Environment-dependent variation in the morphological, physiological, or behavioural expression of a genotype is termed phenotypic plasticity. To test for small-scale morphological plasticity in the Indo-Pacific massive corals Favia speciosa (Dana, 1846) and Diploastrea heliopora (Lamark, 1816), fragments (clone-mates) from 12 colonies of each species were reciprocally transplanted among 6 new habitats located within 2 environmental gradients: a depth cline and a nearshore-to-offshore gradient in sedimentation rates and total suspended solids (TSS). After 7 mo, all fragments were collected, cleared of tissue, and 10 morphometric characters extracted from randomly chosen corallites. Reaction norms, analysis of variance, and canonical discriminant analysis describe environmentinduced changes in corallite architecture. These changes are more pronounced in the depth cline than along the sediment gradient. Similarity of response is suggested by exploratory factor analysis where, for both species, size attributes dominate the first factor, antisymmetry the second, and corallite exsertion the third. Highly significant genotype × environment interactions for F. speciosa indicate that, for this species, genotypes vary in the level of plasticity expressed. Light and TSS emerge as the primary correlates influencing morphological change, although other parameters might act additively, synergystically or antagonistically with them. In shallow waters, increased corallite exsertion may enhance light capture or, alternatively, protect the central (oral disc) area of each polyp from harmful UV radiation. Morphological variability, combined with environment-induced changes in pigmentation, could impede accurate identification of these taxa.

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