Meeting Abstract
Reef corals are symbiotic assemblages that can derive up to 95% of their carbon demands from endosymbiotic dinoflagellates in the ‘genus’ Symbiodinium. The coral-algal symbiosis also relies on other inter-species interactions, including the mutualistic symbiosis between corals in the genus Pocillopora and crabs in the genus Trapezia. The exosymbiotic Trapezia live among branches of the coral, and benefit their host by deterring corallivores and removing sediment and debris. In return Pocillopora not only shelter, but also feed their Trapezia with photosynthetically-derived lipids translocated into tentacle tips, which the crabs clip and consume. As warming seas threaten temperature-sensitive multi-species coral assemblages, we know that genetically distinct Symbiodinium can confer distinct thermal tolerance upon the coral-algal symbiosis, but little work has highlighted the cascading effects of such symbionts on other coral associates. Here we show that in a high-temperature coral reef habitat, physiological distinctions between Symbiodinium genotypes carry effects beyond the coral-algal symbiosis and affect the fitness of associated fauna, including Trapezia. We show that in high temperature back reef pools with high proportions of heat-resistant Symbiodinium D, those Trapezia living with Pocillopora hosting the more thermally sensitive Symbiodinium C1 are smaller and less fecund than those associated with corals hosting the more heat-resistant Symbiodinium D. As smaller Trapezia are less effective at fending off corallivores, by not hosting thermally robust Symbiodinium in this high-temperature habitat a coral not only increases its risk of direct physiological effects from thermal stress, but also undermines its ‘defense force’, thus increasing its risk of predation.
