Global mollusk study finds climate change won't simply shrink ocean life

A four-year analysis of more than 23,000 species of marine mollusks has found that climate change will not simply shrink ocean animals, as researchers have long assumed. Instead, warming, deoxygenating and increasingly less productive oceans are projected to make some groups of clams, snails, squid and their relatives smaller by 2100 while making others larger, depending on where in the world they live.

The study, published August 31 in the Proceedings of the National Academy of Sciences, was led by Isaac Trindade-Santos, who carried out the work as a postdoctoral researcher at the University of Louisiana at Lafayette and has since moved to the University of Helsinki, working with senior author Craig McClain, a biologist at UL Lafayette. It is among the largest efforts yet to test a long-standing rule of thumb in marine ecology: that a warming ocean should, on average, produce smaller animals.

What the data show

The researchers combined 2.79 million species occurrence records with body-size measurements for five major mollusk groups — bivalves, gastropods, cephalopods, chitons and scaphopods, or tusk shells — and modeled how temperature, dissolved oxygen and ocean productivity relate to adult body size. They then projected those relationships forward to 2100 across ten major ocean basins, under both a high-emissions and a low-emissions climate scenario, according to the University of Helsinki's summary of the findings.

Under the high-emissions pathway, the team projected significant declines in mean body size in roughly two-thirds of the group-and-ocean-basin combinations it examined. In the most extreme cases, average body length was projected to fall by as much as 16 percent by century's end. Baltic Sea clams were a striking example: a 16.2 percent decline in average shell length there corresponds to an estimated 41 percent loss of biomass, because volume — and the biomass an animal represents — scales with the cube of length, not linearly with it. Not every group moves in the same direction, however. Arctic cephalopods were projected to grow by 2.9 percent in average length, and Arctic tusk shells by 3.3 percent, as warming in some polar waters loosens metabolic constraints that the cold otherwise imposes.

The emissions pathway mattered less for which groups grew than for how many shrank. Under the lower-emissions scenario, the analysis found significant declines in only 14 of 50 group-by-ocean-basin combinations, compared with far more under high emissions, according to figures reported by Phys.org's account of the paper and corroborated by Acadiana's KATC news.

Why the old assumption held for so long

The idea that warming shrinks marine animals — sometimes called a "third universal response" to climate change, alongside range shifts and altered timing of life events — rests on straightforward physiology. Warmer water speeds up metabolism, and in water-breathing animals, oxygen supply to tissue often cannot keep pace with that faster metabolic demand, a mismatch that tends to favor smaller bodies with more surface area relative to volume. Oxygen availability is also being reshaped independently: as the ocean warms it holds less dissolved gas and stratifies more, a process the National Oceanic and Atmospheric Administration describes among the compounding chemical stresses facing shell-building species, alongside acidification that makes it costlier for mollusks to build calcium carbonate shells in the first place.

What the new study adds is scale and geographic resolution. Earlier work on shrinking marine life often drew on smaller groups of species or single regions. By building what the authors call the Marine Organismal Body Sizes database and applying it across an entire phylum at a global scale, Trindade-Santos and McClain were able to show that temperature, oxygen and productivity do not move together everywhere — and that where they diverge, so do the outcomes for body size.

A patchwork of winners and losers

The unevenness has direct consequences for the ecosystems mollusks anchor. Clams, oysters and mussels filter water and stabilize sediment; snails and chitons graze algae and recycle nutrients; squid and octopuses sit in the middle of ocean food webs and support commercial fisheries. A biomass loss on the order the study projects for Baltic clams would ripple through the animals and industries that depend on them, from wading shorebirds to shellfish harvesters.

Tropical, shallow-water mollusks faced some of the sharpest projected declines, the researchers found, because those species already live close to their thermal limits and have less room to absorb further warming. The North Atlantic stood out as a region where strong warming combined with falling productivity to push down projected sizes across several gastropod groups. Polar and some temperate species, by contrast, were more often projected to hold steady or grow, at least under the scenarios modeled.

"The same group of animals can be projected to shrink in one ocean basin and grow in another, because the ocean itself isn't warming, deoxygenating or changing in productivity uniformly everywhere," said Isaac Trindade-Santos, the study's lead author.

What researchers say happens next

McClain, the senior author, said the findings complicate a narrative that has circulated in both scientific literature and popular science coverage for years. "For years, the idea that warming oceans would simply shrink marine life has been treated almost as a rule of thumb," he said, according to the Helsinki release. "Our results show that's too simple a story. Across an entire phylum and at a global scale, we found that some of the most basic animals in the ocean are going to respond to climate change in strikingly different ways."

Trindade-Santos framed the emissions comparison as evidence that near-term policy choices still shape outcomes. "Higher emissions mainly determine how many groups get smaller, not which few get larger," he said. "How much marine life shrinks is, to a large degree, still a choice we are making today," a point echoed in coverage by Environment, Coastal & Offshore magazine.

The authors say the same modeling approach could next be applied to other invertebrate groups, such as crustaceans or echinoderms, to test whether the uneven, region-by-region pattern they found in mollusks holds more broadly across ocean life. For now, fisheries managers and conservation groups working in the Baltic and other basins flagged as high-risk are likely to be the first to draw on the projections, since body-size shifts of the magnitude described translate directly into changes in harvestable biomass and habitat function.