Descriptions of Polyps and Epidermal Papillomas in Three Bivalve Mollusk Species

Polypoid growths of the foot, mantle, and pericardium have been found repeatedly in bivalve molluscs. These growths (reviewed by Pauley, 1969; Sparks, 1972) generally are composed of essentially normal tissues and have been variously interpreted. Forty-seven (15.7 percent) of 299 freshwater mussels, Anodonta californiensis, with foot lesions were collected by Gilbert B. Pauley from the Columbia River on the Hanford Energy Research and Development Administration Reservation in 1965 or 1966 and have been previously described (Pauley, 1967a, b, 1969). A representative specimen from this series was contributed to the Registry of Tumors in Lower Animals (RTLA 142). The growth was 3-4 mm in diameter and extended slightly more than 2 mm beyond the surface of the foot. The gross surface, as seen with a hand lens, had a slightly sculptured texture, similar to that of the normal areas of the foot. The growth was very slightly constricted at the point of attachment. size, frequency, and the thickness of the vessel walls. The peripheral onefourth of the growth was heavily occupied by normal elongate glands, filled with basophilic granular product, extending up into the surface papillae (Fig. 3). The papilliferous surface was composed of a normal simple ciliated columnar epithelium (Fig. 4). The cause of these lesions is unknown, but the basically normal arrangement of normal cell types rules out neoplasia. The two tissues that had to have proliferated to provide the extra bulk of covering were the fibrous connective tissue and the surface epithelium. The fanning out of muscle from the point of attachment suggests that muscle was pulled into the area as if an injury had caused the surface to Harshbarger