Loggerhead sea turtles not yet safe from extinction risk
Researchers add pathogenic fungus to risk list for already endangered turtles
Rachel Nuwer • January 24, 2011
Each year, about 3,000 female loggerhead sea turtles journey to Boavista Island, a speck of land located off the western coast of Africa in Cape Verde. Its beaches filled with thousands of buried eggs, the island serves as one of the main nesting sites for loggerheads. But a recent steep decline in the number of turtles visiting the beach has caused concern. Since 2007, researchers estimate a decline of more than 40 percent in the number of females returning to the beach.
Although humans have usually been identified as the loggerheads’ chief tormentor — fishing nets, illegal capture and pollution are all threats to the endangered species — recent studies have identified a new villain that may be responsible for the sea turtles’ decline in Boavista and other nesting sites: an aggressive infectious fungus known as Fusarium.
A study published in the Federation of European Microbiology Societies Microbiology Letters in September 2010 identified Fusarium, particularly the strain F. solani, as an important factor in the decline of sea turtles on Boavista, and perhaps elsewhere. “For the past six years [researchers] at the study site noticed a really high mortality in sea turtles,” said co-author Javier Diéguez-Uribeondo, a mycologist, or fungal specialist, at the Royal Botanic Garden in Madrid, Spain. “What we’re seeing is a bit dramatic.”
Fusarium’s identification as a loggerhead killer is not the fungus’ first brush with infamy. In 2006 a fungal outbreak in contact lens solution in the U.S. infected about 100 people, resulting in dozens of hospitalizations and around 55 corneal transplants to prevent vision loss. Fusarium normally occurs in soil but “certain strains of this species can be found almost everywhere,” said Rutgers University mycologist Ning Zhang, who has studied over 1,000 strains of Fusarium in 22 countries. Besides its common occurrence in soil, Fusarium is also found in hospital water tanks, cucumbers, shower curtains and whales, among many other places. Although the fungus can appear almost anywhere, she said, healthy people should not be worried because Fusarium typically causes problems only for those with an already weakened immune system.
The loggerhead researchers identified the fungus as a threat by conducting an experiment infecting healthy loggerhead eggs. Almost 100 percent of infected eggs never hatched, showing that the “fungus is there and it’s killing an endangered species already suffering many threats,” Diéguez-Uribeondo said.
Loggerhead sea turtles live throughout the world’s oceans, making their conservation an international issue. Researchers estimate about 45,000 exist in the wild. Although loggerheads are protected under numerous international treaties, their population is still jeopardized by human activities. Accidental capture in fishing gear accounts for the majority of loggerhead deaths, and illegal harvesting of turtles and eggs for meat still occurs in places like Mexico, the Bahamas and even Boavista.
The addition of Fusarium to all of the other threats facing loggerheads could “risk the extinction of sea turtles,” said Adolfo Marco, a zoologist at the Doñana Biological Station in Seville, Spain, and co-author of the study.
It’s possible that the fungus is a contained problem “happening in a little corner of the world,” Diéguez-Uribeondo said. But he fears that conditions like global warming could trigger its appearance on other beaches, not just Boavista. Like many other emerging diseases, Diéguez-Uribeondo explained, Fusarium’s spread could be triggered by warmer conditions around the globe, though this is still just a hypothesis. The researchers aren’t sure yet why the fungus is spreading or what factors are allowing it to become pathogenic for Boavista turtles.
Though a small island, Boavista’s 30-mile beach accounts for around 90 percent of all loggerhead nests in the entire eastern Atlantic coast, from South Africa to Europe. Because of its significance as a nesting site, the “massive mortalities” of loggerheads on the island can impact the worldwide population, said Melissa Sarmiento, a conservation biology graduate student studying F. solani at Spain’s Royal Botanic Garden.
Humans and other species could also be affected. Egg poachers could be at particular risk, said Marco. Zhang warns that even the researchers themselves must be careful when handling contaminated eggs. Although exposure is normally not a problem for healthy individuals, an open wound could present an invasion opportunity for the fungus, she said. In the future, Zhang thinks that the fungus could spread to other animals such as mice or sea birds via contact with infected eggs.
Little can be done to treat F. solani infections, whether in humans or turtles. Unlike bacterial infections, for which there are many treatment options, there are only a few drug choices to treat fungal infections, according to Zhang. “Fungi are more closely related to animals than bacteria are,” she explained, “so medicine curing fungus can affect humans as well.” For turtles, she said, the best option is to incinerate infected eggs in an attempt to prevent the spread of the fungus “before it’s too late.” Diéguez-Uribeondo speculates that a fungicide to treat Fusarium could be one method of control, though this would first require a complete understanding of the fungus’ life cycle.
The true extent of F. solani’s impacts on turtle populations will not be understood for a while. Although the fungus is indisputably causing mass mortality on Boavista, a loggerhead takes about 25 years to return to her birthplace to lay her first batch of eggs, explained Marco, the Spanish zoologist. In about 10 to 15 years, the true scope of F. solani’s toll on Boavista’s loggerheads will be revealed, Diéguez-Uribeondo thinks. For now, what researchers do know is that the fungus is having an immediate effect on hatchling mortality.
While they wait to see the extent of the damage the fungus is causing, researchers are studying the strain in more detail, said Sarmiento of the Royal Botanic Garden. Once the problems are understood, preventative measures and treatment options can reduce or erase F. solani from turtle nests, she said.
Diéguez-Uribeondo hopes that the newly published study will highlight the issue so that researchers around the world will check turtle nests for similar symptoms. “This might be happening in other places too,” he said. Marco added that the fungus’ worldwide distribution and its almost universal presence in soils make its treatment and control very complicated.
At this point, “all we know is that this fungus can kill the eggs,” Diéguez-Uribeondo said. “Now we need to know how it works so we can help the little sea turtles.”