One would think we have a fairly good idea of how freshwater stingrays lived their lives. Unlike many marine sharks and rays that can travel vast distances across oceans, freshwater species seemed tied to much smaller neighborhoods. Tagging studies repeatedly suggested that obligate freshwater elasmobranchs (species that spend their entire lives in freshwater) rarely moved more than a few kilometers from where they were found. It was a reasonable assumption, based on the evidence available!

Then one stingray decided to challenge that long-standing narrative.

This story starts off in Argentina’s lower Paraná River , researchers tagged and released a female giant freshwater stingray, Potamotrygon brachyura . But when the animal was later detected again? It had traveled an astonishingly over 100 miles (approximately 105 miles or 170 kilometers). It’s not only beyond impressive for a freshwater ray but also the longest movement ever recorded for an obligate freshwater elasmobranch anywhere in the world. We know very little about freshwater sharks and rays, collectively known as freshwater elasmobranchs — believe it or not, these animals are among the least studied vertebrates on Earth! Many inhabit remote river systems, and because of this are difficult to track and/or often live in regions where scientific research resources are limited. At the same time, like other elasmobranchs they face mounting threats from habitat degradation, pollution, overfishing and river modification. Because of this, many freshwater elasmobranch species are already considered threatened with extinction. Yet for some species, researchers still lack basic information about where they move, where they reproduce and how they use different habitats throughout their lives.

If a species spends its entire life within a small area, protecting that local habitat may be enough to safeguard a population. If individuals regularly move across hundreds of kilometers, conservation efforts become… far more complicated. Until now, evidence seemed to support the first scenario for obligate freshwater rays as previous studies generally documented movement distances of less than (6.2 miles) 10 kilometers. That previous data reinforced the idea that management could occur at local scales, focusing on specific stretches of river where the animals were known to occur. But the Paraná River stingray tells a different story. And as such, protecting one section of a river may not be sufficient if the animals depend on habitats spread across an entire watershed. While one animal does not automatically represent the behavior of an entire species, the observation suggests that large-bodied freshwater stingrays may be capable of much broader movements than previously recognized. Which raises fascinating questions, such as have we been underestimating the mobility of freshwater elasmobranchs because tracking studies have been limited in duration or geographic coverage? Are these long-distance movements rare events undertaken by only a few individuals, or are they a regular part of the species’ ecology that researchers simply have not detected before? But the discovery also reminds us that the absence of evidence is not always evidence of absence. When studying elusive animals in complex environments, it can be difficult to capture the full range of behaviors that occur across an individual’s lifetime. A few years of monitoring may reveal important patterns, but it may also miss rare events that turn out to be ecologically significant.

River systems are increasingly fragmented by dams, water diversions and other infrastructure. If giant freshwater stingrays routinely travel long distances, barriers that fragment rivers may pose a greater threat than previously recognized. So, individuals that rely on multiple habitats throughout a river system could be disproportionately affected when those pathways are disrupted. Not to mention that as these animals move through river systems, they connect habitats, interact with different communities and contribute to ecological processes that scientists are only beginning to understand. Understanding those movements is not only essential for building an accurate picture of these animals but how freshwater ecosystems function. So rather than focusing exclusively on localized protected areas, conservation plans should consider entire river corridors.

As researchers continue tracking these river giants, the next discovery may reveal that this long-distance journey is not an exception at all! If that proves true, it’s pretty cool that even giants still have secrets to share with us.