COMPARATIVE HISTOPATHOLOGY OF GONADAL NEOPLASMS IN MARINE BIVALVE MOLLUSKS

Comparative histology of gonadal neoplasms in 14 marine bivalve specie s or hybrids from 5 countries described in the literature and/or archi ved in the Registry of Tumors in Lower Animals (RTLA), Washington, DC, USA, revealed 3 basic histotypes. Hundreds of cases were of germ cell origin with different stages of development. They consisted of undiff erentiated germ cells that filled individual follicles (stage 1), were present throughout the gonadal area (stage 2), or had spread to outly ing tissues (stage 3). Five cases were of stromal origin. The connecti ve tissue comprising these tumors ranged from vesicular to myxoid to s pindle-cell. As these tumors grew, they invaded and destroyed normal f ollicles. Three cases representing a third histotype appeared to be of both germ cell and stromal origin. Two of these 3 were among 15 Crass ostrea virginica recently collected from the Pawcatuck River, Rhode Is land, USA. In the most advanced case, basophilic hypertrophied neoplas tic germ cells were rapidly proliferating along the walls of gonadal f ollicles and the ducts that extended into the mantle, while the centra l region of the tumor mass was densely fibrous. Some neoplastic cells in follicles adjacent to normal ova-bearing follicles were differentia ting into spermatocytes. Tumor cells aggressively crossed the follicul ar basement membrane, invaded the vesicular connective tissue supporti ng the gill axis, and formed a cystic mass along the luminal wall of t he branchial vein. The less advanced C. virginica case had a smaller, less aggressive tumor but its basic features were similar. The third c ase similar in composition, pattern, and behavior was in a C. gigas th at had been collected during the 1960s from the Willapa Bay, Washingto n, USA, and had originally been interpreted as a fibroma. All 3 of the se mixed gonadal-stromal neoplasms are presently diagnosed as gonadobl astomas.