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 Background A brown discolouration has been observed on the shells of blue mussels (Mytilus edulis) grown in the waters around PEI. The discolouration is most noticeable when the shell is dry, which is the usual case when serving the product for consumption. This can be aesthetically unpleasant for consumers and could have negative consequences for the mussel industry. Description Shells of affected mussels appear normal when wet. Upon drying, a tan brown to gray discolouration is apparent on the shell surface. On older portions of the shell, a region of brown discolouration commonly appears. The shell is most affected on the oldest regions of the shell near the umbo. The most recently affected portions of the shell, occurring nearest the shell margins, begin to exhibit very small tan coloured dots. The organism appears to deposit on the shell in the late summer/early fall when mussels are on the collectors or in the first growing season in socks. It attacks the shell initially as small colonies, joining to eventually affect the majority of the shell surface. Growth rate and meat quality are apparently unaffected, as affected animals attain a marketable size at the expected age and the internal anatomy has normal colouration, texture and configuration. Distribution In October 1997 all mussel growing areas on PEI, with the exception of Savage Harbour, had MSP on their mussel crop. By 1998 all mussel growing areas on PEI had MSP on their mussel crop, including Savage Harbour. This shell condition has also been identified in other regions of Atlantic Canada, the eastern and western seaboards of the United States, British Columbia, Europe and Australia. Affected species include: Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus. Identification Previous investigation into this condition had suspected the etiology as a bryozoan. Recent