Climate change won’t simply shrink marine life

Global study of more than 23,000 marine mollusk species projects major shifts in body size by 2100 — including surprising growth in some animals and ocean regions.

Research led by scientists at the University of Louisiana at Lafayette challenges a long-held assumption about the effects of climate change on marine life: warmer oceans won’t simply make animals smaller.

The study was led by Isaac Trindade-Santos, who conducted the research as a postdoctoral researcher in the UL Lafayette laboratory of senior author Dr. Craig McClain. Trindade-Santos is now a postdoctoral researcher at the University of Helsinki.

Researchers analyzed more than 23,000 species of marine mollusks — the group that includes clams, oysters, snails, squid, octopuses and chitons — and found that body size is projected to decline in most ocean regions by 2100. But those changes are far from uniform. Some groups could shrink dramatically, while others are projected to grow larger.

The peer-reviewed study, “Nonuniform resizing of marine life under climate change,” was published Aug. 31 in the Proceedings of the National Academy of Sciences, one of the world’s most-cited multidisciplinary scientific journals.

“For years, the idea that warming oceans would simply shrink marine life has been treated almost as a rule of thumb,” said McClain, a professor in the School of Biological Sciences in the Ray P. Authement College of Sciences. “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 — and a few will actually get bigger. That complexity matters enormously for how we think about the future of ocean ecosystems.”

Researchers combined more than 2.79 million species occurrence records with body-size measurements to examine how temperature, oxygen and ocean productivity affect five major groups of mollusks. They then projected those relationships through 2100 across 10 major ocean basins under high- and low-emissions climate scenarios.

The study draws on centuries of accumulated natural history data, including species records and body-size information assembled in open biodiversity databases. Trindade-Santos helped McClain develop the Marine Organismal Body Sizes database used in the research.

Under the high-emissions scenario, researchers project significant decreases in mean body size in roughly two-thirds of the groups and ocean regions they examined. In some cases, average body length could decline by as much as 16% by 2100.

Those changes in length can translate into much larger losses in biomass. For example, a projected 16.2% decrease in the average length of clams in the Baltic Sea would correspond to an estimated 41% decrease in average body mass.

But not everything gets smaller.

Under high emissions, cephalopods — the group that includes squid and octopuses — and tusk shells are projected to grow significantly larger in the Arctic Ocean, increasing by 2.9% and 3.3% in average length, respectively.

Researchers found that those differences depend heavily on both geography and biology. Cephalopods, for example, have high oxygen demands but also efficient circulatory systems and fast life cycles that may enable them to maintain or even increase their size under warming conditions. Heavily shell-building groups such as bivalves and gastropods appear more consistently vulnerable to shrinkage.

“What surprised me most was just how much geography matters,” Trindade-Santos said. “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. You have to look at the whole picture — the biology of the animal and the specific trajectory of its environment — to understand what’s coming.”

The severity of the projected changes also depends heavily on future greenhouse gas emissions. Under a low-emissions scenario, significant declines were projected in only 14 of the 50 group-and-ocean-basin combinations studied, less than half the number projected under high emissions. The number of projected increases remained roughly the same.

“The two futures we compared differ in a revealing way,” Trindade-Santos said. “Higher emissions mainly determine how many groups get smaller, not which few get larger. How much marine life shrinks is, to a large degree, still a choice we are making today.”

The findings have implications well beyond the animals themselves. Mollusks play critical roles in ocean ecosystems, including filtering water, cycling nutrients, creating habitat, storing carbon and supporting fisheries and food security.

More information:

Isaac Trindade-Santos, Postdoctoral researcher, University of Helsinki
isaac.trindadesantos@helsinki.fi
+358 50 5013 156

Craig McClain, Professor, University of Louisiana at Lafayette
craig.mcclain@louisiana.edu