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MEPS 439:57-71 (2011)  -  DOI:

Symbiont diversity is not involved in depth acclimation in the Mediterranean sea whip Eunicella singularis

D. Forcioli1,*, P.-L. Merle1, C. Caligara1, M. Ciosi1, C. Muti3, P. Francour2, C. Cerrano3, D. Allemand4

1UMR 7138 Université Pierre et Marie Curie/Centre National de la Recherche Scientifique/Université de Nice Sophia-Antipolis/ Muséum National D'Histoire Naturelle/Institut de Recherche pour le Développement, 'Systématique-Adaptation-Evolution', and 2EA 4228 'Ecosystèmes côtiers marins et réponses au stress', Faculté des Sciences, Université de Nice Sophia Antipolis, Parc Valrose, BP 71, 06108 Nice cedex 2, France
3Dipartimento per lo studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26, 16132 Genova, Italy
4Centre Scientifique de Monaco, Avenue Saint Martin, 98000 Monaco

ABSTRACT: In symbiotic cnidarians, acclimation to depth and lower irradiance can involve physiological changes in the photosynthetic dinoflagellate endosymbiont, such as increased chlorophyll content, or qualitative modifications in the symbiont population in favour of better adapted strains. It has been argued that a lack of capacity to acquire new symbionts could limit the bathymetric distribution of the host species, or compromise its long-term survival in a changing environment. But is that always true? To address this question, we investigated the symbiont genetic diversity in Eunicella singularis, a Mediterranean sea whip species with a wide bathymetric distribution (10 to 50 m depth), which has recently suffered from mass mortalities after periods of abnormally high sea temperatures. We measured symbiont population densities and chlorophyll content in natural populations, and followed the response of the holobionts after reciprocal transplantations to deep and shallow depths. A total of 161 colonies were sampled at 2 depths (10 and 30 m) at 5 sites in the northwestern Mediterranean. All colonies harboured a single ribosomal Symbiodinium clade (A’), but a relatively high within-clade genetic diversity was found among and within colonies. This diversity was not structured by depth, even though the deeper colonies contained significantly lower population densities of symbionts and less chlorophyll. We did, however, reveal host-symbiont specificity among E. singularis and other Mediterranean cnidarian species. Transplantation experiments revealed a limit of plasticity for symbiont population density and chlorophyll content, which in turn questions the importance of the trophic role of Symbiodinium in E. singularis.

KEY WORDS: Algal-coral interactions · Symbiodinium · 18S rDNA · Genetic diversity · Depth distribution · Photoacclimation · Eunicella singularis · Mediterranean Sea

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Cite this article as: Forcioli D, Merle PL, Caligara C, Ciosi M and others (2011) Symbiont diversity is not involved in depth acclimation in the Mediterranean sea whip Eunicella singularis. Mar Ecol Prog Ser 439:57-71.

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