Illustrating Oyster Diseases
A new book provides a probing view of pathologies affecting oysters in the wild and on the farm
It is probably safe to say that oyster pathology isn’t “top-of-mind” for the average resident of the Chesapeake Bay region. We simply enjoy eastern oysters (Crassostrea virginica) in whatever manner suits the occasion and the palate; diseases, parasites, and other potential killjoys are, in fact, what we least want to think about.
Fortunately, scientists, resource managers, shellfish pathologists, and aquaculturists do think about pathologies that can affect wild and farm-grown oysters, and this is where a comprehensive, clearly illustrated guide to known and emerging illnesses fills a critical niche. Diseases & Parasites of the Eastern Oyster Crassostrea virginica in Chesapeake Bay , published by Maryland Sea Grant, does just that.
The new book has 18 chapters and more than 100 color figures and diagrams.
“This was an opportunity to create a resource like no other in mollusk health, one that would benefit students and professionals in shellfish health not only in the Bay region but worldwide—everywhere that oysters and other bivalve mollusks matter,” said Ryan B. Carnegie, a research professor at the Virginia Institute of Marine Science (VIMS) and co-author with Christopher F. Dungan and Carol B. McCollough, both of the Maryland Department of Natural Resources. (Dungan has recently retired.)
“Its real power is in the exquisite images we provide, to guide even accomplished users through the interpretation of pathology they may observe at the microscope but not be familiar with,” Carnegie said. “For students and the more inexperienced, there’s fundamental visual background on normal versus abnormal histology that will provide the grounding needed to begin working in the field.”
Histology—the study of the microscopic structures of tissues—is brought into brilliant color and probing detail in more than 100 figures and images that provide clear visuals illuminating everything from bacterial infections and viruses to diseases caused by protists (algae and other single-celled organisms) and parasites. Diseases like Dermo and MSX, familiar in the Chesapeake, as well as those such as Haplosporidia, first discovered in New Zealand and affecting oysters worldwide, are described and then revealed in minute and precise presentation under the microscope.
A page from the chapter on Dermo provides different views of tissues from an oyster infected with the disease. Note the precise locators in the caption text and in the images taken under the microscope.
“When an [oyster] pathologist encounters an unknown or uncertain disease or parasite, their work often screeches to a halt until the unknown can be identified and accurately recorded,” Dungan said. “That’s when an authoritative, comprehensive reference volume is a very welcome friend.”
McCollough said she and her co-authors wanted to present “the best images possible, with consistent color and annotation, often with multiple magnifications to help the viewer see both the relationship of the condition to oyster tissues and the details of the condition itself.” They also wanted to update sparse or outdated information on “the likelihood of encountering a particular condition.”
“This book was an opportunity to present current prevalence of the conditions we addressed,” she said.
Two views of a cestode larvae (a type of flatworm) that has infiltrated an oyster's intestine.
To that end, the authors also provide information about disease patterns for each subject, including seasonal and geographic distribution. They note that although the book focuses on oysters in the Chesapeake and Maryland Coastal Bays, its contents are pertinent to those studying and raising eastern oysters throughout the Atlantic and Gulf coasts in the US, Canada, and Mexico.
While wild populations of the eastern oyster in Chesapeake Bay have crashed due to disease, overharvesting, habitat loss, and poor water quality, aquaculture can reintroduce more of the filter-feeding bivalves. Oyster farms can improve water quality, provide watermen a viable economic option to what remains of the wild harvest, and create opportunities for new industry entrepreneurs.
An aerial view of an oyster farm in Maryland.
Virginia has the largest oyster aquaculture industry on the East Coast, with 135,093 acres under lease. The industry has also grown significantly in Maryland; as of December 2020, there were 486 active shellfish leases in Maryland’s portion of the Chesapeake Bay and Maryland Coastal Bays, comprising 7,539 acres. As the industry grows, so does the need to clearly identify known and emerging threats to oyster survival.
“As with all forms of animal husbandry, success in oyster aquaculture relies on close attention to diseases in order to prevent or mitigate serious impacts on production and survival,” the authors note in the introduction. “This guide is meant to help maintain the disease biosecurity of oysters grown or harvested from Chesapeake Bay and elsewhere.”
An oyster from an aquaculture lease in Maryland.
Shannon Hood, a MDSG Extension specialist who raises oysters at the University of Maryland Center for Environmental Science Horn Point Laboratory, said that as industry growth continues, “so will the demand for interstate transfers or relays of oysters from one waterbody to another, often necessitating histological examination of the oysters.” Along with histopathologists, she said, Extension professionals, students, and lab technicians “will benefit from the detail, care, and clarity of the text and slides, illuminating the microbial world within the oysters. This is really a gorgeous book.”
