Abstract
Mobile intertidal animals exhibit various strategies during emersion to mediate the impact of heat and desiccation, including behavioural adaptations such as moving to lower tidal levels and seeking thermal refuges, which can result in spatial partitioning between species within the intertidal environment. We tested whether the limpets Siphonaria guamensis (Heterobranchia) and Patelloida saccharina (Patellogastropoda) exhibited differential habitat use during tidal emersion by quantifying their abundance and size distribution in various habitats on two rocky shores on the west coast of Thailand. S. guamensis inhabited higher shore levels with hotter average rock temperatures when emersed as compared to P. saccharina. On one of the shores, large S. guamensis lived at higher tidal levels than smaller individuals, whereas large P. saccharina showed the reverse pattern, being found lower on the shore than smaller individuals. The abundance of S. guamensis was positively correlated to the shore slope, with more individuals found on vertical than horizontal rocks, while P. saccharina showed a negative relationship between abundance and algal cover. At the heights where they were most abundant, both species were more often found in bare rock habitats as opposed to crevices and areas dominated by oyster shells, despite the fact that bare rock was as hot as or even hotter than other microhabitats. The exact resting locations of the two species were, however, cooler than the mean temperature of the bare rock. In general, limpets did not exhibit a strong preference for any particular rock orientation, but S. guamensis on one shore was more abundant on east-facing rocks as compared to other aspects. As a result, although thermal stress appears to be a driver for habitat partitioning between species (occupying different tidal heights), temperature alone is unable to explain distribution patterns within species, as limpets were not adopting thermal refuges during tidal emersion. Variations in the physical environments may be mediated by species-specific morphological and/or physiological adaptations which determine the distribution of different limpet species on western Thailand rocky shores.