Parallel phenotypic plasticity and divergent ecological strategies in sympatric sister sponge species

Background/Question/Methods

In organisms that are very different from humans, adaptive significance of design features can elude us, impeding understanding of mechanisms of diversification and coexistence. Experimentally provoking phenotypic plasticity in closely related species can help assess adaptive significance of species differences. Sponges are extreme examples of animals that are very different: stuck in one place for their adult lives, feeding on bacteria-sized plankton filtered from water that they pump through homogeneous bodies that lack organs and nerves. Common Caribbean sponge species, Aplysina fulva and A. cauliformis are nearly indistinguishably molecularly, and share growth form, habitat, and geographic range; raising questions about mechanisms of speciation and coexistence. To distinguish phenotypic plasticity from genetic variation, and learn how plasticity might influence adaptive - and divergent - evolution, I: a) quantified morphological and ecological characters, b) revealed phenotypic plasticity in both species by growing clonemates in different environments, and c) related plastic morphological features to ecological function. Characters included width and pith of individual skeletal fibers, skeletal fiber density, biomechanical properties, vulnerability to parasites and predators, wound healing, transport fibers, propagation by fragments, population dynamics, and growth and survival.

Results/Conclusions

Growing clonemates in environments differing in food availability, sunlight, water movement, and spongivores elicited significant parallel plasticity in the same traits (i.e., transport fibers, skeletal fiber density, growth) in these two species, and lack of plasticity in the same traits (i.e., resistance to predators and to parasites), suggesting that plasticity preceded, rather than impeded, speciation. Combining comparative with experimental data allowed interpretation of adaptive significance of plastic characters that may underlie divergence. Density of skeletal fibers, not previously used to distinguish sponge species, not only clearly distinguishes these otherwise very similar species, but also appears to influence fragmentation, fragment survival, access to light for photosymbionts, and growth rate. Experimentally provoking phenotypic plasticity can be a powerful tool for revealing adaptive significance of traits and for clues about mechanisms of diversification and coexistence.