Diversity of algal endosymbionts (zooxanthellae) in octocorals: the roles of geography and host relationships

Abstract

The presence, genetic identity and diversity of algal endosymbionts (Symbiodinium) in 114 species from 69 genera (20 families) of octocorals from the Great Barrier Reef (GBR), the far eastern Pacific (EP) and the Caribbean was examined, and patterns of the octocoral-algal symbiosis were compared with patterns in the host phylogeny. Genetic analyses of the zooxanthellae were based on ribosomal DNA internal transcribed spacer 1 (ITS1) region. In the GBR samples, Symbiodinium clades A and G were encountered with A and G being rare. Clade B zooxanthellae have been previously reported from a GBR octocoral, but are also rare in octocorals from this region. Symbiodinium G has so far only been found in Foraminifera, but is rare in these organisms. In the Caribbean samples, only Symbiodinium clades B and C are present. Hence, Symbiodinium diversity at the level of phylogenetic clades is lower in octocorals from the Caribbean compared to those from the GBR. However, an unprecedented level of ITS1 diversity was observed within individual colonies of some Caribbean gorgonians, implying either that these simultaneously harbour multiple strains of clade B zooxanthellae, or that ITS1 heterogeneity exists within the genomes of some zooxanthellae. Intracladal diversity based on ITS should therefore be interpreted with caution, especially in cases where no independent evidence exists to support distinctiveness, such as ecological distribution or physiological characteristics. All samples from EP are azooxanthellate. Three unrelated GBR taxa that are described in the literature as azooxanthellate (Junceella fragilis, Euplexaura nuttingi and Stereonephthya sp. 1) contain clade G zooxanthellae, and their symbiotic association with zooxanthellae was confirmed by histology. These corals are pale in colour, whereas related azooxanthellate species are brightly coloured. The evolutionary loss or gain of zooxanthellae may have altered the light sensitivity of the host tissues, requiring the animals to adopt or reduce pigmentation. Finally, we superimposed patterns of the octocoral-algal symbiosis onto a molecular phylogeny of the host. The data show that many losses/gains of endosymbiosis have occurred during the evolution of octocorals. The ancestral state (azooxanthellate or zooxanthellate) in octocorals remains unclear, but the data suggest that on an evolutionary timescale octocorals can switch more easily between mixotrophy and heterotrophy compared to scleractinian corals, which coincides with a low reliance on photosynthetic carbon gain in the former group of organisms.

From population connectivity to the art of striping Russian dolls: the lessons from Pocillopora corals

“…The evolution of coral hosts and their endosymbionts remains unclear. As an illustration, studying 69 genera (20 families) of octocorals and their endosymbionts, Van Oppen, Mieog, Sánchez, and Fabricius (2005) revealed that the symbiotic associations (at the level of phylogenetic clades) are not easily explained by taxonomic affiliation of the hosts. Nevertheless, host–symbiont association could be, on the contrary, more specific at the genus level.…”

Distribution of lipids and fatty acids in corals by their taxonomic position and presence of zooxanthellae

“…The unique octocoral Heliopora coerulea, which has a rigid aragonitic skeleton similar to stony hexacorals, was also studied. Among the corals collected, 2 genera of stony corals (Balanophyllia and Tubastrea), 1 genus of alcyonarians (Dendronephthya), 6 genera of gorgonians (Annella, Melithaea, Parisis, Menella, Paracis, and Viminella), and Siphonogorgia were azooxanthellate; other genera contained zooxanthellae (Fabricius & Alderslade 2001, van Oppen et al 2005.…”

Symbiont diversity is not involved in depth acclimation in the Mediterranean sea whip Eunicella singularis

“…harboured a single Symbiodinium clade). It should however be noted that heteroplasmic colonies are not found in tropical octocorals either, even in places where significant Symbiodinium diversity exists (Van Oppen et al 2005). Whether the homoplasmic status of E. singularis is intrinsic to symbiotic octocorals or due to the apparent absence of an alternative symbiont in the Mediterranean is still an open question; but in any case this observation means that E. singularis appears to be homoplasmic, along with the vast majority of symbiotic cnidarians (Goulet 2006).…”