title: Productivity links morphology, symbiont specificity and bleaching in the evolution of Caribbean octocoral symbioses.
abstract: Many cnidarians host endosymbiotic dinoflagellates from the genus Symbiodinium. It is generally assumed that the symbiosis is mutualistic, where the host benefits from symbiont photosynthesis while providing protection and photosynthetic substrates. Diverse assemblages of symbiotic gorgonian octocorals can be found in hard bottom communities throughout the Caribbean. While current research has focused on the phylo- and population genetics of gorgonian symbiont types and their photo-physiology, relatively less work has focused on biogeochemical benefits conferred to the host and how these benefits vary across host species. Here we examine this symbiosis among 11 gorgonian species collected in Bocas del Toro, Panama. By coupling light and dark bottle incubations (P/R) with (13)C-bicarbonate tracers, we quantified the link between holobiont oxygen metabolism with carbon assimilation and translocation from symbiont to host. Our data show that P/R varied among species, and was correlated with colony morphology and polyp size. Sea fans and sea plumes were net autotrophs (P/R>1.5), while nine species of sea rods were net heterotrophs with most below compensation (P/R<1.0). (13)C assimilation corroborated the P/R results, and maximum δ(13)Chost values were strongly correlated with polyp size, indicating higher productivity by colonies with high polyp SA:V. A survey of gorgonian-Symbiodinium associations revealed that productive species maintain specialized, obligate symbioses and are more resistant to coral bleaching, whereas generalist and facultative associations are common among sea rods that have higher bleaching sensitivities. Overall, productivity and polyp size had strong phylogenetic signals with carbon fixation and polyp size showing evidence of trait covariance.
authors.affiliation: School of Biological Sciences, Department of Earth Science, and Swire Institute of Marine Science, The University of Hong Kong, Hong Kong, PRC.|Smithsonian Marine Science Network, National Museum of Natural History, Washington, DC, USA.|Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Baker
authors.affiliation: Department of Biology, University of Alabama at Birmingham, Birmingham, AL, USA.|Smithsonian Marine Station, Fort Pierce, FL, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Freeman
authors.affiliation: Smithsonian Marine Science Network, National Museum of Natural History, Washington, DC, USA.
authors.forename: Nancy
authors.initials: N
authors.lastname: Knowlton
authors.affiliation: Department of Biology, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Thacker
authors.affiliation: Department of Environmental Science, American University, Washington, DC, USA.
authors.forename: Kiho
authors.initials: K
authors.lastname: Kim
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA.
authors.forename: Marilyn L
authors.initials: ML
authors.lastname: Fogel
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000502:physiology*; D005075:Biological Evolution; D057486:Carbon Cycle; D017691:Caribbean Region; D004141:Dinoflagellida / Q000145:classification / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D010788:Photosynthesis; D010802:Phylogeny; D013559:Symbiosis