Medusan morphospace: phylogenetic constrains, biomechanical solutions, and ecological consequences. Invertebrate Biology

Abstract. Medusae were the earliest animals to evolve muscle-powered swimming in the seas. Although medusae have achieved diverse and prominent ecological roles throughout the world’s oceans, we argue that the primitive organization of cnidarian muscle tissue limits force production and, hence, the mechanical alternatives for swimming bell function. We use a recently developed model comparing the potential force production with the hydrodynamic requirements of jet propulsion, and conclude that jet production is possible only at relatively small bell diameters. In contrast, production of a more complex wake via what we term row-ing propulsion permits much larger sizes but requires a different suite of morphological features. Analysis of morphometric data from all medusan taxa independently confirms size-dependent patterns of bell forms that correspond with model predictions. Further, mor-phospace analysis indicates that various lineages within theMedusozoa have proceeded along either of two evolutionary trajectories. The first alternative involved restriction of jet-pro-pelled medusan bell diameters to small dimensions. These medusae may be either solitary individuals (characteristic of Anthomedusae and Trachymedusae) or aggregates of small in-dividual medusan units into larger colonial forms (characteristic of the nectophores of man