Trematode Metacercariae of Scallops


Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Metacercarial infection of scallops, Pimientilla.

Scientific name or taxonomic affiliation

Encysted metacercaria identified as Stephanostomum sp. in the family Acanthocolpidae.

Geographic distribution

Both coasts of Baja California Sur, México.

Host species

Argopecten ventricosus. Various other species of trematode metacercaria have been reported in various bivalves including oysters, mussels and clams.

Impact on the host

Infection results in the production of tiny melanized spots (about 0.5 mm in diameter) in the adductor muscle. Argopecten ventricosus from Bahía Magdalena and Bahía Concepción had high prevalences of infection (98%) with between a few to more than 800 cysts per adductor muscle. The effect of the parasite on the health of natural and cultured scallops is unknown but the visibility of infection is likely to impact on market value.

Diagnostic techniques

Gross Observations: Small brown spots, which resemble pepper granules, on the surface and throughout the adductor muscle.

Squash Preparations: Thin slices (about 3 mm thick) of the adductor pressed between two Plexiglas plates or glass slides and examined under a dissecting microscope (50 ×) show metacercariae. Metacercaria extracted from the cysts can be preserved in AFA (80 parts 70% ethanol, 10 parts formaldehyde and 10 parts glacial acetic acid). Preserved specimens stained with Gomori Trichromic and mounted in synthetic resin were about 764 µm long, 280 µm wide, had two rows of tegumental oral spines (28 spines per row) on the terminal oral sucker and a preequatorial ventral sucker about 264 µm in diameter (Pérez-Urbiola et al. 2001).

Histology: Cross sections of metacercariae can be observed within the adductor muscle tissues.

Methods of control

Methods for prevention and control have not been investigated. However, the exclusion of the final host (unknown but likely to be fish) or first intermediate host (possibly a species of marine snail) should prevent the infection from occurring in cultured scallops.


Pérez-Urbiola, J.C. and S.F. Martínez-Díaz. 2001. Stephanostomum sp. (Trematoda: Acanthocolpidae), the cause of "pimientilla" disease in catarina scallop Argopecten ventricosus (circularis) (Sowerby II, 1842) in Baja California Sur, México. Journal of Shellfish Research 20: 107-109.

Citation Information

Bower, S.M. (2003): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Trematode Metacercariae of scallops.

Date last revised: June 2003
Comments to Susan Bower

Date modified: