During the full and new moons in the months of May and June, thousands of horseshoe crabs line Delaware Bay beaches to spawn along the shoreline.

Horseshoe crabs date back 445 million years and although they resemble crustaceans such as crabs and lobsters, they are more closely related to arachnids (i.e., spiders and scorpions).

The Delaware Bay has the largest spawning population of horseshoe crabs in the world. They play a key role in the preservation of the Delaware Bay’s ecosystem and are an important food source for marine animals and shorebirds. The red knot, for example, relies on horseshoe crab eggs for their nutritional welfare during migration from nesting grounds in the Arctic and/or South America. Sea turtles and sharks also eat horseshoe crab eggs and larvae.

This summer, three University of Delaware students are conducting research on these “living fossils” under the supervision of Danielle Dixson, assistant professor of marine science and policy in UD’s College of Earth, Ocean, and Environment.

Microbes, sunscreen, and sea level rise, oh my!

Naomi Gilbert, a junior studying biotechnology at James Madison University and participant in UD’s research experience for undergraduates (REU) program, is exploring the effect microbial communities found on horseshoe crab eggs have on the eggs’ survival in the sand prior to hatching.

“Microbial communities are populations of microorganisms that inhabit a single environment,” Gilbert says. “The microbes within these communities can exist in the presence of one another and may interact with each other.”

According to Gilbert, this symbiotic relationship between the microbe and its host can be positive, negative or neutral. Microbes “can influence the survival or development of the host, in this case the horseshoe crab egg, by competing with harmful bacteria that may attempt to colonize on the eggs’ surface.”

In a laboratory setting, Gilbert is growing and culturing bacteria found on the eggs’ surface and sand to determine which group of bacteria out-perform the other. Then, she will sequence and analyze DNA from bacteria present in the eggs’ biofilm to determine the composition of the microbial communities to see if the bacteria are carrying the DNA sequence that allows for the production of antimicrobial enzymes.

In contrast, Megan Cain, a junior environmental science major and UD Summer Scholar, is studying the effects of sunscreen pollution on the horseshoe crab population.

The active ingredients in sunscreen help guard our skin from the sun. For example, zinc oxide reflects UVA rays while oxybenzone — which has proven to be harmful to marine ecosystems such as coral reefs — absorb both UVA and UVB rays.

Cain is curious about how sunscreen affects the development and survival the Atlantic horseshoe crab.

“We are running the study because the Atlantic horseshoe crab’s mass spawn — when they come to shore to mate — and when the eggs hatch aligns with the tourist season on Delaware’s beaches. Meaning more sunscreen is potentially in the water than usual,” she says.

Cain placed three clutches of eggs, or families, each into four different concentrations of seawater — three containing sunscreen and one plain seawater as a control. She is tracking the eggs’ development through photographs to measure their size, survivorship and nutrient levels. At the same time, she is testing different brands of sunscreen to determine which active ingredients have the most detrimental effects.

Similarly, Lucas Pensinger, a senior marine science and policy major and UD Summer Scholar, is looking at pollution through a different lens.

He is studying the effects of ocean acidification, the ongoing decrease in pH in the global ocean due to the absorption of excess carbon dioxide from the atmosphere, and rising sea levels on future horseshoe crab spawning behaviors.

Horseshoe crabs prefer to spawn on shallow sloping beaches. But, changes in sea level are beginning to physically alter the beaches.

“With higher sea levels, berms — which are a level space, shelf, or raised barrier separating two areas — may become more permanent features on our beaches. There will also be less physical space on the beach; as sea levels rise in response to warming temperatures, low tide will be at increasingly higher points on the beach meaning more of the beach itself will be constantly inundated with water,” he says.

Additionally, Pensinger believes that the natural human response to higher sea levels may be to build “more, higher, steeper dunes” to protect their property. He is exploring what effect these naturally occurring changes may have on horseshoe crab spawning.

In the lab, Pensinger is studying how ocean acidification affects the development of horseshoe crab eggs from birth to juvenile age. Then, to understand the effect of sea level rise, he plans to conduct field experiments using fences to herd mating pairs along the beach to observe and characterize their spawning behavior under changing beach conditions.

“Very little research has been done on horseshoe crabs, in terms of human induced effects such ocean acidification, climate change and pollutants,” says Dixson. “These are important, unexplored topics that we wanted to investigate.” 

To see how small baby horseshoe crabs are, see this video courtesy of Dixson's laboratory.