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First published online February 13, 2009
Journal of Experimental Biology 212, 662-667 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.022624

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Phototropic growth in a reef flat acroporid branching coral species

Paulina Kaniewska^1,*, Paul R. Campbell², Maoz Fine³ and Ove Hoegh-Guldberg¹

¹ ARC Centre of Excellence, The University of Queensland, Centre for Marine Studies, St Lucia, QLD 4072, Australia
² Queensland Department of Primary Industries and Fisheries, Horticulture and Forestry Science, Indooropilly Research Centre, QLD 4068, Australia
³ Faculty of Life Sciences, Bar-Ilan University, The Interuniversity Institute for Marine Science, Eilat, POB 469, Eilat 88103, Israel

^* Author for correspondence (e-mail: p.kaniewska{at}uq.edu.au)

Accepted 9 December 2008

Many terrestrial plants form complex morphological structures and will alter these growth patterns in response to light direction. Similarly reef building corals have high morphological variation across coral families, with many species also displaying phenotypic plasticity across environmental gradients. In particular, the colony geometry in branching corals is altered by the frequency, location and direction of branch initiation and growth. This study demonstrates that for the branching species Acropora pulchra, light plays a key role in axial polyp differentiation and therefore axial corallite development – the basis for new branch formation. A. pulchra branches exhibited a directional growth response, with axial corallites only developing when light was available, and towards the incident light. Field experimentation revealed that there was a light intensity threshold of 45 µmol m^–2 s^–1, below which axial corallites would not develop and this response was blue light (408–508 nm) dependent. There was a twofold increase in axial corallite growth above this light intensity threshold and a fourfold increase in axial corallite growth under the blue light treatment. These features of coral branch growth are highly reminiscent of the initiation of phototropic branch growth in terrestrial plants, which is directed by the blue light component of sunlight.

Key words: axial polyp differentiation, coral morphology, light quantity, light quality, Acropora pulchra

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