SEBENS, Kenneth P: Energetic Constraints and Size Gradients in Intertidal and Subtidal Marine Invertebrates.

Marine invertebrate populations generally span habitats with a range of physical and biological characteristics. For example, energetic stress increases with temperature and aerial exposure, and prey intake increases with immersion time. Wave action can have similar effects, although certain species benefit from wave dislodgment of their prey. The difference between energy intake and metabolic (and/or behavioral) costs determines an energetic optimal size for individuals in such populations. The comparison of this optimal individual size, based on energetics, to the maximum predicted size based on mechanical constraints, provides a mechanism to explain organism size gradients in intertidal and subtidal habitats. For species whose energetic optimal size is well below the maximum size for the wave/flow conditions, energetic constraints should control size. When the opposite is true, populations of small individuals may dominate habitats with strong dislodgment or deformation probability. Finally, when the maximum size of individuals is far below either energetic optima or mechanical limits, other sources of mortality (e.g. predation) may favor energy allocation to early reproduction rather than to continued growth. For modular organisms, the size of individual units (e.g. polyps, zooids) affects the energetics of the entire colony or clone. Individual unit size can be determinate across a broad range of environmental conditions. However, the growth rate of a colony or clone can still depend on habitat-dependent energetic costs and prey availability, and on mechanical properties of the aggregate organism. Predictions based on optimal size models have now been tested for a variety of intertidal and subtidal invertebrates including sea anemones, corals, and octocorals.