crustacean Briarosaccus callosis - Parasitic

I. Causative Agent and Disease have an enlarged, raised abdominal Briarosaccus callosus is a para- flap and exaggerated growth of coxal sitic barnacle belonging to the setae. Histological examination confirms Arthropoda, subphylum Crustacea, the extensive infiltration of tissues by the , subclass , basophilic branching tubules of the root- infraclass Cirripedia () and lets generally causing no inflamma- the superorder (parasitic tory response. However, encapsulation barnacles). Rhizocephalan barnacles are of rootlets has been observed when the primarily parasites of decapod crus- inflammatory response of the host taceans but also occur in free-living is stimulated by a concurrent rickettsial barnacles and bivalve molluscs. They are infection. noted for their invasiveness but general non-lethality in the natural host and IV. Transmission endocrinological suppression of host The complete life history of B. cal- fecundity at the population level. losus is not known but a portion of larval biology has been established to allow II. Host analogy with other known rhizocepha- Briarosaccus callosus is a cos- lans. The externa is a double cylinder mopolitan rhizocephalan parasitizing with an internal chamber containing ova several species of lithodid crabs most that move into the outer chamber as the notably from the southeast of North embryos mature causing the externa col- America, Antarctica and sub Antarctic or to become pale orange. Stage I nauplii areas, southwest Indian Ocean, the Ber- released from the externa become plank- ing , the Gulf of Alaska, southeast tonic during which successive molts Alaska and British Columbia, Canada. occur to the final stage IV before molting Known crab species parasitized by B. into a cyprid. As with other rhizocepha- callosus in Alaska include the red, blue lans, these cyprids are presumed to be and golden king crabs. the infectious stage and are sexually di- morphic in that one externa may produce III. Clinical Signs cyprids that are small and female while External clinical signs include a another produces larger male cyprids. large orange/red sausage-shaped sac The smaller female cyprids attach to the known as an externa that is attached by lamellae of a crab host and molt to a stalk underneath the abdominal flap. the kentrogon stage which injects a cell The externa contains ova and/or larvae mass through a hollow stylet. These cells of the parasite. Radiating into the crab proliferate in the crab host becoming the viscera from the externa is an emerald female interna and reproductive externa. green dendritic mass or root system Newly emerged virginal externa do not called the interna which largely replaces reach sexual maturity until fertilized by the hepatopancreas extending rootlets larger male cyprids to begin the cycle into all major organs and tissues of the again. visceral cavity including nerves and the bases of and muscle within the V. Diagnosis coxal joints. Gonads of both crab sexes Diagnosis is made by observation of are atrophied or absent. Parasitized male the typical orange/red externa attached 108 109 Arthropods

by a stalk underneath the abdominal flap and necrotic causing an intense inflam- in lithodid crabs and the emerald green matory foreign body response that may interna within the visceral mass. Parasit- kill the crab host. The parasite alters the ized crabs, including males, have femi- endocrinology of the host crab causing nized traits of an enlarged abdominal atrophy of the gonads (castration) in both flap and a thick ventral growth of coxal sexes and suppressed growth which ef- setae, atrophied gonads and a protective fectively limit recruitment. The negative behavior towards the attached externa. impact of this parasite on a population The externa utilizes hemoglobin as a scale may be significant in certain areas respiratory pigment and bleeds red when depending on prevalences which have damaged. Histological examination varied in southeast Alaska from less than demonstrates the dendritic branches of 1% in red king crabs, 20% in golden king the interna that retain the green pigment crabs and up to 76% in blue king crabs. granules in fixed material. VII. Human Health Significance VI. Prognosis for Host There are no zoonotic human Crabs are able to molt successfully health concerns regarding with attached externa but the likelihood of king crabs by Briarosaccus callosus. for long-term survival is not known once However, the rootlets may cause green the parasite externa is lost or becomes discoloration of knuckle meats despite senescent. Externa do not regenerate and cooking. the interna rootlets become atrophied

Dual parasitism of by Bria- Cyprid of Briarosaccus callosus from the rosaccus callosus showing viable externa externa of parasitized blue king crab (orange) and second senescent externa that has become necrotic (arrow)

108 109 crustacean Arthropods Briarosaccus callosis - Parasitic Barnacle

 Histological section of blue king crab Histological section of Briarosaccus callosus thoracic ganglion invaded by Briarosaccus in another blue king crab infected with a callosus rootlets (arrow) of the interna with rickettsia-like organism showing encapsula- no host inflammatory response tion of rootlets (arrow) by host cells

Briarosaccus callosus in blue king crab with typical green rootlet mass from healthy parasite externa

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crustacean Arthropods 

Parasitized blue king crab with missing externa resulting in black, atrophied and necrotic internal rootlet mass (arrow)

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Blue king crab parasitized by Briarosaccus callosus showing atrophied yellow ovary (arrow) and obvious green rootlets of the interna in the hepatopancreas; compare with normal ovary on next page; note coagulated hemolymph nodule (arrowhead) resulting from concurrent infestation by the ciliate , Mesanophrys sp.

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