Low symbiont diversity as a potential adaptive strategy in a marginal non-reefal environment: a case study of corals in Hong Kong

Symbiosis with genetically diverse Symbiodinium has been shown to affect host coral physiological responses to environmental stresses. Hong Kong, located in a subtropical region, is a marginal environment for coral growth largely due to its wide annual temperature fluctuation with low mean winter sea water temperature (similar to 16 A degrees C) and variable salinity conditions. The symbiont diversity in Hong Kong corals is therefore worth investigating to enrich our understanding on symbioses in marginal and fluctuating environments. Examination of 56 scleractinian coral species and five soft coral species using denaturing gel gradient electrophoresis of the internal transcribed spacer region 2 found only five distinct subclades of Symbiodinium with C1 the dominant type occurring in all but one scleractinian coral and all soft coral species investigated. C15 and C21 Symbiodinium were found in Porites spp. and Montipora peltiformis, respectively, both of which are vertical transmitters. D8-12 was found in Oulastrea crispata, a stress-tolerant species, and D1 in a single sample of Goniastrea aspera. No spatial differences in Symbiodinium composition were found among different regions of Hong Kong. Seasonal monitoring of tagged Platygyra acuta and Porites spp. colonies also revealed no changes in their symbiont types despite wide ranges of in situ temperature fluctuation. Hong Kong scleractinian corals hosted a remarkably low symbiont diversity compared with corals in the surrounding regions. The predominance of a single subclade, C1 Symbiodinium, suggests that this subclade is best acclimatized to local fluctuating conditions and/or low winter temperature. Forming symbiosis with the best acclimatized symbiont, instead of with a diverse group of symbionts with different physiological performances, either sequentially or simultaneously, may be a strategy used by Hong Kong corals to cope with stressful conditions.