Parasites Defy Biodiversity Rules, Thriving Far from the Equator
By tracking infections across snails, crabs and fish, FAU researchers found that host movement, local conditions and temperature shape where parasites thrive.
Study Snapshot: For decades, scientists have observed that biodiversity is highest near the equator and decreases toward the poles, a pattern called the latitudinal diversity gradient, which occurs across ecosystems and all forms of life. A new FAU study reveals a surprising twist: certain trematode parasites – worm-like organisms that cycle through snails, crabs and fish – are actually more common in cooler, temperate waters than in the tropics. While most species and parasites peak near the equator, infections in crabs, small fish and adult fish increase with latitude, showing that these parasites can defy traditional biodiversity rules.
The researchers found that temperature, how far hosts move, and local conditions all affect parasite infections. In the tropics, hosts live closer to their heat limits, so parasites are more likely to kill them. In cooler temperate waters, hosts handle infections better, letting parasites survive and complete their lifecycles. These results help explain how parasites shape ecosystems and evolution – and could give scientists clues about how diseases might respond to changes in climate.
For decades, scientists have observed a clear pattern across the natural world: biodiversity tends to be higher near the equator and lower toward the poles. Known as the latitudinal diversity gradient (LDG), this trend holds across ecosystems – from forests to oceans – and across life forms, from bacteria and plants to animals.
One major reason appears to be temperature: warmer tropical regions support more life by boosting productivity and speeding up evolution, allowing species to adapt and diversify more quickly. Other interactions, like herbivory, predation and mutualistic relationships, are also stronger in the tropics, creating vibrant, dynamic ecosystems.
A new Florida Atlantic University study overturns conventional thinking about where parasites flourish. While biodiversity is typically highest near the equator, researchers found that certain trematode parasites – worm-like organisms that cycle through snails, crabs and fish – are actually more abundant in cooler, temperate waters. Infections in intermediate hosts, such as crabs and small fish, increase with latitude, and adult trematodes in larger fish show the same surprising trend.
“This study reveals that parasite distributions can defy global biodiversity patterns,” said Christopher Moore, Ph.D., first author and a former post-doctoral researcher at FAU’s Harbor Branch Oceanographic Institute. “We found that trematode infections don’t just peak in snails, as previously documented, they also increase in crabs and fish farther from the equator. It’s a striking example of how ecological patterns we think we understand can have fascinating exceptions.”
LDGs are among the most well-known patterns in ecology, showing that species diversity is typically highest in tropical regions and declines toward the poles. Many host–parasite relationships follow this trend, with tropical hosts usually carrying more parasites. Trematodes, however, break the rule. These parasites rely on complex life cycles, moving from a first-intermediate host, like a snail, through second-intermediate hosts such as crabs or small fish, before reaching their definitive hosts, including larger fish and birds.
The study, published in the Journal of Biogeography, analyzed 29 surveys of intertidal ecosystems, focusing on 23 studies reporting data on larval or adult-stage trematodes. Altogether, the data span approximately 2,500 kilometers of coastline, from subtropical to temperate zones, covering roughly 23 degrees of latitude. The findings consistently show higher parasite prevalence at higher latitudes, revealing an inverse LDG that extends beyond the first-intermediate host stage.
Temperature seems to play a major role. In tropical regions, warmer and more constant temperatures push hosts closer to their limits. Parasite infections in these hosts often lead to higher mortality, which makes it harder for parasites to persist. In cooler temperate waters, hosts can tolerate infections better, giving parasites a chance to survive, reproduce and complete their complex lifecycles.
The types of animals that parasites infect also matter. Crabs, for example, are abundant and tough; they can handle parasite infections and are a favorite snack for birds and larger fish, making them important “middle hosts” in the parasites’ life cycle. Small, bottom-dwelling fish, like gobies and blennies, also carry more parasites at higher latitudes. These fish tend to stay close to home, which makes it easier for parasites to complete their lifecycles.
In contrast, open-water fish that swim long distances, like many pelagic species, show little change in parasite numbers with latitude because their wide-ranging habits make it harder for parasites to stick with them.
“Parasites don’t just follow numbers – they follow their hosts,” said Michael W. McCoy, Ph.D., senior author, associate director, FAU School of Environmental, Coastal, and Ocean Sustainability, and professor of quantitative ecology, Department of Biological Sciences, FAU Charles E. Schmidt College of Science and FAU Harbor Branch. “Where parasites thrive depends on a combination of factors: how far hosts move, how long they live, and the local environmental conditions they experience. Some hosts, like crabs and small bottom-dwelling fish, create ideal conditions for parasites, while wide-ranging species make it harder for them to persist. By studying these relationships, we can better understand how ecosystems function, how species interact, and even how diseases might respond to changes in climate.”
Moving forward, researchers plan to track trematode infections across a wider range of latitudes, especially in less-mobile hosts.
“Parasites are often overlooked in biodiversity studies, yet they are key players in ecosystems,” McCoy said. “By uncovering these unexpected patterns, we gain a window into the complex interplay between hosts, parasites and their environment – a dynamic that will become even more important as global temperatures rise and ecosystems shift.”
Study co-author is Krista A. McCoy, Ph.D., director of research and conservation at Florida Oceanographic Society.
This work was supported by the Harbor Branch Oceanographic Institute Foundation’s Specialty License Plate fund.
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