The influence of irradiance on the severity of thermal bleaching in sea anemones that host anemonefish |
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Authors: | R Hill A Scott |
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Institution: | (1) Plant Functional Biology and Climate Change Cluster, School of the Environment, University of Technology, Sydney, PO Box 123, Broadway, NSW, 2007, Australia;(2) National Marine Science Centre, School of Environmental Science and Management, Southern Cross University, PO Box 4321, Coffs Harbour, NSW, 2450, Australia |
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Abstract: | Entacmaea quadricolor is a geographically widespread species of sea anemone that forms a three-way symbiosis with anemonefish and Symbiodinium. This species dominates the reef substrata at North Solitary Island, Australia, which is located in a region identified as
a climate change hot spot. Their geographic location places these anemones under significant threat from rising ocean temperatures,
although their upper thermal limit and risk of bleaching are unknown. To address this knowledge gap, anemones were exposed
to one of four temperatures (23, 25, 27, or 29°C) and one of two irradiance treatments (high or low light) over 6 days. At
moderate temperatures (27°C, 1°C above summer average), anemone bleaching was characterised by symbiont expulsion, while extreme
temperatures (29°C) resulted in an additional loss of photosynthetic pigments from within symbionts, and in some cases, host
mortality. Irradiance influenced the susceptibility to thermal stress with high light promoting the bleaching response, along
with significant reductions in the effective quantum yield of anemone symbionts. The long-term loss of photosystem II photochemical
efficiency within in hospite symbionts was observed during exposure to temperatures exceeding the summer average, indicating photosynthetic damage. The
resident Symbiodinium, identified as clade C using 28S rRNA gene sequences, therefore represents the partner within the symbiosis that is likely
to be most vulnerable to rising seawater temperatures. Results suggest that E. quadricolor is living within approximately 1°C of the upper thermal maximum at the Solitary Islands, and given the predictions for rising
seawater temperature on Australia’s east coast, the thermal threshold at which bleaching will occur is expected to be reached
and exceeded more frequently in the future. |
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