The tropical sea anemone Aiptasia pallida as a lab model for the study of coral bleaching

Bleaching is still among major events threatening coral reefs. New tools have to be developped
to better understand the mechanisms leading to this pathology : we studied the use of the
hermatypic anemone Aiptasia pallida as experimental model for coral bleaching. Aiptasia
appears as a good candidate as it is easy to maintain in aquarium and subjected to bleaching like
corals.
Both morphological and physiological approaches were performed to investigate the
ultrastructure of the anemone tissues (TEM) and the zooxanthellae photophysiology
(chlorophyll a fluorescence, respiration and pigmentation).
Experiments under light and dark stress reveal that anemone tissues ultrastructure can be
differently affected. In darkness, the ectoderm activity is reoriented to capture prey by
increasing cnidocyte density. In contrast, intense light affects especially the gastroderm :
intercellular spaces increase, the expulsion of intact algae in the gastric cavity and the
degradation of zooxanthellae inside vacuoles seem to reduce the zooxanthellae density,
chloroplast thylakoids lose their parallel arrangement.
The analysis of the fluorescence induction curve appears as a powerful tool to analyse the
physiological events series previous to bleaching. Although no significant zooxanthellae density
reduction was observed, the decrease of pigments concentrations indicates that light or dark
stresses induce anemone bleaching. Under strong light intensity, A. pallida zooxanthellae show
an increased proportion of PSII QB non reducing, leading to partial photoinhibition. This
phenomenon favours the ROS production that damages cellular structures of host and
zooxanthellae. In darkness, there is no photosynthesis; anemones have therefore to find other
feeding sources, as suggested by the ultrastructural approach.
As the present results confirm some of those obtained on scleractinians, A. pallida can be
regarded as a good model for coral bleaching studies and has numerous advantages for
experimentation.