Sylon (Rhizocephalan Disease) of Shrimp and Prawns


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Sylon infection, Rhizocephalan disease of shrimps.

Scientific name or taxonomic affiliation

Sylon sp.; the only recognized species is Sylon hippolytes.

Geographic distribution

Widely distributed in shrimp and prawn populations of the northern hemisphere: North Atlantic Ocean including the coasts of Iceland, Spitzbergen, Greenland, Shetland Isles, and Franz Josef Land; coastal areas of the North Pacific Ocean including the adjacent Bering Sea, Chukchi Sea, Sea of Okhotsh and North Sea of Japan.

Host species

Known from 21 species of Caridea shrimp belonging to three families: Crangonidae, Pandalidae and Hippolytidae.

Impact on the host

Roots of the parasite invade all tissues of the cephalothorax. Infection causes stunting of growth, modification of the secondary sexual characteristics (i.e. morphological alterations of the appendix masculina and appendix interna on the endopod of the second pleopods) and castration. Infected prawns usually die when the parasite has completed its life cycle. The few surviving prawns are marked by obvious brownish to black scars in tissues formerly inhabited by the parasite. Sylon sp. usually occurs in low prevalences (less than 5%) on many species of shrimp throughout the northern hemisphere. However, high prevalences (up to 47%) were found on some stocks of prawns (Pandalus platyceros) in northern British Columbia while stocks of P. platyceros in southern British Columbia are not infected.

Diagnostic techniques

Gross Observations: The externa is usually attached to the ventral surface of the abdomen and depending on the stage of infection can be a pinkish to white coloured plump ovoid sac (growing to about 1.5 cm in length and 1.0 cm in width) or can be grayish coloured and wrinkled when spent. Mature cyprid larvae expelled from the externa are ovoid in shape and approximately 170µm long, this stage has been observed to swim rapidly with a flicking action of the posterior appendages and to walk on solid surfaces using its antennal appendages with their suction cup-like extremities.

Figure 1. Pandalus platyceros with a large white coloured Sylon sp. externa (arrow) protruding from the ventral surface of the abdomen.

Figure 2. Pandalus platyceros showing a black scar (S) on the ventral surface of the abdomen at the attachment site of a lost Sylon sp. externa. Also visible are brownish to black coloured interna (I) that are the necrotic and melanized roots of the parasite.

Figure 3. Three specimens of Spirontocaris holmesi showing the developmental progression of Sylon sp. externae. The top specimen has a recently emerged small bright pink externa containing developing ova. The middle specimen has a large plump externa containing cyprid larvae in an advanced stage of development. The bottom specimen has a grayish coloured wrinkled (spent) externa from which most of the cyprid larvae have been expelled.

Figure 4. Wet mount preparation of a Sylon sp. cyprid larva showing the posterior appendages (P) and antennal appendages (A).

Histology: Parasite roots called interna can be observed within and around the organs of the cephalothorax. If the parasite is intact, there is usually no cellular response by the host to the infection. When the externa is lost, a massive melanotic inflammatory response occurs around the interna causing the interna to die and acquire a brown colour.

Figure 5. Histological section through branches of a healthy interna (arrows) of Sylon sp. between the hepatopancreas tubules (T) and gonad (G) within the cephalothorax of Pandalus platyceros. Haemotoxylin and eosin stain.

Methods of control

No known methods of prevention or control. In British Columbia, there is great concern that the Sylon infecting prawns (P. platyceros) is different from that affecting other species of shrimp. Until the risk of introducing the abundant prawn Sylon found in prawn stocks from northern B.C. to uninfected southern prawn stocks can be accurately assessed, there will have to be restrictions/controls placed on commercial fisherman who wish to transport live prawns from northern stocks to southern markets.


Bower, S.M. and J.A. Boutillier. 1988. Enigma of Sylon (Crustacea: Rhizocephala) infections on the shrimp Pandalus platyceros, in British Columbia. Abstracts Third International Colloqium on Pathology of Marine Aquaculture, Gloucester Point, VA. 2-6 October 1988, p. 59-60.

Bower, S.M. and J.A. Boutillier. 1989. Extent of castration of prawns (Pandalus platyceros) by Sylon (Crustacea: Rhizocephala). Abstract. Journal of Shellfish Research 8(2): 467.

Bower, S.M. and J.A. Boutillier. 1990. Sylon (Crustacea: Rhizocephala) infections on the shrimp in British Columbia. In: F.O. Perkins and T.C. Cheng (eds.). Pathology in Marine Science. Academic Press, p. 267-275.

Butler, T.H. 1980. Shrimps of the Pacific coast of Canada. Canadian Bulletin of Fisheries and Aquatic Sciences 202, p. 11-15.

Høeg, J. and J. Lützen. 1985. Marine Invertebrates of Scandinavia, No. 6. Crustacea Rhizocephala. Norwegian University Press, Oslo, p. 24-27.

Jensen, P.G., J.T. Høeg, S. Bower and A.V. Rybakov. 1994. Scanning electron microscopy of lattice organs in cyprids of the Rhizocephala Akentrogonida (Crustacea: Cirripedia). Canadian Journal of Zoology 72: 1018-1026.

Lützen, J. 1981. Observations on the rhizocephalan barnacle Sylon hippolytes M. Sars parasitic on the prawn Spirontocaris lilljeborgi (Danielssen). Journal of Experimental Biology and Ecology 50: 334-347.

Citation Information

Bower, S.M., Meyer, G.R. (1998): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Sylon (Rhizocephalan Disease) of Shrimp and Prawns.

Date last revised: August 1998
Comments to Susan Bower

Date modified: