• Oysters​
  
• Freshwater Mussels​
• Clams​
  

• MSX and Dermo are 2 diseases common in Chesapeake Bay oysters. These diseases can cause death in oysters but are harmless to people.
• Each fall MD DNR examines adult oysters from throughout the Maryland portion of the Chesapeake Bay to determine how many have these diseases.
• MD DNR is also starting to study the health of oyster larvae.

The annual Maryland fall survey is an oyster survey that was started in 1939 to track spatfall intensity and observed mortality. In 1990. a sub-set of the sites in the survey were selected to be used as disease monitoring bars in response to the oyster mortality events caused by the disease MSX. The fall survey continues to be carried out annually to this day and provides regular information on the disease status of adult wild Maryland Chesapeake Bay oyster populations. Each fall, at least 30 oysters are collected from each of 53 sites around Maryland’s Chesapeake Bay and examined for disease. Oysters are checked microscopically via histology for Haplosporidium nelsoni, the parasite responsible for causing MSX disease in oysters. The oysters are also checked via rectal tissue incubation in Ray’s fluid thioglycollate medium for Perkinsus marinus, the parasite that causes Dermo disease in oysters. These diseases can both lead to death in oysters however they do not affect humans. Disease data from the fall survey is used by MD DNR’s Shellfish Division to determine if import requests of oysters into Maryland can be allowed through by comparing the disease prevalence of the 2 locations. Reports containing the results of each annual survey can be found here​.

Larval oyster health is also being studied at the Cooperative Oxford Laboratory. A reference guide with histological and whole animal pictures of healthy larvae at various ages was created. This makes it possible to examine larvae in a state of decline from local hatcheries via light microscopy to help determine what the cause of the problem is. Bacteria swarms in or around the larvae is the primary possible pathogen indicator that is checked for. The swimming activity, soft tissue content, and gut content are also examined to help determine the health status of the larvae. This does not always determine the cause of the larval mortality but it can rule out some of the possible disease causes.

 
Necropsy of eastern oysters (Crassostrea virginica) from Maryland. A) Oyster just after being opened. B) Shellfish pathologist Brian Preziosi cuts a slice of oyster tissue for histology. C) Slice of oyster tissue to be preserved and made into a slide so it can be checked for diseases.

• Freshwater mussels help keep rivers clean but are in decline.
• Maryland DNR is working to restore their populations and establish disease tests for them.​
  

There are over 300 species of FW mussels in North America yet nearly all of these are declining. Maryland has 17 (16 native and 1 non-native) species. Freshwater mussels are generally not used for human consumption since some species can be distasteful, but they do provide ecological services. These include nutrient cycling, biofiltration, and structural habitat. To help restore those species in decline, a freshwater mussel hatchery will be built in Brandywine, MD. Freshwater mussel propagation has already started at Maryland DNR’s Joseph Manning Hatchery and will continue there until construction of the freshwater mussel hatchery is complete. To support this propagation effort, the Cooperative Oxford Lab is working on a freshwater mussel histology atlas for Maryland species. This will help establish diagnostic procedures needed to test for freshwater mussel diseases in case a freshwater mussel mortality event occurs. The atlas will provide a reference guide of what healthy tissues look like for each Maryland freshwater mussel species so shellfish pathologists can identify when tissue looks abnormal.

 

A) The preserved soft tissue of the Maryland freshwater mussel species alewife floater (Utterbackiana implicata). The base of the arrow indicates where the gill is. B) Alewife floater gill cross section on a histological slide that has been stained with Hematoxyloxin and Eosin to make the structure more visible.

A collaborative project with NOAA is currently underway to determine the prevalence of diseases in clams in Alaska, specifically in the Kachemak Bay. The project aims to determine disease burden and thereby test the hypothesis that an increase in disease prevalence is a driver of declines in the intertidal hardshell clam populations in that part of Alaska. Disease analysis will be done via histology by DNR staff at the Cooperative Oxford Laboratory. Any abnormal conditions found during the necropsy of the animals are also recorded. This project is one example of the many projects that DNR collaborates with NOAA on.​

 

Necropsy of clams from Alaska. A) Butter clams (Saxidomus gigantea) placed on a numbered tray.
B) Little neck clams (Leukoma staminea) on a numbered tray. C) Butter clam just after being opened.
​D) Little neck clam with soft tissue removed from shell.