Light-induced plasticity in plant morphology is considered adaptive in terrestrial habitats that vary in light, but remains unexplored for marine habitats. This is despite similar modes of growth, development and photosynthetic equipment in terrestrial and marine photoautotrophs and similarly dynamic light environments. We tested whether manipulations of light quantity and quality induce morphological plasticity in the marine macroalga, Asparagopsis armata. Using multivariate analyses (principal components analyses and multivariate analyses of covariance), we show that correlated morphological traits underlie a fundamental growth strategy characterized by the production of phalanx and guerrilla phenotypes in environments that mimic light and shade respectively. This foraging response is not under simple genetic or environmental control, but influenced by interactions between genotype and environment. Evidence of plasticity and genetic variation in plasticity in a marine modular organism generates additional, testable hypotheses on the ecological consequences of variation in growth form that may further explain the evolution of plasticity.