MEPS 253:145-154 (2003)  -  doi:10.3354/meps253145

Effects of hypo-osmosis on the coral Stylophora pistillata: nature and cause of Œlow-salinity bleaching¹

Ailsa P. Kerswell1,2,*, Ross J. Jones1

1Centre for Marine Studies, The University of Queensland, St Lucia, Queensland 4072, Australia
2Present address: School of Marine Biology and Aquaculture, James Cook University, Townsville, Queensland 4811, Australia

ABSTRACT: The response of the scleractinian coral Stylophora pistillata to short-term (min to h) and long-term (d) exposure to reduced-salinity seawater was examined. Pulse Amplitude Modulated (PAM) chlorophyll fluorescence techniques were used to assess the photosynthetic efficiency of the symbiotic dinoflagellates (dark-adapted Fv/Fm) in the coral tissues (in hospite) before, during and after exposure. Exposure to reduced-salinity seawater caused a marked reduction in efficiency (the ratio of variable [Fv] to maximal [Fm] fluorescence), and there was an apparent link between a reduction in dark-adapted Fv/Fm and a loss of symbiotic dinoflagellates from the corals. The reduction in Fv/Fm of the symbiotic algae and subsequent dissociation of the coral-algal symbiosis (coral bleaching) occurred during exposure to reduced-salinity seawater in either the light or dark. The results demonstrate that bleaching in response to low-salinity seawater is a truly sublethal response, contrary to a recent suggestion. The study also suggests that bleaching of corals in response to low-salinity seawater may not involve the passive loss of algal symbionts and that an impairment of the capacity of the algal symbionts for photosynthesis represents a common Œcue¹ initiating the dissociation of the coral-algal symbiosis during exposure to sub-optimal conditions. This study demonstrates how exposure to low-salinity seawater alone can cause some of the symptoms commonly attributed to temperature anomalies and anthropogenic pressures on coral reefs.

KEY WORDS: Coral bleaching · Symbiotic dinoflagellates · Salinity · Chlorophyll fluorescence

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