|Morphological alterations of zooxanthellae in bleached cnidarian hosts|
Ladriere, O.; Compere, P.; Decloux, N.; Vandewalle, P.; Poulicek, M. (2008). Morphological alterations of zooxanthellae in bleached cnidarian hosts. Cah. Biol. Mar. 49(3): 215-227
In: Cahiers de Biologie Marine. Station Biologique de Roscoff: Paris. ISSN 0007-9723, more
zooxanthellae; ultrastructure; alterations; symbiosis; coral bleaching;
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Studying the morphological changes of zooxanthellae in the host gastroderm is essential to understand the mechanisms of bleaching. Transmission electron microscopy was used to observe samples from four coral species-three collected from a barrier reef in Madagascar (Acropora digitifera (Dana, 1846), Echinopora hirsutissima Milne-Edwards & Haime, 1849 and Porites (Synaraea) rus Forskal, 1775)) and one cut from an aquarium-grown coral (Pocillopora damicornis (Linnaeus, 1758)-and from the hermatypic (zooxanthellae-containing) sea anemone Aiptasia pulchella (Carlgren, 1943). Zooxanthellae from bleached animals showed different stages of degradation or disorganization. Some were free, detached from the host gastroderm, associated or not with host-cell remains. Others were vacuolated, with abundant reserve material globules and angular holes probably created by the loss of crystalline material during cutting. Experimentally heat-shocked P. damicornis harboured, moreover, a greater number of dividing algae. Bleached individuals were found to vary as regards their response to stress, and zooxanthellae expelled from heat-shocked anemones showed a greater mitotic index and a higher survival rate than algae extracted or naturally externalized from healthy individuals. We propose a combination of morphological criteria for use in diagnosing the health state of algae-cnidarian symbiosis, so vulnerable in the case of bleaching.