Psorospermium spp. (Protozoan Infection) of American Crayfish


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Psorospermium sp. of North America, Protozoan infection.

Scientific name or taxonomic affiliation

Psorospermium sp. probably more than one species.

Geographic distribution

North America including Canadian provinces of Ontario and Alberta, and United States states of Minnesota, Wisconsin, Louisiana, Florida as well as west coast USA.

Host species

Orconectes rusticus, Orconectes virilis, Orconectes immunis, Orconectes limosus, Orconectes propinquus, Procambarus clarkii, Procambarus zonangulus, Procambarus alleni, Procambarus fallax, and Pacifastacus leniusculus. Other species ( or morphotypes) of Psorospermium occur in various species of crayfish in Europe and Australia.

Impact on the host

In some cases prevalence of infection was 100% and intensity varied from 0-510 spores per mg of tissue. Seven different morphological forms were described from P. clarkii by Henttonen et al. (1997). The 1st amoeboid forms (about 10 µm in diameter) were covered by a flexible membrane and containing one or two groups of small globules. The 2nd round naked forms (25-30 µm in diameter) contained one or two nuclei. The 3rd ellipsoid forms had a gelatinous-looking cell wall. The 4th elongated forms were longer than the 3rd form, the thin cell wall had a reticular pattern and the cell contents are uniform globules. The 5th mature forms are the typical most common form of Psorospermium spp. and are described below. In addition, large naked forms and abnormal forms have been reported (Henttonen et al. 1997). Some abnormal forms consist of two or three lobes and other abnormal forms, which increase rapidly when a crayfish dies, have an innermost membranous layer that loses its transparency. Apart from occasional evidence of a weak melanization response, no pathology was reported.

Diagnostic techniques

Squash Preparations: Long (120-200 × 35-60 µm) and short (70-120 × 35-60 µm) ovoid spores with multi-layered refractive cell wall (with one layer consisting of numerous jointed plates) and with contents varying between small uniform globules to large globules located in one continuous line and surrounded by a few small globules (Henttonen et al. 1997). Henttonen et al. (1994) reported four distinct mature forms: long with pointed ends, long with rounded ends, short with pointed ends and short and slightly curved . The nuclei within the spores are difficult to observe because stains penetrate through the cell wall and into the cytoplasm very weakly. Spores predominantly occur in the abdominal muscle and/or in the epidermal connective tissue below the carapace and sometimes in the gills. Spores are visible in crayfish preserved in 5-10% formalin or cooked when tissue samples were treated with 10% KOH on the slide prior to applying the coverslip.

Methods of control

Methods of transmission are not understood and experimental exposure by feeding Psorospermium-infected crayfish tissues to juvenile P. clarkii and P. zonangulus has had limited success (Henttonen et al. 1997). No known methods of control.


Aiken, D. 1989. Marron fungus? World Aquaculture 20: 6-7.

Evans, L.H. and B.F. Edgerton. 2002. Pathogens, parasites and commensals; Chapter 10. In: Holdich, D.M. (ed.) Biology of Freshwater Crayfish. Blackwell Sciences Ltd., Oxford, England. pp. 377-438.

Henttonen, P. and O.V. Lindqvist. 1992. Incidence of Psorospermium sp. in several cultivated populations of crayfishes, Procambarus spp. (Decapoda, Cambaridae), in southern Louisiana. Journal of the World Aquaculture Society 23: 31-37.

Henttonen, P., J.V. Huner and O.V. Lindqvist. 1994. Occurrence of Psorospermium sp. in several North American crayfish species, with comparative notes on Psorospermium haeckeli in the European crayfish, Astacus astacus. Aquaculture 120: 209-218.

Henttonen, P., J.V. Huner, P. Rata and O.V. Lindqvist. 1997. A comparison of the known life forms of Psorospermium spp. in freshwater crayfish (Arthropoda, decapoda) with emphasis on Astacus astacus L. (Astacidae) and Procambarus clarkii (Girard) (Cambaridae). Aquaculture 149: 15-30.

Citation Information

Bower, S.M. (2006): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Psorospermium sp. (Protozoan Infection) of American Crayfish.

Date last revised: June 2006
Comments to Susan Bower

Date modified: