Pea Crabs in Mussels


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Oyster crab, Pea crab.

Scientific name or taxonomic affiliation

Tumidotheres (=Pinnotheres) maculatus, Pinnotheres pisum, Pinnotheres spp., Fabia subquadrata (Decapoda: Pinnotheridae).

Geographic distribution

Eastern and western U.S., Europe, Argentina, coast of Japan, British Columbia.

Host species

Mytilus edulis, Mytilus californianus, Modiolus modiolus and other species of bivalves including oysters, clams, cockles and scallops.

Impact on the host

The mantle cavity may contain several crabs thereby reducing market value. Infestation prevalances between 54.3 to 72.6% were reported for T. maculatus in M. edulis from an offshore bank near Quequén, Argentina. Numbers are limited by mantle space with no evidence of direct pathology. The crabs feed on food items collected by their hosts. The association of P. pisum with M. edulis was described as amensalism for male pea crabs (of benefit to the facultative male pea crab at the expense of the mussel) and parasitism for females (again of benefit to the pea crab and detrimental to the mussel but in this case the relationship is obligatory) (Haines et al. 1994). Mytilus edulis harbouring late stage and adult female T. maculatus had significantly lower shell length and meat dry weight (Tablado and Gappa 1995).

Diagnostic techniques

Gross Observations: Pea crabs are visible to the naked eye within the mantle cavity of their hosts.

Methods of control

Precautions should be taken to prevent their introduction. Note: They can invade other commercially important bivalves such as Mya arenaria, Placopecten magellanicus, Argopecten irradians and oysters.


Cheng, T.C. 1967. Marine molluscs as hosts for symbioses with a review of known parasites of commercially important species. In: F.S. Russell (ed.) Advances in Marine Biology. Volume 5. Academic Press Inc., London, p. 315-335.

Haines, C.M.C., M. Edmunds and A.R. Pewsey. 1994. The pea crab, Pinnotheres pisum (Linnaeus, 1767), and its association with the common mussel, Mytilus edulis (Linnaeus, 1758), in the Solent (UK). Journal of Shellfish Research 13: 5-10.

Hart, J.F.L. 1982. Crabs and their relations of British Columbia. British Columbia Provincial Museum, Victoria. 267 pp.

Kubota, S. 1983. Studies on life history and systematics of the Japanese commensal hydroids living in bivalves, with some reference to their evolution. Journal of the Faculty of Science, Hokkaido University, Series VI, Zoology 23: 296-402 (specifically see pages 328-329).

Pearce, J. B. 1964. On reproduction in Pinnotheres maculatus (Decapoda: Pinnotheridae). Biological Bulletin Marine Biology Laboratory Woods Hole 127: 384.

Tablado, A. and J.L. Gappa. 1995. Host-parasite relationships between the mussel, Mytilus edulis L., and the pea crab, Tumidotheres maculatus (Say), in the southwestern Atlantic. Journal of Shellfish Research 14: 417-423.

Citation Information

Bower, S.M. (1996): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Pea Crabs in Mussels.

Date last revised: September 1996
Comments to Susan Bower

Date modified: