Clamshells show oval-shaped pits indicative of trematode infestation. Credit: John Huntley

Invertebrates in estuaries could be at a greater risk of parasitism as climate change causes sea levels to rise. A new paper published 8 December in Proceedings of the National Academy of Sciences of the United States of America (doi:10.1073/pnas.1416747111) describes how rapid sea level rise in the Holocene affected the population of parasitic flatworms called trematodes.

“We are creating conditions for trematodes to be very successful in estuarine environments,” John Huntley, assistant professor of paleobiology at the University of Missouri and lead author on the paper, told Eos. This could have implications for both human health and the health of the world’s fisheries, he explained.

While en route to their desired host, which is often the digestive tract of birds and mammals, modern-day trematodes first infest invertebrates such as snails, crabs, or clams. The trematodes do major damage in these invertebrates by gobbling up protein-rich gonads or severely altering growth rates. Some scientists have even speculated that the parasite can affect behavior by inducing clams to climb to the shore’s surface and crawl around on the sand, often in plain view of hungry predators such as seagulls, which provide the perfect home for the trematode.

If trematodes behaved in the past the same way they do now, shells of infected clams would hold important information, Huntley recognized.

The soft bodies of trematodes do not leave physical fossils, but the infected clamshells develop oval-­shaped pits where the shell grew in an attempt to keep the parasite out. Trematodes are the only parasite that leaves these uniquely shaped pits, Huntley explained. Could a record of pitted shells through marine strata reflect past environmental changes?

While studying how climate change affects the fossil record, Huntley previously analyzed clamshells from a core in the Adriatic Sea and found that as sea level rose, concentrations of infected shells also rose. In the new paper, he took advantage of previous researchers’ use of radiocarbon dating in China’s Pearl River Delta that provided a highly detailed description of environmental changes through the last 10,000 years.

Huntley found that from the time sea levels rose and flooded river valleys to about 300 years later when sea level had reached its maximum height, the prevalence of trematodes—­evidenced by the rising numbers of pitted clamshells in subsequent layers of strata—­significantly increased. This pattern matched his previous research in marine environments in Italy.

“If we see a similar pattern in a completely different setting, really quite far away in the Adriatic, in a marine setting, that seems to speak of perhaps a more broadly applicable phenomenon,” he said.

“The clear temporal covariation between parasite prevalence and indirect measures of sea level change is a clear warning sign that current ongoing climate change can potentially impact marine parasitic diseases and alter their effects on populations, communities, and food webs,” said Robert Poulin, head of the Evolution and Ecological Parasitology research group at the University of Otago in New Zealand; he was not involved in the research.

“I cannot think of any comparable long-term data set that provides this sort of compelling evidence,” Poulin continued.

Poulin said that the research could expand to consider how the populations of the parasite’s final hosts—in particular, some bird species—changed during that time as well. However, Huntley noted that finding a good proxy to document such effects may prove tricky.

Other next steps include documenting how increases in temperature and the overall productivity of the environment affected the prevalence and geographic spread of the parasite.

“Predicting the future is a difficult game,” Huntley said; to anticipate what changes may come, “we have any number of natural experiments in Earth’s history with climate change and biotic responses on any manner of scales. We can actually go back into the fossil record and see how the biota responded in the past to various environmental perturbations.” Armed with this information, communities may be able to better prepare for future changes.

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2014), Estuaries may face increased parasitism as sea levels rise, Eos Trans. AGU, 95(50), 476, doi:10.1002/2014EO500003.

© 2014. American Geophysical Union. All rights reserved.

© 2014. American Geophysical Union. All rights reserved.