For decades, the latitudinal diversity gradient (LDG) has been a foundational principle in ecology, revealing a consistent pattern in the natural world: species diversity peaks near the equator and diminishes progressively toward the poles. This trend, long accepted as a universal ecological pattern, has been documented across countless ecosystems, from dense tropical forests to vast ocean expanses, and across a myriad of life forms including bacteria, plants, and animals. The underlying drivers of this phenomenon are closely linked to temperature, with warmer tropical climates promoting higher productivity and accelerated evolutionary rates, thus fostering greater diversification and adaptation. Moreover, complex biotic interactions such as herbivory, predation, and mutualism further intensify ecological dynamics in tropical regions, contributing to rich biodiversity hotspots.
However, a groundbreaking study conducted by Florida Atlantic University challenges this conventional wisdom by focusing on a lesser-studied group: parasites, specifically trematodes. Trematodes are parasitic flatworms that infect a range of hosts in aquatic environments, completing complex life cycles that typically involve multiple intermediate hosts, such as snails, crabs, and small fish, before reaching their definitive hosts, which often include larger fish and birds. Contrary to the expected pattern set by the LDG, this research found that certain trematode parasites are more prevalent in cooler, temperate waters farther from the equator. This striking reversal of parasite distribution calls into question long-held assumptions about biodiversity and ecological gradients.
Christopher Moore, Ph.D., lead author and former postdoctoral researcher at FAU’s Harbor Branch Oceanographic Institute, highlights that parasite prevalence not only peaks in snails but also shows increased infection rates in crabs and fish as one moves away from tropical latitudes. This suggests an intriguing exception to the LDG, illustrating that ecological patterns are more nuanced than previously thought. The research underscores the complex relationship between hosts and parasites and how these organisms adapt differently across latitudinal gradients.
The latitudinal diversity gradient is acknowledged as a fundamental ecological pattern, understood to result from persistent environmental and physiological constraints. Most host-parasite relationships follow the trend, with hosts in tropical regions typically harboring a greater diversity and abundance of parasites. Trematodes, however, represent a compelling outlier due to their intricate life histories and dependence on multiple hosts to complete their life cycles. Their distribution and prevalence intimately depend on the biology and ecology of each host stage, as well as environmental conditions like temperature and host movement patterns.
The study synthesized data from 29 comprehensive surveys of intertidal ecosystems along approximately 2,500 kilometers of coastline, spanning both subtropical and temperate zones across about 23 degrees of latitude. Meta-analysis of 23 studies focusing on larval and adult stages of trematodes consistently revealed higher parasite infection rates at higher latitudes. This inverse latitudinal gradient pattern was particularly pronounced not only in primary hosts such as snails but extended to secondary intermediate hosts like crabs and small fish, as well as to adult parasites inhabiting larger fish. Such patterns imply that environmental conditions in temperate zones may facilitate more effective parasite transmission and persistence.
Temperature emerges as a critical factor influencing this counterintuitive pattern. Tropical waters, characterized by warm and relatively stable temperatures, impose physiological stresses on hosts, which approach their tolerance limits under parasite burden. Increased mortality in these hosts curtails parasite survival and lifecycle completion. Conversely, cooler temperate waters provide a more hospitable environment where hosts can better tolerate infections, allowing trematodes to survive, develop, and reproduce successfully. This thermal tolerance dynamic plays a pivotal role in shaping parasite-host interactions and their spatial distribution.
Host traits further mediate parasite prevalence. Crabs, being abundant and possessing robust defenses against infections, serve as key “middle hosts” in the trematode lifecycle. Their resilience enables them to maintain infections while remaining vulnerable to predation by birds and larger fish, which are definitive trematode hosts, thereby sustaining the parasite’s transmission cycle. Similarly, small, bottom-dwelling fish such as gobies and blennies, which exhibit limited dispersal and site fidelity, create ideal conditions for parasite transmission to occur locally, enhancing infections at higher latitudes.
In contrast, pelagic fish that traverse long distances tend to exhibit more stable and lower parasite loads regardless of latitude. Their wide-ranging movement patterns disrupt parasite transmission and lifecycle completion, emphasizing the significance of host mobility in determining spatial parasite dynamics. These findings collectively emphasize the interplay between host ecology, parasite life history, and environmental factors in shaping biogeographic patterns.
Senior author Michael W. McCoy, Ph.D., associate director of FAU’s School of Environmental, Coastal, and Ocean Sustainability, notes that understanding these complex relationships is vital for unraveling ecosystem function and anticipating disease dynamics amid environmental change. The research highlights how parasites track their hosts’ ecology and behaviors, and how subtle environmental gradients can redefine expected global biodiversity patterns. This novel insight into parasite distributions underscores the importance of integrating parasitology with macroecological perspectives.
Looking forward, the research team plans to expand their investigations into trematode infections across broader latitudinal ranges and in less mobile host species. By deepening the geographic and ecological scope, this work aims to illuminate the intricate mechanisms driving parasite biogeography and address how future climate shifts may alter disease ecology on a global scale.
Parasites, often overlooked in biodiversity and ecological studies, play crucial roles as regulators of host populations and participants in ecosystem dynamics. This research fundamentally reshapes our understanding of parasite distribution and diversity, revealing that their ecological patterns can diverge markedly from their hosts and from other forms of life. As global temperatures rise and climate zones shift, these findings may hold important implications for disease management, conservation biology, and the maintenance of ecological resilience in marine and coastal systems.
The Florida Atlantic University study stands as a vivid reminder that ecological theories are not immutable laws but frameworks that evolve with scientific discovery. By shedding light on the unexpected inverse gradient of trematode prevalence, this research opens new frontiers in macroecology and parasitology, emphasizing the intricacy and variability inherent in natural systems.
Subject of Research:
Animals
Article Title:
Latitudinal Variation in Trematode Prevalence Across Regions in North and South America: Evidence of an Inverse Gradient in Second-Intermediate and Final Hosts
News Publication Date:
3-Feb-2026
Web References:
https://www.fau.edu/
https://www.fau.edu/hboi/
https://onlinelibrary.wiley.com/doi/full/10.1111/jbi.70157
http://dx.doi.org/10.1111/jbi.70157
Image Credits:
Alex Dolce, Florida Atlantic University
Keywords:
Biodiversity, Ecological diversity, Biogeography, Animal dispersal, Latitudinal gradients, Species distribution, Species richness, Parasites, Parasitic diseases, Equator, Geography, Climate zones, Polar climates, Tropical climates, Ecology, Evolutionary ecology, Ecological adaptation, Wild populations, Climate change, Climate change adaptation
