Latitudinal variation in the symbiotic dinoflagellate Symbiodinium of the common reef zoantharian Palythoa tuberculosa on the Saudi Arabian coast of the Red Sea

AimThe Red Sea presents an ideal setting to explore the variability of Symbiodinium over environmental, latitudinal and geographical gradients. We used sequences from two molecular markers to examine genetic diversity of Symbiodinium associated with the widely distributed zoantharian Palythoa tuberculosa in the northern and central Red Sea. LocationNorthern and central Red Sea. MethodsSpecimens (n=329) were collected from 15 locations. Sequence data from nuclear ribosomal ITS2 (n=269) and chloroplast minicircle psbA(ncr) (n=173) were phylogenetically analysed (maximum likelihood, neighbour joining), and Symbiodinium types identified for each P. tuberculosa colony. To establish whether environment was a strong predictor of Symbiodinium psbA(ncr) lineage, SST, chlorophyll-a, salinity, and depth data were fit into a multinomial logistic regression using the package VGAM in the R statistical environment. ResultsBased on ITS2 and psbA(ncr) results, P. tuberculosa colonies were shown to be in symbioses with Symbiodinium clade C (n=172) and clade D (n=1). Within clade C, four psbA(ncr) lineages were observed; closely related lineages designated Pt-1-a and Pt-1-b, and closely related lineages Pt-3-a and Pt-3-b. By location, Pt-1-a dominated the sites within the Gulf of Aqaba (c.86%, 37/43 colonies). At the entrance to the Gulf of Aqaba, Pt-3-a dominated (c.88%, 15/17), while the more southern remaining sites in the Red Sea were dominated by Pt-3-b (c.78%, 89/113). Main conclusionsMultinomial logistic regression analyses established that predictions based on the combination of temperature, chlorophyll-a and salinity accurately reflected symbiont distributions in the central and northern Red Sea. Palythoa tuberculosa host Pt-1-a in the coldest region, the Gulf of Aqaba (annual average SST=24.5-25.0 degrees C), while immediately to the south Pt-3-a dominates (SST=26.0-26.5 degrees C), with warmest southern sites dominated by Pt-3-b (SST >26.5 degrees C). The Gulf of Aqaba is a unique environment, and more research on Symbiodinium outside the Gulf is required to understand symbiont diversity patterns within the Red Sea.