3.2 Endosymbiosis with photosynthetic dinoflagellates (Symbiodiniaceae)

Many members of the phylum Cnidaria, including corals, octocorals, sea anemones and hydrocorals, host unicellular dinoflagellate endosymbionts (i.e. zooxanthellae) belonging to the family Symbiodiniaceae [110]. These associations are often obligatory and of fundamental importance to coral reef ecosystems as they enhance the growth of calcifying corals that form the reef. For instance, the zooxanthellae contribute to host nutrition (up to 95% of the energy requirements in scleractinian corals [111]) and skeletogenesis by providing photosynthetically fixed carbon, while the cnidarian host provides inorganic nutrients and refuge from herbivory to its symbionts [112–114]. Previous studies have demonstrated that the association of Cnidaria–Symbiodiniaceae is not stochastic, but mostly determined by host phylogeny and geography [115, 116]. Like scleractinian corals, hydrocorals feed heterotrophically on a variety of resources (mostly planktonic feeders [51, 117]) and rely on a mutualistic symbiosis with Symbiodiniaceae algae for autotrophic nutrition and calcification [118, 119]. While coral-Symbiodiniaceae associations have been extensively studied over the last decades (reviewed in [120]), only two studies have recently investigated hydrocoral-Symbiodiniaceae associations on Caribbean reefs [121, 122]. Rodriguez and colleagues [122] showed that Symbiodiniaceae species that associate with M. alcicornis vary as a function of its geography, with Symbiodinium sp. (formerly clade A) found in samples from Mexico and Breviolum sp. (formerly clade B) in the eastern Atlantic, with the exception of samples from the Canary Island and Cape Verde Islands that comprised Cladocopium sp. (formerly clade C). Unpublished data collected across M. cf. platyphylla Indo-Pacific range showed that this species can associate with the genera Symbiodinium (dominant symbiont), Cladocopium and more rarely with Brevolium in French Polynesia, Papua New Guinea and the south-western Indian Ocean (Dubé et al. in prep.; Boissin et al. in prep.). The other Indo-Pacific species (M. cf. dichotoma and M. cf. exaesa) investigated so far show the same Symbiodiniaceae associations (Boissin et al. in prep.).
