What is it about?
Analyzing 23,000+ marine mollusk species, we show climate change might shrink the mean size in 68% of cases under high emissions. Yet, this response is highly idiosyncratic: a small number of cases actually show an increase in both climate scenarios.
Featured Image
Photo by Pascal van de Vendel on Unsplash
Why is it important?
Body size is perhaps the most fundamental of all biological traits—a unifying dimension that shapes how an organism moves, constrains its metabolic rate, and positions it within the trophic hierarchy of its community. For decades, ecology has worked under a simplifying assumption: the general expectation that a warming planet will systematically reduce the size of the animals that inhabit it. Yet, our global analysis of over 23,000 marine mollusk species reveals a far more complex reality. The future ocean is not shrinking uniformly; instead, its diverse assemblages are undergoing an uneven, highly idiosyncratic restructuring governed by both evolutionary history and local environmental trajectories. The physical reality of this uneven reorganization is amplified by a basic biological rule of scaling: body mass does not change linearly with length. Because weight scales exponentially with linear dimensions, even a minor reduction in average length has a compounding effect. For instance, the projected 16.2% decline in the average length of Baltic Sea bivalves under high emissions corresponds to a striking 41% decrease in average individual mass. Such non-linear shifts in size structure carry the potential to fundamentally alter trophic energy transfer, reorganizing consumer-resource dynamics within marine food webs. Beyond these trophic reconfigurations, this restructuring has major socioecological implications. Shelled mollusks are not passive spectators on the seafloor; they are active ecosystem engineers that filter water, cycle nutrients, and sequester carbon. Altering their size distributions directly compromises the efficiency of these services and challenges marine food provisioning, a resource upon which millions of coastal people rely. Ultimately, these findings convey a calm but clear message regarding ecological complexity and human influence. A comparison of our two future projections shows that while mitigating greenhouse gas emissions does not freeze biological systems in place, it significantly reduces the geographic extent and severity of body-size declines. The degree to which the ocean's fauna contracts is not predetermined, but is tied directly to our global emissions pathway. This reality invites us to look beyond simple taxonomic censuses and begin strategically monitoring the functional dimensions of biodiversity to understand how our marine ecosystems are actually being rearranged.
Perspectives
Long before the advent of modern ecological databases, naturalists such as Alexander von Humboldt, Charles Darwin, and Alfred Russel Wallace were captivated by the spatial variation of life across our planet. Since beginning my own journey compiling biodiversity records in Brazil in 2010, I have felt a deep appreciation for the centuries of patient taxonomic labor that followed those early naturalists. Generous researchers worldwide described species one publication at a time, slowly building a vast mosaic of natural history. Our study was only made possible by synthesizing this collective legacy. Helping Craig McClain assemble the Marine Organismal Body Sizes (MOBS) database was both a privilege and a reminder of a larger truth: when we compile individual observations of names, occurrences, and physical traits into open-access repositories, we transform three centuries of natural history into a living infrastructure capable of answering fundamental questions that previously could not even be asked. In analyzing this shared data, what became most striking to me was the profound influence of geography. We often look for simple, universal rules in ecology, yet our models indicate that the very same group of marine animals can be projected to shrink in one ocean basin and grow in another. This spatial complexity arises because the global ocean does not warm, deoxygenate, or experience shifts in primary productivity uniformly. Understanding what lies ahead requires us to move away from broad generalizations and instead examine the delicate intersection between an organism's unique physiology and the highly localized trajectory of its environment. The severity of these future shifts is ultimately tied to our greenhouse gas emissions, and comparing our two projected scenarios reveals an important asymmetry. Under a low-emissions future, significant declines in body size are restricted to just 14 of the 50 taxon-basin combinations studied—less than half of the 34 declines projected under high emissions. Meanwhile, the few projected increases remain virtually unchanged between the two pathways. This indicates that higher emissions do not determine which groups will grow, but rather how many groups are forced to shrink. Widespread biological contraction in our oceans is not an inevitability; it remains a trajectory we still have the capacity to choose. For me, this reflection is the most vital message of our study.
Isaac Trindade Santos
University of Helsinki
Read the Original
This page is a summary of: Nonuniform resizing of marine life under climate change, Proceedings of the National Academy of Sciences, August 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2606099123.
You can read the full text:
Contributors
The following have contributed to this page
