Large-scale study shows that ocean warming has reduced marine animal populations over 450 million years
When ocean temperatures soar, marine animals don’t just struggle, they actually shrink, a pattern that has been repeated for nearly 450 million years, a massive new study shows. The researchers used nearly 9,000 recorded changes and more than 1.6 million individual measurements taken from fossils, historical records and living animals to build one of the largest datasets ever of the sizes of marine organisms. What they found is shocking: marine life shrinks much more during periods of intense global warming than during crises caused by cooling or falling oxygen levels. This suggests that today’s decline in fish and shellfish is not some new, isolated trend but part of a pattern deeply written into Earth’s history.
what is villain effect among marine animals
In fact, paleontologists have given this pattern of shrinkage a name, calling it the “Lilliputian effect,” a nod to the tiny characters in “Gulliver’s Travels.” This pattern tends to follow most mass extinctions in the fossil record, in which surviving animals became noticeably smaller for a period, then slowly rebounded once things stabilized. Paleontologist Paulina S Nätscher, who led the study at the Friedrich-Alexander University of Erlangen-Nuremberg in Germany, and her team divided thousands of records into three categories: a calm background period, an acute environmental crisis, and a period of recovery that followed each period.
Why the climate warming crisis is hitting marine life harder
So far, most evidence for this shrinkage pattern comes from single species studied at a single location over a fairly short period of time, leaving a lot of room to wonder whether this is a true biological rule or just a sporadic coincidence. By combining records from oceans around the world, this new data set helps resolve this debate. Cold-blooded marine life, including mussels, crustaceans and fish, shrink every time a crisis strikes, whether it’s triggered by warming, cooling or declining oxygen. But when researchers specifically separated crises related to warming, the shrinkage effect was about twice as large as other types of disasters.
True dwarfing and species replacement
An important detail here is that the researchers are describing true dwarfing, meaning that individual animals within a species are actually growing to smaller adult sizes, rather than simply the larger species going extinct and leaving the smaller species behind. Kenneth De Baets, co-author of the study, noted that compared with other types of environmental stress, these effects tend to be twice as strong during warming events. When the team compared each ancient warming event to how steep the temperature rise was at the time, the largest jumps in temperature were always matched by the largest decreases in body size, although this relationship wasn’t entirely accurate, suggesting that other factors associated with warming, notably declining oxygen levels in seawater, may also have made the shrinkage more severe.
Why warm water makes animals smaller
Scientists have a pretty solid explanation for why this happens in today’s living oceans. Warmer water simply contains less dissolved oxygen, and at the same time, warmer bodies burn oxygen more quickly to keep functioning. For a cold-blooded animal that keeps growing throughout its life, this creates a real bottleneck, as its gills eventually can’t take in enough oxygen to support its larger body. Staying small becomes an easy way to relieve this pressure, which is why many marine species stop growing more quickly and settle at smaller adult sizes as waters warm.
What this means for fish sizes and fisheries
Wolfgang Kießling, who leads the Paleoenvironmental Research Group at Florida State University, believes that ancient warming events like this could almost serve as a preview of ocean warming in the coming decades. This isn’t just a theoretical concern, either: an existing study that looked at thousands of reef fish across Australia found that for every 2 degrees Fahrenheit rise in ocean temperatures, the average fish length decreased by about 5 percent. Because body size directly affects how much an animal eats, how many offspring it can produce, and how much food it provides to other species, a widespread shift toward smaller body size has the potential to ripple throughout the marine food web, ultimately affecting the fisheries that coastal communities and commercial industries rely on for food.
A warning written into the fossil record
The study is particularly significant because it is the first to demonstrate on a truly global scale that ocean warming is leaving a distinct imprint on marine life that is stronger and more volatile than the effects of other environmental crises, and has remained stable over hundreds of millions of years of Earth’s history. The shrinkage already occurring in today’s warming ocean appears to follow a pattern deep within geological time, and researchers believe that as ocean temperatures continue to climb, animals may continue to shrink until warming eventually stops merely stunting growth and begins pushing species toward extinction entirely. The full findings have been published in the journal Proceedings of the National Academy of Sciences.
