Scientists say the sunscreen we slather on when we head to the beach may be deadly for horseshoe crabs.

At a March 13 symposium on emerging contaminants – substances not yet regulated that have adverse effects on human health or the environment – University of Delaware assistant professor and researcher Danielle Dixson highlighted a study that found horseshoe crab eggs and larvae underdevelop or die when exposed to sunscreen-polluted water.

“The Delaware tourism and beach season coincides with the horseshoe crabs,” Dixson said. “Their mass spawning event and their eggs are laid pretty shallow in the sand, right where tourists are enjoying the sun and putting their sunscreen on.”

Dixson and other researchers examined eggs collected from the shorelines of the Delaware Bay, dividing them into four groups: A control group placed in water, and three groups placed in water with varying amounts of Hawaiian Tropic sunscreen.

All the eggs in the polluted water were stunted and never hatched, while the control group developed normally.

“Sunscreen exposure strongly influences the development, growth and survivorship of the horseshoe crab egg and larvae,” she concluded. Dixson said she and other researchers are ramping up the project this year by experimenting with smaller concentrations of different types of sunscreen, as well as testing the chemicals found in sunscreen independently to see how smaller quantities of chemicals affect horseshoe crabs. They also will survey beachgoers to see, among other things, whether people would choose a sunscreen based on its environmental impact.

The chemical found in Hawaiian Tropic used in Dixson's study is oxybenzone, which has been found to alter the DNA of coral, increase susceptibility to coral bleaching and disrupt the endocrine systems of marine animals. Research has found the chemical impacts aquatic life at a concentration of 62 parts per trillion – the equivalent of one drop of water in about six Olympic-size pools, Dixson said.

Dixson said sunscreen pollution has gained quite a bit of attention as a new pollutant. “It's estimated that 4,000 to 6,000 metric tons of sunscreen gets washed off of beachgoers annually worldwide.”

Oxybenzone is one of dozens of chemicals of concern to scientists and public health officials that have not yet been federally banned or regulated. The symposium aimed to set a baseline to identify contaminants found in Delaware's waterways.

Experts outlined a massive list of contaminants – pyrethroid insecticides, polycyclic aromatic hydrocarbons and phthalates, just to name a few – but the day concluded with a bit of good news.

Researchers from the University of Maryland found local osprey are no longer suffering from long-banned pollutants such as DDT. Once a widely used pesticide, DDT was banned in the United States about 40 years ago after it was found to have adverse affects on public health and cause thin eggshells in predatory species like osprey, killing off new generations of birds of prey.

“Historically, there have been some serious problems in fish-eating birds,” said researcher Barnett Rattner. He said little research on the effect of toxic chemicals on biological organisms has been conducted in the last decade.

Decades later, evidence of pollutants still appears in the eggs and blood of tested birds, but not enough to hurt the animals, researchers said. They compared the results of their 2015 study to a similar 2002 study, and found pollutants such as flame retardants were present, but in much smaller amounts.

“It's good news,” Rattner said. “Ospreys have demonstrated their resilience.”

Event organizer and Delaware Center for the Inland Bays Watershed Coordinator Emily Seldomridge said scientists call osprey and horseshoe crabs “charismatic megafauna,” the critters the average person can actually see, feel and make connections with, she said.

“That's why there's so much interest in it, but I don't think that's any less important than some of the chemical compounds that you can't see in the water,” she said.

Nutrient pollution – excess concentrations of nitrogen and phosphorous in water bodies – historically has been at the forefront of local water impairment discussions, but there's a need to identify what other pollutants are causing problems, or may post threats in the future, in Delaware's waterways, Seldomridge said.

“We're shifting from what we used to think of as bacteria and nutrients being the contaminants in water to this more human impact,” she said. “Every time we have these new chemicals or contaminants there's something new that's popping up. And because it's new, we don't know enough about it to know what level is a toxic level, what level doesn't matter. I think the jury is still largely out. There's a lot we don't know.”

That's exactly why Dixson plans to conduct a more detailed study about horseshoe crabs and sunscreen. The chemical initially tested has already been replaced, and some brands are marketing products as  “oxybenzone-free,” she said.

“They're replacing oxybenzone with avobenzone, but we don't really know if avobenzone is any better,” she said. “Sunscreen pollution potentially has cascading effects, especially in Delaware Bay because of the importance of horseshoe crabs. Future studies are obviously necessary to develop a horseshoe crab-safe sunscreen.”

Editor’s note: The public relations firm representing Hawaiian Tropic did not immediately respond to a request for comment.