A novel reef coral symbiosis


Reef building corals form close associations with unicellular microalgae, fungi, bacteria and archaea, some of which are symbiotic and which together form the coral holobiont. Associations with multicellular eukaryotes such as polychaete worms, bivalves and sponges are not generally considered to be symbiotic as the host responds to their presence by forming physical barriers with an active growth edge in the exoskeleton isolating the invader and, at a subcellular level, activating innate immune responses such as melanin deposition. This study describes a novel symbiosis between a newly described hydrozoan (Zanclea margaritae sp. nov.) and the reef building coral Acropora muricata (=A. formosa), with the hydrozoan hydrorhiza ramifying throughout the coral tissues with no evidence of isolation or activation of the immune systems of the host. The hydrorhiza lacks a perisarc, which is typical of symbiotic species of this and related genera, including species that associate with other cnidarians such as octocorals. The symbiosis was observed at all sites investigated from two distant locations on the Great Barrier Reef, Australia, and appears to be host species specific, being found only in A. muricata and in none of 30 other species investigated at these sites. Not all colonies of A. muricata host the hydrozoans and both the prevalence within the coral population (mean = 66%) and density of emergent hydrozoan hydranths on the surface of the coral (mean = 4.3 cm−2, but up to 52 cm−2) vary between sites. The form of the symbiosis in terms of the mutualism–parasitism continuum is not known, although the hydrozoan possesses large stenotele nematocysts, which may be important for defence from predators and protozoan pathogens. This finding expands the known A. muricata holobiont and the association must be taken into account in future when determining the corals’ abilities to defend against predators and withstand stress.

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This work was supported by a grant from the Natural Environment Research Council, UK (NE/E006949/1) and travel support from the GEF-World Bank Bleaching Working (BWG). We would like to thank BWG members for comments on the original manuscript, particularly Ove Hoegh-Guldberg, Bill Fitt and Rob van Woesik. We thank Clay Winterford of the Joint University of Queensland, Queensland Institute of Medical Research Histotechnology Facility and Robyn Webb of the Centre for Microscopy and Microanalysis at the University of Queensland for their help with preparing and sectioning tissue for histological examination.

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Correspondence to O. Pantos.

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Pantos, O., Bythell, J.C. A novel reef coral symbiosis. Coral Reefs 29, 761–770 (2010).

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  • Coral reef
  • Holobiont
  • Hydrozoan endosymbiont
  • Scleractinia