Author Annmarie Fearing with a largetooth sawfish specimen for DNA testing.
Kelcee Smith
There’s something almost magical about wandering through the collections of a natural history museum.
Row after row of shelves hold glass jars, carefully labeled drawers and weathered cabinets filled with animals collected decades, sometimes even centuries, ago. To many people they look like relics of the past, but to many others, they’re time capsules. Scientists have taken advantage of this DNA time machine for one of the world’s most endangered marine animals, revealing a story that can’t be told by population counts alone.
Largetooth sawfish (Pristis pristis) once cruised tropical rivers, estuaries and coastal waters across much of the globe, but today, they have disappeared from vast stretches of their former range. Every one of the world’s five sawfishes are now Critically Endangered, largely because fishing gear easily entangles their iconic tooth-lined rostra and because the habitats they depend on continue to disappear. But while we know sawfish numbers have plummeted, this new study asked a deeper question: what has happened to their genetic diversity? That question matters more than many people realize because biodiversity goes beyond how many species exist and includes the genetic variation within those species and the diversity of the ecosystems they inhabit. Genetic diversity is essentially a species’ toolkit for surviving change — it gives populations the ability to adapt to new diseases, changing climates and unexpected environmental challenges. But if you lose enough of that diversity, even populations that appear healthy can become increasingly vulnerable. “Once gone, genetic diversity does not bounce back quickly, even when the numbers of individuals increase,” senior author Dr. Nicole Phillips, an Associate Professor at The University of Southern Mississippi at the time of the study, explains. Measuring these changes isn’t easy, especially for long-lived animals like sawfish. Looking at DNA from living animals only provides a snapshot of today, so it doesn’t tell us what has already been lost.
That’s where museums came to the rescue.
In a recent study published in Proceedings of the Royal Society B, a global team of researchers collected tissue from 375 historic sawfish specimens dating back as far as the 1800s, with most coming from dried rostra sitting in museum collections, while others were donated from private collections. After carefully selecting specimens with reliable dates and locations, the team extracted DNA using techniques designed for highly degraded historical samples. For lead author Dr. Annmarie Fearing of the University of Southern Mississippi, one of the most remarkable aspects of the project was giving these objects — that once hung on the walls of bars or sat on museum shelves — a second life: “What were once remnants of the exploitation that contributed to the decline of sawfishes have now become valuable tools for conservation. Some of these rostra are all that remain of extinct populations and our only way of obtaining DNA from these populations.”
The researchers recovered usable DNA from 95 historical specimens spanning nearly 140 years and compared them with modern samples collected in the 2000s, and the results painted a sobering picture. Historically, largetooth sawfish possessed remarkably high genetic diversity spread across different regions of the world. Populations in the Eastern Pacific, Western Atlantic, Eastern Atlantic, and Indo-West Pacific each carried unique genetic signatures, reflecting long periods of genetic isolation. Today, much of that diversity is simply gone. Globally, genetic diversity was found to have declined significantly between the late nineteenth century and the early twenty-first century. Of the 35 historical genetic variants (known as “haplotypes”) identified in older specimens, only six were still found in modern populations. While the researchers anticipated a decline, seeing just how much unique genetic diversity had vanished was remarkable. Phillips explains that largetooth sawfish naturally form genetically distinct populations because they don’t cross ocean basins, and females often return to the same rivers to give birth generation after generation. “The loss of genetically distinct populations at both large and small spatial scales to the extent that we have seen… means that the cumulative loss of unique genetic diversity is staggering.”
A Largetooth Sawfish in Australia.
Peter Kyne
Australia’s story, however, offers a rare bit of good news. Unlike many other regions, Australian largetooth sawfish appear to have maintained relatively stable genetic diversity since at least the 1960s. According to co-author Dr. Peter Kyne of Charles Darwin University, Australia became one of the first countries in the world to legally protect sawfish when federal protections were introduced in 2000. Combined with relatively intact northern river systems and low human population densities across much of the region, those protections have helped preserve one of the species’ last strongholds. But Kyne cautions against complacency, as bycatch in fisheries, expanding agricultural development and proposals to extract freshwater from northern rivers continue to threaten the nursery habitats young sawfish depend on. “The future of Largetooth Sawfish in Australia is far from secure despite decades of protection, conservation and research focus,” he says. Brazil, on the other hand, tells a more uncertain story. The country’s remaining population still appears to hold an important reservoir of genetic diversity, representing one of the last surviving populations outside Australia. Phillips says protecting that population is now a conservation priority. “Further declines in the species in Brazil need to cease, with adequate protection and enforcement of the species and habitats to prevent further losses,” she says, also noting that recent sightings in countries such as Panama and Costa Rica offer hope that additional populations may still survive, though their size and long-term viability remain unknown.
One of the study’s most important findings is that the surviving populations represent two deeply distinct evolutionary lineages — one lineage occurs in the Indo-West Pacific while the other includes populations from the Atlantic and Eastern Pacific. And Phillips says recognizing these lineages as distinct Evolutionarily Significant Units (ESUs) helps conservationists prioritize what remains: “Conserving diversity within each ESU protects against putting your eggs all in one basket in a genetic sense and increases the likelihood of protecting genetic variation to support future capacity for adaptation in the global population.”
Conservation success is often measured by the number of animals left in the wild. That’s understandable because counting individuals is straightforward. But a population can recover in size while still carrying only a fraction of its historical genetic diversity, making it less resilient to whatever challenges come next. So, what does recovery really mean? Is preventing extinction enough? Or should our goal be to preserve the evolutionary potential that allows species to keep adapting long after we’re gone?
One thing for certain is that this research experience fundamentally changed how Fearing views museum collections. “Most of these sawfish rostra were donated to museums decades ago with no expectation that they would one day be used for genetic research, yet they became essential to our study,” she says. “Our research is just one example of the incredible potential museum specimens have to answer future questions.” And as Phillips notes, this project itself was only possible because of an extraordinary global collaboration among researchers, museum staff, citizen scientists, aquarists and volunteers who worked together to piece this evolutionary puzzle back together.
