Mechanisms of habitat segregation between corallimorpharians: Photosynthetic parameters and Symbiodinium types

Corallimorpharians are evolutionarily important relatives to reef-building corals, yet little is known about their ecophysiology. We demonstrate that physiological mechanisms determine, in part, the light-dependent distributional patterns of 2 corallimorpharians, Rhodactis rhodostoma and Discosoma unguja, on coral reefs in the northern Red Sea. Field measurements of the physiological parameters related to photosynthetic activity revealed that zooxanthellae abundance, chlorophyll a concentration, maximum quantum yield (F _v/F_m), and excitation pressure of Photosytem II (Q _m) varied equally for both species among 3 depths. In contrast, laboratory measurements in 3 experimental light treatments - low light (LL), medium light (ML) and high light (HL) - revealed that D. unguja was more sensitive to HL than R. rhodostoma. Genetic characterization of their endosymbiotic algae using a PCR for the internal transcribed spacer region (ITS2) of nuclear ribosomal DNA showed that both R. rhodostoma and D. unguja contain Symbiodinium ITS2-type C1 symbionts in shallow areas, but that the algal assemblage were either Type C1 or Type D1a in both species in deeper areas. In reciprocal depth, light, and temperature manipulation experiments, bleached R. rhodostoma never shuffled Symbiodinium ITS2 types, however, some R. rhodostoma in the control treatment did. In contrast, some individuals of D. unguja shuffled Symbiodinium ITS2 types both in field-reciprocal experiments and laboratory-control treatments. Both species contained UV radiation absorbing compounds over a wide-depth range. We concluded that the photoacclimation mechanisms of R. rhodostoma and D. unguja were influenced by both host- and symbiont-mediated factors. We hypothesize that a low abundance of D. unguja in shallow water was due to decreased tolerance to the high irradiance found in host factors.