Trematode Metacercariae of Crabs


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Metacercarial infection of crabs.

Scientific name or taxonomic affiliation

Various species of Digenea including Microphallus spp., Megalophallus spp.

Geographic distribution

Global, although each species probably has a confined distribution.

Host species

Many species of crabs including Callinectes sapidus and Cancer magister. Metacercariae have also been reported from lobsters and crayfish.

Impact on the host

Majority of the species are innocuous. However, very heavy infections may result in lysis of host muscle tissue surrounding the parasite and ataxia by species that encyst in the nervous system. Hyperparasitism by the haplosporidian Urosporidium crescens causes the larval fluke to become enlarged (0.4-0.6 mm in diameter) and darkly pigmented resulting in the condition called buckshot, pepper spots, or pepper crabs and decreased marketability.

Diagnostic techniques

Gross Observations: Invisible to the unaided eye unless hyperparasitized by U. crescens.

Squash Preparations: Metacercaria can be observed in thin slices (about 3 mm thick) of tissue pressed between two glass plates and examined under a dissecting microscope (50 × magnification).

Histology: Cross sections of metacercariae can be observed in stained tissue sections.

Methods of control

Prevention and control impractical.


Messick, G.A. and C.J. Sindermann. 1992. Synopsis of principal diseases of the blue crab, Callinectes sapidus. NOAA Technical Memorandum NMFS-F/Nec-88. 24 p.

Sparks, A.K. 1985. Synopsis of Invertebrate Pathology Exclusive of Insects. Elsevier Science Publishing Co., Amsterdam. p. 352-353.

Citation Information

Bower, S.M., McGladdery, S.E., Price, I.M. (1994): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Trematode Metacercariae of Crabs.

Date last revised: Fall 1994
Comments to Susan Bower

Date modified: