Symbiodinium diversity in the soft coral Heteroxenia sp. and its nudibranch predator Phyllodesmium lizardensis

We examined the diversity of the photosynthetic dinoflagellate, Symbiodinium, over a 2-year period in two invertebrates from Australia’s Northern Great Barrier Reef: the nudibranch Phyllodesmium lizardensis and an octocoral of the genus Heteroxenia. In years one and two, we used denaturing gradient gel electrophoresis with internal transcribed spacer 2 (ITS2) region amplicons and identified two nearly identical genotypes of clade C (C64 and a variant) in all samples of each species. We examined the secondary structure of both sequences and found that each had predicted ∆G values within the range of stable free energy values for Symbiodinium ITS2 sequences. In year two, we also used real-time quantitative polymerase chain reaction assays (qPCR) with clade-specific internal transcribed spacer 1 primers to determine whether there were cryptic clades (A, B, and/or D) associated with either host in addition to clade C. qPCR revealed that clades B, C, and D were present in all animals of both species and that all but two nudibranch samples also harbored clade A. These findings suggest that there may be more flexibility in this host/symbiont interaction than has previously been assumed.