Morphological alterations of zooxanthellae in bleached cnidarian hosts

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. Resume : Alterations morphologiques des zooxanthelles d'hotes cnidaires atteints par le blanchissement. L'etude des changements morphologiques s'operant au sein des zooxanthelles presentes dans le gastroderme hote est essentielle pour comprendre les mecanismes du blanchissement. Quatre especes de coraux - trois provenant d'un recif barriere a Madagascar (Acropora digitifera (Dana, 1846), Echinopora hirsutissima Milne-Edwards & Haime, 1849 et Porites (Syneraea) rus Forskal, 1775)) et une produite par bouturage en aquarium (Pocillopora damicornis (Linnaeus, 1758)) - et l'anemone hermatypique Aiptasia pulchella (Carlgren, 1943) ont ete echantillonnees et observees au microscope electronique a transmission. Les zooxanthelles des animaux blanchis presentent differents stades de degradation ou de desorganisation. Certaines sont vacuolees, avec de nombreux globules de reserve et des trous anguleux crees par la perte de cristaux mineraux durant la coupe. Les echantillons de P. damicornis soumis a un stress thermique experimental hebergent plus d'algues en division. Dans cette etude, nous avons montre une certaine variabilite de reponse au stress parmi Cah. Biol. Mar. (2008) 49 : 215-227