Parallel phenotypic plasticity and divergent ecological strategies in morphologically and molecularly similar sympatric sponge species

How can phenotypic plasticity promote or impede adaptive change and diversification? Answering this key question can be experimentally intractable, but closely related clonal species allow a direct approach: experimentally provoking phenotypic plasticity to assess the adaptive significance of both plastic traits and species differences. Two common Caribbean sponge species, Aplysina fulva and A. cauliformis, are nearly indistinguishable molecularly, and share growth form, habitat, and geographical range. This raises questions about species boundaries, within-species variability, and mechanisms of speciation. To distinguish phenotypic plasticity from genetic variation, and learn how plasticity might influence adaptive—and divergent—evolution, I: (i) quantified morphological and ecological characters, (ii) revealed phenotypic plasticity by growing clonemates in different environments, and (iii) related plastic morphological features to ecological function. Characters included skeletal fibre density, biomechanical properties, vulnerability to parasites and predators, wound healing, transport pathways, propagation by fragments, population dynamics, and growth and survival in settings differing in food, sunlight, predators, and water motion. Transplanting both species to a different environment elicited parallel plasticity in the same traits. Combined comparative and experimental data reveal integrated suites of ecologically relevant characters that clearly distinguish these species and allow interpretation of adaptive significance of plastic characters that may underlie divergence.