Invasive seaweed enhances recruitment of a native bivalve: roles of refuge from predation and the habitat choice of recruits

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Invasive species may have a range of negative effects on native species in the region invaded. The invasive green alga Caulerpa taxifolia has invaded several temperate regions worldwide and now occurs in 9 estuaries in temperate eastern Australia. Despite the threat posed by C. taxifolia, virtually nothing is known of its effects on native estuarine infauna. In the present study, we investigated the distribution and abundance, habitat choice and predation of recruits (post-set juveniles) of the native Sydney cockle Anadara trapezia at 2 sites invaded by C. taxifolia in Lake Conjola, New South Wales, Australia. Recruitment of A. trapezia was significantly higher in C. taxifolia (both with sparse [30 %] and with dense [100 %] cover) than in Zostera capricorni and bare sediment. Up to 680 recruits m-2 were observed in C. taxifolia, with the highest recruit densities occurring at intermediate C. taxifolia densities. However, in habitat choice experiments, recruits showed no preference for C. taxifolia over the seagrasses Z. capricorni and Halophila ovalis, but a strong preference for adult A. trapezia over all macrophytes when A. trapezia were included as treatments in experiments. Field data showed that adult A. trapezia in bare sediments had very few recruits attached to them (<0.07 recruits adult-1) compared to adults found within C. taxifolia (0.95 ± 0.25 and 1.23 ± 0.31 recruits adult-1 at the 2 sites). Given that recruits displayed a strong preference for adults, but were rare on adults living in bare sediments, we tested whether C. taxifolia provided a refuge from predation. Recruits on adults placed into a bare sediment habitat were all consumed by predatory fish within 15 min, whereas none of the recruits on adults in an adjacent C. taxifolia habitat were eaten. Our results show that recruits do not reject C. taxifolia as a recruitment substratum despite showing preference for adult conspecifics, and that C. taxifolia may, by providing recruits with a refuge from predation enhance the recruitment of A. trapezia compared to uninvaded sediment. Although the long-term consequences of this enhanced recruitment are unknown, our study demonstrates that, contrary to commonly held views, the effects of C. taxifolia are not always negative. © Inter-Research 2006.