Sustaining in vitro cultures of endosymbiotic dinoflagellates in the genus Symbiodinium is important, addressing questions relating to Symbiodinium function and Symbiodinium dependent host fitness. Difficulties in establishing representative Symbiodinium cultures from Pacific coral isolates limit the availability of diverse Symbiodinium types, especially in the C clade. While this clade exhibits high subcladal diversity (over 100 types), and represents the ecologically dominating phylotype in Indo–Pacific corals, only two ancestral types C1 and C3 are currently in permanent culture. This study attempted to cultivate Symbiodinium C15, a derived C clade type, from the Hawaiian coral Porites compressa. We tested a number of basic media in combination with defined organic supplements, as well as host factor derived additives and ion-manipulated media, in order to mimic the intracellular ion regime of a zooxanthellate host. While basic media did not support cell survival at all, the use of organic supplements such as amino acids plus taurine or host derived tissue homogenate had a positive effect on survival and stabilized in vitro densities temporarily. However, none of the conditions tested supported a proliferating motile Symbiodinium C15 culture. In two independent experiments, a potentially free-living Symbiodinium A clade strain was successfully cultured, which exhibited phylogenetic separation from endosymbiotic A clade strains. This study demonstrates the effectiveness of host mimics or host derived supplements to study otherwise ‘unculturable’ Symbiodinium strains. Our results imply that Symbiodinium C15 is incapable of surviving ex hospite unless a host-derived or potential host mimicking feature is present. This potential host dependency has important implications for post-bleaching recovery of the endemic coral P. compressa and suggests a coevolutionary link between vertical transmission mode, host dependency of the symbiont and bleaching resistance of this holobiont.