can minimize heat gain (Garrity 1984; Bauwens et al. 1996, Munoz et al. 2005), can reduce the physiological stress experienced by organisms. Despite these responses, periodically individuals are killed when they are in conditions which exceed their physiological tolerances (e.g. on hot summer days; Wolcott 1973; Chan et al. 2006) especially on tropical shores where species live closer to their thermal limits than their temperate counterparts (Somero 2002, 2010; Tewksbury et al. 2008). Mobile species utilize a variety of behavioural responses to alleviate thermal stress. Mobile gastropods, for example, forage when washed by waves and then hide in cool, damp refuges or aggregate together during emersion (Garrity 1984; Williams and Morritt 1995; Chapman and Underwood 1996). Topographic features such as crevices and rockpools are typically used as refuges; however, the importance of species which act as biogenic habitats (ecosystem engineers, sensu Jones et al. 1997) is becoming increasingly acknowledged (e.g. Seed 1996; Thompson Communicated by F. Bulleri. S. R. Cartwright G. A. Williams (&) The Swire Institute of Marine Science and Division of Ecology & Biodiversity, The School of Biological Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China e-mail: hrsbwga@hkucc.hku. hk 123 Mar Biol (2012) 159:2323–2332 DOI 10.1007/s00227-012-2017-3