Coloration in relation to ecology in the Asian spiny-backed spider, Thelacantha brevispina (Araneae, Araneidae) on Moorea, French Polynesia

COLORATION IN RELATION TO ECOLOGY IN THE ASIAN SPINY-BACKED SPIDER, THELACANTHA BREVISPINA (ARANEAE, ARANEIDAE) ON MOOREA, FRENCH POLYNESIA HUA S. TRUONG Integrative Biology, University of California, Berkeley, California 94720 USA Abstract. Color polymorphism is a widespread phenomenon in many arthropods, including spiders. The persistence of these visible, readily measured polymorphisms over time suggests that they carry adaptive functions and thus are maintained by natural selection. Studying polymorphism is useful for examining evolution in action and the different modes of selection operating to maintain different morph patterns. The color polymorphic spiny-backed spider, Thelacantha brevispina, was surveyed on Moorea, French Polynesia to understand distribution, prey abundance, and density across coastal and forest habitats. In addition, split-brood and laboratory reciprocal transplant experiments were conducted to determine whether coloration was due to genes or a result of phenotypic plasticity. Quantitative field surveys revealed a negative relationship between spider density and prey abundance, with lower prey counts in coastal habitats but prey yielded higher energy value. Spiders were found on a variety of habitats with diverse prey species. The split-brood design showed that a genetic origin of coloration is likely with strong maternal effects and distinct color differences among sites. The reciprocal transplant confirmed the lack of a plastic response to rearing habitats. The results underscore the importance of ecological aspects in understanding the evolution and maintenance of coloration in T. brevispina spiders. Key words: color polymorphism; Thelacantha brevispina; genetic polymorphism; phenotypic plasticity; density; prey abundance; Moorea, French Polynesia I NTRODUCTION Polymorphism is the occurrence of multiple discontinuous forms or morphs among individuals within an interbreeding population (O’Neill and Beard 2011). Visible polymorphisms, which can be readily observed and measured, have long been the focus of research in evolutionary biology to understand intraspecific variations (Huber and Hopf 2004). Examples of this widespread phenomenon include color and banding in the land snails (Jones et al. 1977), wing patterns in the butterflies (Baxter et al. 2008), and melanism in the peppered moth (Bishop and Cook 1980). The persistence of these polymorphisms over time suggests that they carry adaptive functions thus are maintained by natural selection (Gillespie and Oxford 1998). Studying polymorphism and its adaptive values may be useful for examining the role of evolutionary processes in maintaining them and also the different modes of selection operating on different morph patterns. Color and pattern may vary considerably among and within animal species. One striking example is color polymorphism, the occurrence of multiple discrete color variants within a population (Agrawal 2001). Such variation in color may be a direct reflection of differences among individuals as a consequence of genetic variation (i.e. genetic polymorphism) or may reflect a plasticity of