How Body Plans Limit Acclimation: Responses of a Demosponge to Wave Force

The biomechanical basis of morphological acclimation to wave force was studied in intertidal demosponges. Colonies of Halichondria panicea were found to be stronger and stiffer in high wave force habitats than in low wave force habitats. These biomechanical changes are due to increase spicule number and size in sponges from areas of high wave action. The spicule changes follow the predictions of theories developed for particulate composite materials (e.g., those comprised of a flexible matrix with ridge imbedded stiffeners), suggesting that the habitat—dependent changes observed in H. panicea are engineering solutions to environmental stresses. An additional constraint imposed upon the basic Porifera body plan is the necessity of pumping water through the skeleton for feeding and respiration. In H. panicea, piping elements decrease in diameter in high wave force environments. This increases the resistance of oscular systems to water flow, thereby increasing the costs of water pumping. Environments with extremely high wave force are not inhabited by H. panicea, possibly because the high cost of pumping water through a skeleton dense enough to persist would limit or preclude growth. This limitation, however, is peculiar to the engineering trade—offs required by the body plan and feeding mode of H. panicea. Species in other taxa with different basic body plans may be immune to these trade—offs, and may be ecologically limited over identical environmental gradients in different ways.