Scientists at the University of Georgia Skidaway Institute of Oceanography are investigating black gill in shrimp, a condition Georgia shrimpers are blaming for an ongoing downturn in shrimp harvests. Very little is known about black gill, so professors Marc Frischer and Dick Lee are working with shrimpers and a number of agencies in a collaborative project to answer some key questions about the condition.
R/V Savannah crewman Chris Keene prepares to dump a load of shrimp and other marine life for inspection and study.
Black gill is a symptom of a health problem in the shrimp. The affected shrimp are easy to identify because they exhibit large black areas on their gills, which are right behind their head. The black gill has no effect on the edible qualities of the shrimp. Shrimp affected with black gill are perfectly safe to eat, and the condition has no effect on the taste of the shrimp.
Black gill has been an issue for pond-raised shrimp for more than a decade, but it has only been within the last several years it has become a problem for wild shrimp fishermen. Black gill can be triggered by several factors among pond-raised shrimp. Skidaway researchers believe black gill in wild Georgia shrimp is caused by a microscopic parasite classified as a ciliate—a single cell animal with tiny hairs called cilia that help them move. The scientists don’t know yet exactly which ciliate is to blame. The blackened gills are the result of the shrimp’s immune system reacting to the ciliate invasion. It creates black nodules around the invasive ciliates in the shrimp’s gills.
Three shrimp with black gill.
Beyond the blackened gills, it is not known how the condition affects the health and morbidity of the shrimp. Shrimp shed their gills through their normal molting process. Scientists suspect the parasite triggers a molting response, causing the shrimp to shed their shells and gills repeatedly in an effort to rid themselves of the parasite. This may cause them to use up extra energy and leave them stressed and vulnerable to predators. Examination of infected gill tissue also reveals the ciliate can damage the shrimp gill and directly impact the ability of the shrimp to breathe.
The Georgia Department of Natural Resources statistics indicate that at its peak in October 2013, 40 percent of the shrimp captured in its surveys had black gill. Shrimpers are blaming black gill for reduced catches last year and so far this season.
“That may turn out that is the case, or it may not,” said Frischer. “As of right now, we have no scientific evidence to support it. That would be a good question to address in an additional research project beyond this one.”
Frischer, Lee and their collaborators will try to determine how black gill is transmitted, and if it is infectious. They also want to determine the distribution of the condition and its causative agent, and also see if the parasite exists in other crustaceans, in sediments or in the water.
According to Frischer, the black gill ciliate may always be present in the shrimp and probably other places too. For most of the year, shrimp are able to handle it. “However, in the late summer the water warms and the oxygen level drops, the shrimp may become stressed,” he said. “This may stress the shrimp and allow the parasite to proliferate.”
The two-year project will be sponsored by a $140,000 grant from Georgia Sea Grant, a unit of UGA Public Service and Outreach. The funding is not yet official, but Skidaway Institute scientists began their work early because this is the time of year when black gill is prevalent.
Anna Walker of Mercer University is working with the Skidaway Institute researchers to conduct pathological tissue studies.
Other collaborators on the project include UGA Marine Extension Service, Georgia Department of Natural Resources – Coastal Resources Division, the Georgia Shrimp Association, the South Carolina Department of Natural Resources, Southern Shrimp Alliance, the Florida Fish and Wildlife Conservation Commission and the North Carolina Division of Marine Fisheries.