Rhizocephalan Parasites of Crabs


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Rhizocephalan parasites of crabs, Parasitic barnacles of crabs.

Scientific name or taxonomic affiliation

Several species in the genera Lernaeodiscus, Sacculina, Loxothylacus, Thompsonia, and Briarosaccus.

Geographic distribution

Global with various species occurring in discrete locations while other species such as Briarosaccus callosus are thought to be cosmopolitan.

Host species

In various species of economically important crabs including Carcinus maenas, Callinectes sapidus, and species of Lithodes and Paralithodes.

Impact on the host

The root system of the parasite (called internae) invades the cephalothorax and may grow into the muscle tissues of the limbs and abdomen. The parasite often castrates its host and usually alters the hosts endocrinology which affects, moulting, behaviour and secondary sexual characteristics and may stunt growth. High prevalences of infection can conceivably reduce commercial stocks significantly, especially if populations were already severely depressed.

Diagnostic techniques

Gross Observations: Ovoid to sausage-shaped sac (called the externa) that is usually attached to the ventral surface of the abdomen of the crab host. The externa contains a mantle cavity (that is usually packed with eggs or developing nauplii larvae) and an ovary. The ovary is attached to the mantle (outer tissue layer of the externa) near the stock. The stock attaches the externa to the host. Roots (called internae) disseminate from the stock. Depending on the species, the internae may be colourless or green. Rhizocephala with green internae usually also have red coloured haemolymph that contains haemoglobin. The gonad of the host is usually atrophied.

Figure 1. Ventral view of a king crab Lithodes aequispina with a large externa of Briarosaccus callosus attached to the surface of the abdomen.

Figure 2. Wet mount preparation of a recently hatched Briarosaccus callosus nauplius larva.

Figure 3. Dorsal view of a king crab Lithodes aequispina with the carapace removed showing the green coloured Briarosaccus callosus internae (arrows) intertwined amongst the organs of the cephalothorax: tubules of the hepatopancreas (H), atrophied female gonad (G) and heart (He).

Histology: Internae, covered by a thin cuticle, permeate the organs in the cephalothorax. If the parasite is healthy there is usually no cellular response by the host to the internae. If the externa is lost, a massive melanotic inflammatory response occurs around the internae resulting in a visible brown colouration of the root system.

Figure 4. Histological section through healthy internae (arrows) of Briarosaccus callosus between the tubules of the hepatopanreas (T) within the cephalothorax of Lithodes aequispina. Haemotoxylin and eosin stain.

Figure 5. Histological section through healthy internae (arrows) of Briarosaccus callosus adjacent to and within the ventral nerve ganglion (N) of Lithodes aequispina. Haemotoxylin and eosin stain.

Figure 6. Histological section through necrotic internae (I) of Briarosaccus callosus surrounded by an intense accumulation of host haemocytes within the hepatopancreas of a Lithodes aequispina which had lost the parasite externa. Haemotoxylin and eosin stain.

Methods of control

No known methods of prevention or control. Infected crabs of all sizes should be removed from the water and destroyed.


Bower, S.M. and N.A. Sloan. 1985. Morphology of the externa of Briarosaccus callosus Boschma (Rhizocephala) and the relationship with its host Lithodes aequispina Benedict (Anomura). The Journal of Parasitology 71: 455-463.

Høeg, J.T. 1995. The biology and life cycle of the Rhizocephala (Cirripedia). Journal of the Marine Biological Association of the United Kingdom 75:517-550.

Meyers, T.R. 1990. Diseases of Crustacea. Diseases caused by protistans and metazoans. In: O. Kinne (eds.). Diseases of Marine Animals. Volume III: Introduction, Cephalopoda, Annelida, Crustacea, Chaetognatha, Echinodermata, Urochordata. Biologische Anstalt Helgoland, Hamburg, p. 350-423.

Overstreet, R.M. 1983. Metazoan symbionts of crustaceans. In: A.J. Provenzano Jr. (ed.). The Biology of Crustacea. Volume 6, Pathobiology. Academic Press, New York, p. 155-250.

Citation Information

Bower, S.M., Meyer, G.R. (1999): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Rhizocephalan Parasites of Crabs.

Date last revised: March 1999
Comments to Susan Bower

Date modified: