Nonuniform resizing of marine life under climate change

Global warming is often predicted to drive universal declines in animal body size, but empirical evidence remains equivocal. This is particularly true in marine systems, where body size distributions are influenced not only by temperature but also by oxygen and productivity, all of which are expected to be greatly altered in future oceans. Differences in regional climate trajectories and the physiology of marine taxa also suggest that body size responses will not be spatially or ecologically uniform. We address these complexities by projecting future changes in mean body size across species within ocean basins using a database of 23,329 marine mollusc species. We quantify how body size distributions change across energetic gradients and forecast assemblage level shifts of the five major classes of molluscs across all 10 oceanic basins. Under high greenhouse gas emission scenarios, we project a significant decline in body size in 68% of class-basin combinations (34 of 50), with some clades expected to shrink by ~16% in mean length by 2100. However, these responses are not universal but are governed by clade-level differences in dominant ecological and physiological strategies and the geography of climate change. Because biomass scales allometrically with length, these trends can correspond to substantial changes in the mass of the average species, from a 46% decline to a 15% increase depending on clade and region. Such widespread functional disruptions may threaten critical ecosystem services (carbon sequestration, nutrient cycling, and human food provisioning), with implications for global marine ecosystem functioning and services.