It isn't always easy being a sea turtle. Predators and poachers dig eggs out of their sandy nests; birds and crabs wait and pounce as little hatchlings flop as rapidly as their flippers can move them toward the waiting sea; sharks lurk in the ocean.
Of late, turtles' lengthy oceanic lives have brought them into conflict with all manner of human shenanigans, from plastic debris to balloons to fishing gear. Small wonder that all species of sea turtles are listed as threatened or endangered, three of them critically endangered.
As if that wasn't bad enough, the early 1980s saw a huge rise in reports of turtles – particularly green turtles – suffering from fibropapillomatis. This disease had first been reported in 1938 but had been little observed since; suddenly green turtles around the world, particularly in warmer climes, were being found with the disease's signature cauliflower-like tumors growing on their heads. When they grew externally, the tumors themselves were not directly deadly, but their size and location – predominantly around the eyes and on the neck and shoulders – interfered with turtles' ability to see and eat, with frequently fatal consequences. If afflicted turtles were captured and the tumors removed, the animals could ultimately recover enough to be released back into the wild; if the tumors grew internally, however, then death was inevitable.
Initially, the cause of the disease was a mystery. Studies by researchers with the University of Florida and the Turtle Hospital on Marathon Key established that it could be spread from turtle to turtle and thus was an infectious agent; that agent was then identified as a form of herpes virus.
But why did the disease suddenly spike in the 1980s? Was there an environmental cause, some kind of change that created conditions more favorable for the virus' development or transmission? A recent paper in the journal PLoS One appears to have found the answer.
A team of three researchers led by Kyle van Houten of the National Oceanic and Atmospheric Administration (NOAA) examined records of almost 4,000 green turtle strandings along the coast of Hawaii over 28 years, to see if they could discern any patterns. Sure enough, several emerged.
There was a strong correlation between turtles' size and age and the incidence of fibropapillomatitis: by some margin, the disease occurred most frequently in juveniles and young adults. It is during that period of their growth that green turtles return to coastal environments after spending their early years in pelagic waters; furthermore, van Houten and colleagues found that some specific coastal areas had high incidence of the disease, while others had none. Those areas where the disease was found with greater frequency were the ones where agricultural development had resulted in high rates of runoff of nutrients, particularly nitrogen.
That increase in nitrogen in the water stimulated the growth of non-native species of algae that had begun to gain a toehold in the 1950s and, by the 1970s, had so successfully out-competed native species that in some places they now comprise more than 90 percent of green turtles' diet. The key to the sudden success of these algae species was that they were able to absorb and store excess nitrogen in the form of arginine, an amino acid that virological studies have shown to be essential to the growth of herpes viruses.
Finding the cause of a problem is one thing; enacting a solution, particularly to entwined issues as vexing as nutrient pollution and invasive species, is another. But van Houten and colleagues are determined to answer and solve this 30-year-old mystery.
Photograph of green turtle swimming off Hawaii, by Mila Zinkova, via Wikimedia Commons