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Red Algae May Help Blue Crabs as Seagrass Declines

Juvenile blue crab. ©Megan Wood/VIMS

©VIMS

Megan Wood researchers juvenile blue crab use of exotic red algae as a nursery habitat. ©VIMS

By Katharine Sucher, Virginia Sea Grant Correspondent

In and around the Chesapeake Bay, juvenile blue crabs may be getting a new kind of nursery.

Normally, young blue crabs use seagrass beds to survive their formative days in the York River. With seagrass beds in decline, however, blue crabs must rely on alternative nurseries. Virginia Sea Grant Graduate Research Fellow Megan Wood is investigating whether Gracilaria vemiculophylla, an exotic red alga, can serve as a nursery habitat for blue crabs. Understanding how habitat affects juvenile blue crab survival could make it easier to predict adult populations.

“People need to realize that you can’t just focus on the adult crabs you buy in the market. If you want to preserve the adult segment of the population, you need to focus on life cycle phases that aren’t economically profitable, too,” Wood, a Virginia Institute of Marine Science (VIMS) graduate student, explains. “Maybe if we focus on juveniles, we can figure out what they’re doing in nursery habitats that gets them to be harvestable adults.”

Juvenile blue crab. ©Megan Wood/VIMS

Juvenile blue crab. ©Megan Wood/VIMS

Before becoming juveniles, blue crabs start their lives as tiny larvae that float in ocean waters. Ocean currents, wind direction, and other factors can all affect larvae’s rates of survival and their ability to return to the bay’s nursery habitats.

“There are a lot of random events that happen during the larval stage that we can’t predict,” Wood says. “So maybe if the wind blows the wrong way, you lose half of the larvae.”

After hatching and spending four or five weeks floating in the ocean, the surviving larvae mature into megalopa. These megalopa, which are about the size of a pinhead, ride currents back into the Chesapeake Bay. At this stage, predicting survival rates is still nearly impossible, a fact that can be frustrating to fishery scientists, managers, and policymakers trying to design management strategies for the bay’s blue crab population.

That’s why Wood sees value in studying juvenile blue crabs. One week after reentering the Chesapeake Bay, megalopae molt into juvenile blue crabs, which seek refuge from predators in seagrass and other protective habitats. As seagrass becomes more and more scarce in the Chesapeake Bay, however, young blue crabs that rely on these habitats could find themselves more exposed than ever before.

“If seagrass is in decline, there needs to be something else for juvenile blue crabs to live in and to help them survive,” Wood says.

According to preliminary findings, an exotic red alga appears to be an effective alternative to native seagrasses.

No one knows for sure how this exotic red alga came to the Chesapeake Bay, but one theory is that it was brought in when Asian oysters were transplanted in the Bay. Despite its foreign origins, this non-native alga is considered exotic, not invasive, because it hasn’t had any negative effects on native species in Chesapeake Bay.

In fact, exotic red algae seem to be a good habitat alternative for more than just juvenile blue crabs. So far, data suggests that they increase an area’s biodiversity. This is good news for blue crabs that feast on a variety of small prey. Still, Wood cautions against spreading the exotic alga freely. An abundance of exotic red algae could discourage already limited seagrass growth in the Chesapeake Bay.

Rom Lipcius, a VIMS professor and Wood’s advisor, says Wood’s project hits on a question of major global interest. Fishery ecologists worldwide are paying special attention to the role of habitat and how it affects population abundance and fishery yield.

“These types of findings will feed into management practices, the protection of habitat, and eventually into stock assessment models,” Lipcius says.

Overall, Wood believes that determining the value of exotic red algae as a nursery habitat will help organizations like the Virginia Marine Resources Commission (VMRC) define clearer goals when planning management strategies. Working directly with VMRC Director of Fisheries Rob O’Reilly as her outreach mentor, Wood attends blue crab meetings and learns how her work could be useful for blue crab management. She sees potential in looking into other ways to manage blue crab, too.

“By focusing on habitat-explicit population rates, we might find that we shouldn’t be focused only on protecting adults,” Wood says. “Maybe we should be focused on protecting some of these habitats, too.”