Typical manifestation of MSP on dry mussel shell.

Electron micrograph of fungal colony on mussel shell.

AIN 01.2001

Mycotic Sloughing Periostracum (MSP)in Mussels

BackgroundA brown discolouration has been observed on theshells of blue mussels (Mytilus edulis) grown inthe waters around PEI. The discolouration is mostnoticeable when the shell is dry, which is theusual case when serving the product forconsumption. This can be aesthetically unpleasantfor consumers and could have negativeconsequences for the mussel industry.

DescriptionShells of affected mussels appear normal whenwet. Upon drying, a tan brown to graydiscolouration is apparent on the shell surface. On older portions of the shell, a region of browndiscolouration commonly appears. The shell ismost affected on the oldest regions of the shellnear the umbo. The most recently affectedportions of the shell, occurring nearest the shellmargins, begin to exhibit very small tan coloureddots. The organism appears to deposit on the shellin the late summer/early fall when mussels are onthe collectors or in the first growing season insocks. It attacks the shell initially as smallcolonies, joining to eventually affect the majority

of the shell surface. Growth rate and meat qualityare apparently unaffected, as affected animalsattain a marketable size at the expected age andthe internal anatomy has normal colouration,texture and configuration.

DistributionIn October 1997 all mussel growing areas on PEI,with the exception of Savage Harbour, had MSPon their mussel crop. By 1998 all mussel growingareas on PEI had MSP on their mussel crop,including Savage Harbour. This shell conditionhas also been identified in other regions ofAtlantic Canada, the eastern and westernseaboards of the United States, British Columbia,Europe and Australia. Affected species include:Mytilus edulis, Mytilus galloprovincialis andMytilus trossulus.

IdentificationPrevious investigation into this condition hadsuspected the etiology as a bryozoan. Recent

Electron micrograph of fungal hyphae protruding fromshell.

Electron micrograph of a mussel shell.

studies conducted by the Atlantic VeterinaryCollege and the PEI Department of Fisheries,Aquaculture & Environment have definitivelyidentified the causative agent as a fungus.Electron microscopy and histopathology hasdemonstrated the eroding nature of the organismto the periostracum and the prismatic layer of theshell, hence causing the sloughing of theperiostracum, and the condition’s name, MycoticSloughing Periostracum(MSP). This loss of theoutermost layer of the shell accounts for thebrown discolouration. The fungus, although notdefinitively identified, has been classified asWallemia, a marine salt loving fungus. Efforts arecurrently ongoing by a mycotic expert to culturethe fungus under laboratory conditions.

Meat QualityThere is no histological evidence thatdemonstrates that MSP affects the quality orhealth of the mussel. The affecting organism doesnot penetrate through the shell, which leaves the

internal organs of the mussel unaffected.

ConclusionsAlthough aesthetically unpleasant to the consumerpurchasing PEI mussels, this shell condition doesnot affect the quality or safety of the meat of the

mussel. It appears that MSP is more prevalent inareas of intense cultivation, but is also seen inmany mussel populations around the world.

This information sheet was made possible by thefollowing partners:

� University of Prince Edward Island (AVC)� PEI Department of Fisheries, Aquaculture

and Environment� PEI Aquaculture Alliance� PEI Aquaculture and Fisheries Research

Initiative (AFRI)

For further information contact:

Dr. Jeff DavidsonAtlantic Veterinary College/UPEI550 University AveCharlottetown, PEI C1A 4P3902-566-0666/Dav idson@upei .ca