Impact of invasive Tubastraea spp. (Cnidaria: Anthozoa) on the growth of the space dominating tropical rocky-shore zoantharian Palythoa caribaeorum (Duchassaing and Michelotti, 1860)

Competition for space directly affects the structure of the sessile benthic communities on hard substrates. On the Brazilian coast Palythoa caribaeorum is an abundant shallow water mat-forming zoantharian and has fast growth rates. The objective of the present study was to assess the zoantharian's biotic resistance by investigating changes in growth rates when interacting with the invasive sun corals Tubastraea tagusensis and T. coccinea on a tropical rocky shore in southeast Brazil. In the experimental design, eight blocks were arranged along the rocky shore where the growth treatment was applied with four different levels: empty, skeleton, live T. coccinea and live T. tagusensis. We opened up 10 x 10 cm(2) spaces in the P. caribaeorum mat into which treatments were transplanted with epoxy putty. Growth measures of the zoantharian were obtained using photographs. During the 184 days of the experiment, P. caribaeorum had the highest total growth rate in the empty treatment with an average value of 2.124 +/- 0.189 mm.mo(-1); the treatment with the lowest total growth rate was T. coccinea with mean value of 1.104 +/- 0.117 mm.mo(-1). The presence of T. coccinea influenced the growth rate of the zoantharian, unlike its congeneric T. tagusensis, which did not significantly influence the growth rate of P. caribaeorum. The negative interactions between P. caribaeorum and the invasive Tubastraea spp. were species specific in nature, and the native zoantharian exhibited quite some biotic resistance to the invasive species, especially to T. tagusensis. This was due to competition, probably mediated both through physical and/or chemical mechanisms and preemptive occupation of space. Growth rates of P. caribaeorum were dependent on the degree of competitive interaction as well as seasonal differences in light and/or temperature which remain to be elucidated. From a management perspective, the manual removal of Tubastraea spp. will allow native species such as P. caribaeorum to recover space and thus foster ecosystem recovery.