Gonadal Neoplasia of Clams


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Gonadal neoplasia of clams, Germinomas of clams, Epizootic seminomas and dysgerminomas of clams.

Scientific name or taxonomic affiliation

Cause unknown.

Geographic distribution

  1. Penobscot Bay, Maine, USA to Passamaquoddy Bay, New Brunswick, and low prevalence (less than 4%) in Nova Scotia and Prince Edward Island, Canada (Barber et al. 2002)).
  2. Maryland, Rhode Island, and Florida, USA.
  3. South Carolina and Florida, USA.
  4. Rhode Island, USA.
  5. Baffin Island, Canada

Host species

  1. Mya arenaria.
  2. Mercenaria mercenaria.
  3. Mercenaria mercenaria/campechiensis (hybrid quahog).
  4. Arctica islandica.
  5. Macoma calcarea.

Impact on the host

In some clams, greminomas are confined to one or more gonadal follicles with little apparent effect on surrounding follicles, tissues or their physiological function. In other clams, more follicles are involved and gonadal architecture and function are affected. In some clams the tumor cells break through the follicle wall and metastasize, involving distant tissues (e.g., gills) and presumably, causing mortalities. Although not yet demonstrated, the proliferation of undifferentiated germ cells first in the gonad and then into surrounding tissues suggests a progressive and lethal nature. Both male and female clams are affected but results of some surveys indicate that the diesease may be more prevalent in females and gonadal neoplasms have a direct negative impact on the number of eggs produced. The minimal size at which M. arenaria becomes infected is believed to be between 25.4 and 45.7 mm in shell height (2-7 years of age). Transmission of the disease in the field to transplanted naïve M. arenaria was not achieved by cohabitation with affected softshell clams (Barber et al. 2002). It has been proposed that gonadal neoplasia in clams may be induced by anthropogenic environmental contaminants such as oil spills and herbicides. In M. arenaria, a parallel in the distribution and incidence of germinomas and blooms of toxin producing dinoflagellates (Alexandrium spp.) has been reported (Landsberg 1996).

Hybrids of Mercenaria mercenaria and Mercenaria campechiensis in a coastal Florida lagoon had a persistent, unusually high frequency of gonadal neoplasia (21.6% compared to 6.5% in M. mercenaria and 11.8% in M. campechiensis) which may act as a barrier to gene flow between the species (Bert et al. 1993).

Diagnostic techniques

Histology: The presence of monomorphic, basophilic, undifferentiated germ cells that proliferate rather than mature in gonadal follicles. The nucleus of each affected cell is often enlarged and eccentric with a nucleolus that is frequently difficult to distinguish in the abundantly clumped chromatin. Margination of the chromatin occurs in some cases. Neoplastic cells can occur in follicles with developing oocytes or spermatocytes. In some cases, follicles are completely packed with neoplastic cells and sexual identity is lost. In these cases, metastasis, in which clusters of neoplastic cells have broken through the basement membrane of the follicles and have become established at a remote site, can occur. Neoplastic cell at the metastatic site are the same in morphology as those seen in the follicles, except that mitotic figures are more prevalent.

Electron Microscopy: Rough endoplasmic reticulum and Golgi are sparce in neoplastic cells (characteristics of immature or undifferentiated cells such as stem cells, embryonic cells and tumor cells). Desmosomes are common on the cellular membranes of the neoplastic cells.

Methods of control

No known methods of prevention or control.


Barber, B.J. 1996. Effects of gonadal neoplasms on oogenesis in softshell clams, Mya arenaria. Journal of Invertebrate Pathology 67: 161-168.

Barber, B.J. 2004. Neoplastic diseases of commercially important marine bivalves. Aquatic Living Resources 17: 449-466.

Barber, B.J. and G.S. Bacon. 1999. Geographic distribution of gonadal neoplasms in softshell clams, Mya arenaria, from Maine and Altantic Canada. Journal of Shellfish Research 18: 295. (Abstract).

Barber, B.J., G.S. MacCallum, S.M.C. Robinson and S.E. McGladdery. 2002. Occurrence and lack of transmissibility of gonadal neoplasia in softshell clams, Mya arenaria, in Maine (USA) and Atlantic Canada. Aquatic Living Resources 15: 319-326.

Barry, M. and P.P. Yevich. 1975. The ecological, chemical and histopathological evaluation of an oil spill site. Part III. Histopathological studies. Marine Pollution Bulletin 6: 171-173.

Bert, T.M., D.M. Hesselman, W.S. Arnold, W.S. Moore, H. Cruz-Lopez and D.C. Marelli. 1993. High frequency of gonadal neoplasia in a hard clam (Mercenaria spp.) hybrid zone. Marine Biology 117: 97-104.

Gardner, G.R., P.P. Yevich, J. Hurst, P. Thayer, S. Benyi, J.C. Harshbarger and R.J. Pruell. 1991. Germinomas and teratoid siphon anomalies in softshell clams, Mya arenaria, environmentally exposed to herbicides. Environmental Health Perspectives 90: 43-51.

Hesselman, D.M., N.J. Blake and E.C. Peters. 1988. Gonadal neoplasms in hard shell clams Mercenaria spp., from the Indian River, Florida: occurrence, prevalence, and histopathology. Journal of Invertebrate Pathology 52: 436-446.

Landsberg, J.H. 1996. Neoplasia and biotoxins in bivalves: is there a connection? Journal of Shellfish Research 15: 203-230.

Peters, E.C., P.P. Yevich, J.C. Harshbarger and G.E. Zaroogian. 1994. Comparative histopathology of gonadal neoplasms in marine bivalve molluscs. Diseases of Aquatic Organisms 20: 59-76.

Van Benden, R.J. 1994. Molecular analysis of bivalve tumors: models for environmental/genetic interactions. Environmental Health Perspectives 102: 81-83.

Van Beneden, R.J., G.R. Gardner, N.J. Blake and D.G. Blair. 1993. Implications for the presence of transforming genes in gonadal tumors in two bivalve mollusk species. Cancer Research 53: 2976-2979.

Yevich, P.P. and C.A. Barszcz. 1976. Gonadal and hematopoietic neoplasia in Mya arenaria. Marine Fisheries Review 38: 42-43.

Citation Information

Bower, S.M. (2006): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Gonadal Neoplasia of Clams.

Date last revised: December 2006
Comments to Susan Bower

Date modified: