Environment-induced Colour Changes in the Massive Corals Diploastrea Speciosa and Diploastrea Heliopora

— Environment-dependent variation in the morphological, physiological or behavioural expression of a genotype is termed phenotypic plasticity. Twelve colonies of both Diploastrea speciosa and Diploastrea heliopora, two common Indo-Pacific massive corals, were tested for plastic responses by transplanting palm-sized fragments (clonemates) over six stations (three reefs  two depths) in Singapore’s southern waters. After 15 weeks, there were no major changes in colour among the fragments transplanted to the shallow stations, with the typical grey of Diploastrea heliopora, and the bright green oral discs of the Diploastrea speciosa polyps, clearly represented. The fragments at the two deepest stations, however, changed dramatically. The oral discs of Diploastrea speciosa lost their bright green pigmentation and the surrounding tissue turned pale brown. The response from Diploastrea heliopora was even more pronounced, with yellow-pinks and pale greens replacing its usual grey pigmentation. Plasticity in colour was evident for both species, and probably light-induced, however, further work is required to determine whether such changes are adaptive.