Symbiodinium photosynthesis fuels the growth of an array of marine invertebrates, including scleractinian corals and octocorals. Studies examining the symbiosis between Caribbean octocorals and Symbiodinium are sparse, even though octocorals blanket the landscape of Caribbean coral reefs. Here, I compared the photosynthetic characteristics of Symbiodinium in four comCaribbean octocoral species (Pseudoplexaura porosa, Pseudoplexaura wagenaari, Eunicea tourneforti, and Pterogorgia anceps) in outdoor aquaria at ambient temperature. The four octocoral species exhibited similar photochemical efficiencies despite significant differences in Symbiodinium density, chlorophyll a per cell, light absorption by chlorophyll a, and rates of photosynthetic oxygen evolution. Differences in photosynthetic performance between octocoral species could not be easily attributed to the physiology of either symbiotic partner, as the four octocoral species associated with one of three Symbiodinium internal transcribed spacer 2 (ITS2) types. P. porosa and P. anceps harbored the same Symbiodinium type but exhibited markedly different photosynthetic characteristics, which highlights the importance of host morphology on Symbiodinium performance. I also compared the photosynthetic performance of Symbiodinium in discrete growth forms of Briareum asbestinum at ambient and at elevated temperature. B. asbestinum grows in either an encrusting or branching morphology, each of which co-occurs and associates with distinct Symbiodinium types (B19 in encrusting, B21 in branching). The two morphologies had different symbiotic characteristics at ambient temperature, as encrusting fragments had greater Symbiodinium and chlorophyll densities cm-2; however, photosynthetic oxygen evolution was not significantly different. In addition, branches had higher photochemical and light absorption efficiencies than encrusting fragments. At elevated temperature, more negative impacts were seen in branches than in encrusting fragments, including impaired photochemical efficiency and a decreased ratio of photosynthesis to respiration. Neither morphology exhibited coral bleaching at elevated temperature, despite negative effects of elevated temperature on photosynthesis by Symbiodinium. A decreased ratio of photosynthesis to respiration represents a potential fitness cost to branches, as the energetic contribution of their Symbiodinium may be reduced at elevated temperature. Understanding the symbioses between gorgonian corals and Symbiodinium and how they respond to elevated temperature will aid in understanding why gorgonian corals dominate many Caribbean reefs
