Warming oceans will reshape marine body sizes in surprising and uneven ways

For decades, one of the most repeated predictions in marine science has been deceptively simple: as the oceans warm, the animals living in them will shrink. The idea, often called the temperature-size rule, has been treated almost as an ecological law, appearing in textbooks and climate models alike. But a sweeping new analysis of more than 23,000 marine mollusk species suggests that this tidy assumption badly undersells the complexity of what is actually happening beneath the waves. The study, led by Isaac Trindade-Santos, who conducted the research as a postdoctoral researcher in the University of Louisiana at Lafayette laboratory of senior author Craig McClain and is now a postdoctoral researcher at the University of Helsinki, was published on August 31 in the Proceedings of the National Academy of Sciences under the title Nonuniform resizing of marine life under climate change.

The research team assembled an extraordinary evidence base to test the shrinking-ocean hypothesis. They combined more than 2.79 million species occurrence records with body-size measurements spanning five major groups of mollusks: bivalves such as clams and oysters, gastropods such as snails, cephalopods such as squid and octopuses, chitons, and the less familiar scaphopods, or tusk shells. By examining how temperature, oxygen availability and ocean productivity correlate with body size across this entire phylum, the researchers built statistical relationships that could then be projected forward in time. Those projections were run through 2100 across 10 major ocean basins under both high- and low-emissions climate scenarios, producing one of the most geographically and taxonomically detailed forecasts of body-size change ever attempted for a single animal phylum.

The headline finding confirms part of the conventional wisdom. Under the high-emissions scenario, mean body size is projected to decline significantly in roughly two-thirds of the group-and-region combinations the team examined, amounting to decreases in 68 percent of class-basin pairings by the end of the century. In the most extreme cases, average body length could fall by as much as 16 percent by 2100. The Baltic Sea emerged as a particular hotspot: clams there are projected to shrink by about 16.2 percent in average length. Because an animal’s mass scales with the cube of its length, that seemingly modest linear change translates into a staggering estimated 41 percent decrease in average body mass, a figure with profound implications for how much living tissue these ecosystems can support.

Yet the study’s most striking contribution is what it found in the other direction. Not everything gets smaller. Under high emissions, cephalopods and tusk shells in the Arctic Ocean are projected to grow significantly larger, with average length increasing by 2.9 percent and 3.3 percent respectively. In a field where uniform shrinkage has been the default expectation, the prospect of some animals expanding in the fastest-warming ocean on Earth is a genuine surprise, and one that forces a rethink of how body size responds to environmental change.

The explanation for these divergent trajectories lies in the interplay between geography and biology. Cephalopods carry high oxygen demands, which might seem to make them vulnerable in warming, deoxygenating waters, but they also possess highly efficient circulatory systems and fast life cycles that may allow them to maintain or even increase their size as conditions change. Heavily shell-building groups such as bivalves and gastropods appear more consistently vulnerable to shrinkage, likely reflecting the energetic costs of calcification in warmer, more acidic waters. The lesson, the authors argue, is that no single physiological rule can capture the responses of an entire phylum spread across radically different ocean environments.

Geography, in fact, proved to be as important as taxonomy. The same group of animals can be projected to shrink in one ocean basin while growing in another, because the ocean itself is not changing uniformly. Warming, deoxygenation and shifts in productivity each follow their own regional patterns, and body size responds to the combination of all three rather than to temperature alone. This regional patchiness means that global averages, the currency of most climate impact summaries, can obscure the true biological story unfolding in any given sea.

The research also demonstrates the enduring scientific value of natural history collections and open biodiversity databases. The species records and body-size measurements underpinning the analysis were accumulated over centuries of observation and assembled into openly accessible resources. Trindade-Santos helped McClain develop the Marine Organismal Body Sizes database used in the study, an effort that turned scattered measurements from museums, literature and field surveys into a machine-readable foundation for global forecasting. Without that painstaking accumulation of data, questions of this scale would simply be unanswerable.

The stakes of these projections extend far beyond the animals themselves. Mollusks are ecological workhorses: they filter water, cycle nutrients, create habitat for other species, store carbon in their shells and tissues, and support fisheries and food security for hundreds of millions of people. A 41 percent drop in the average body mass of Baltic clams, if it materializes, would ripple through food webs, alter nutrient processing and affect commercial harvests. Changes in cephalopod abundance and size, meanwhile, could reshape predator-prey dynamics in Arctic ecosystems that are already being transformed faster than any other ocean region.

Critically, the study shows that the future is not yet written. Under the low-emissions scenario, significant declines were projected in only 14 of the 50 group-and-basin combinations studied, less than half the number expected under high emissions, while the number of projected increases remained roughly the same. In other words, the emissions path humanity chooses mainly determines how many groups get smaller, not which few get larger. As Trindade-Santos put it, how much marine life shrinks is, to a large degree, still a choice being made today.

For McClain, a professor in the School of Biological Sciences in the Ray P. Authement College of Sciences at UL Lafayette, the takeaway is a call for intellectual humility. For years, he noted, the idea that warming oceans would simply shrink marine life has been treated almost as a rule of thumb, but the results show that this is too simple a story. Across an entire phylum and at a global scale, some of the most basic animals in the ocean are responding to climate change in strikingly different ways, and a few will actually get bigger. That complexity, he argues, matters enormously for how scientists and policymakers think about the future of ocean ecosystems, and it suggests that the coming decades will demand forecasts as varied and regionally specific as the oceans themselves.

Subject of Research: Projected nonuniform changes in body size of marine mollusks under climate change

Article Title: Climate change won’t simply shrink marine life

Article References: Climate change won’t simply shrink marine life. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: climate change, marine mollusks, body size, ocean warming, bivalves, cephalopods, Baltic Sea, Arctic Ocean, ocean deoxygenation, biodiversity, PNAS, emissions scenarios