One of the most interesting findings of our study was that gross appearance of shell disease lesions did not closely reflect the severity of tissue damage or host response. For example, small, 1 mm 2 , black foci could range from grade 1 (mild lesion) to grade 4 (severe). Conversely, relatively large lesions (e.g. 10*15 mm) could be as mild as grade 2. Similarly, while PRSD lesions were highly damaging by definition, all having large shell defects, many of these lesions were only grade 3 in histological severity and in some cases appeared to be in a very benign, healing phase. Thus, diagnosis of lesion severity based solely upon gross observation is highly inaccurate