In April 2015, hundreds of diamondback terrapin turtles and tens of thousands of fish were found dead on a beach near Flanders Bay, Long Island. This threatened species has already experienced steep population declines around Long Island, and the unprecedented die-off had potential to impact the survival of the terrapin population in the area. Through collaboration with the New York State Department of Environmental Conservation (NYSDEC) biologists and a local wildlife rehabilitator, Cornell’s Wildlife Health Program (WHP) researchers determined that saxitoxin, a potent neurotoxin from algal blooms, was behind the die-off; their discovery was published in the April 2017 issue of the journal Toxicon.

Turtles in trouble
Diamondback terrapins are the only turtle species that live in coastal salt marshes, and already face serious challenges. The species is listed as threatened or a “species of concern” in multiple states, and populations are declining--in Jamaica Bay, Long Island, one of the largest known populations has declined by 60% in recent years. That made the mass die-off all the more concerning for scientists. “A turtle die-off of this scale is a significant concern,” says Dr. Elizabeth Bunting, WHP director and senior extension associate with the Cornell University College of Veterinary Medicine. “The first step to preventing it from happening again is to pinpoint the cause.”

An unusual suspect
At the same time the terrapins were turning up dead, the NYSDEC closed the shellfish beds around the bay due to an algal bloom which was producing a potent neurotoxin known as saxitoxin—at levels that were the highest ever recorded in the area. Shellfish concentrate the toxin and are a human health concern if consumed, in turn creating an economic impact on commercial and recreational shellfish harvesting in temperate coastal water systems. “This algal bloom was a red flag,” says NYSDEC Biologist Kevin Hynes. “We immediately wondered if it was also the cause of the turtle deaths.”

Karen Testa, a rehabilitator at Turtle Rescue of the Hamptons, Inc., collected and submitted over 25 terrapins for examination to the WHP and the NYSDEC. The terrapins were examined by WHP staff including pathologist Dr. Rob Ossiboff and Hynes. Others were necropsied on Long Island by the Riverhead Foundation and Dr. Chris Gobler’s lab at SUNY Stonybrook. No turtles had any physical evidence of disease or trauma. The animals were also tested for ranavirus, a common cause of mortality events, but results were negative—which added to the case that saxitoxin was to blame.  However, because a saxitoxin-caused mortality in a temperate turtle species had never happened before, the scientists had to ensure the evidence was indisputable. “We had to show with a high degree of certainty that the toxin was present in the animals’ tissue, and that the animals were exposed from eating the local shellfish,” says Hynes.

Case closed
The WHP and Gobler’s lab tested for saxitoxin in the terrapin tissue, while also collecting and identifying the turtle’s gut contents in order to identify the species of shellfish found in the affected turtles. The results confirmed the scientists’ suspicions—the saxitoxin was present in the turtles’ tissues, as well as the corresponding local mussel species in the stomach contents. “Although more information regarding the effect of saxitoxin on reptiles is still needed, we concluded that saxitoxin was the likely cause of the deaths of hundreds of diamondback terrapins in Flanders Bay, Long Island,” says Bunting. “The impact on this fragile and declining population from this event may threaten the survival of the species in this area.”