JOHNSON ET AL.: RHIZOCEPHALAN INFECTION IN BLUE KING CRABS 



The lack of obvious externas on the parasitized 

 crabs is puzzling. One possibility is that externas are 

 produced but are inconspicuous and/or evanescent. 

 Most rhizocephalans produce easily detected exter- 

 nas that emerge from the venter of the abdomen. 

 Species of Thompsonia, however, produce multiple 

 small externas 1-4.5 mm long and no more than 1.1 

 mm in diameter. These externas occur on the ap- 

 pendages and venters of the thorax and abdomen, 

 depending on the species, and those of at least one 

 of the species are easily dislodged (Hafele 1911; Potts 

 1915; Phang 1975). If few and scattered externas 

 of the Thompsonia type were present, they could 

 have escaped notice on animals as large as the blue 

 king crabs investigated. The second possibility is that 

 externas are not developed in the blue king crab. 

 Host ranges of rhizocephalans are often broad, but 

 some of the host/parasite associations may be acci- 

 dental or not fully evolved. Sacculina carcini is 

 known to react differently in different species of 

 crabs. In Carcinus maenas multiple broods of lar- 

 vae are produced by S. carcini, but if the host is Por- 

 tunus holsatus, it breeds but once and then is shed, 

 which suggests that C. maenas is a natural host but 

 P. holsatus is an adventitious and not entirely com- 



petent one (Baer 1951). Perhaps the Olga Bay para- 

 site is not a usual parasite of the blue king crab, and 

 although the interna develops extensively and causes 

 severe damage to female gonads, externas cannot 

 be produced in this species. The fact that some roots 

 of the parasite were degenerating or necrotic in most 

 infected crabs suggests that parasites do die within 

 the blue king crab, and that infections might be lost 

 before externas are formed. 



ACKNOWLEDGMENTS 



We are grateful to E. Munk, J. Bowerman, and R. 

 Otto of the Kodiak Laboratory for assistance with 

 fieldwork; to S. Meyers, also of the Kodiak staff, for 

 laboratory assistance; to G. Roe and C. Smith of the 

 Oxford Laboratory for preparing tissues for histo- 

 logical examination; to R. Otto for reviewing the 

 manuscript; to T R. Meyers, University of Alaska, 

 Juneau, for providing tissues of blue king crabs in- 

 fected with Briarosaccus callosus; and finally, to Bill 

 Pinnell and Morris Talifson of Olga Bay, without 

 whose logistic support and hospitality the fieldwork 

 would have been twice as difficult and infinitely less 

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FIGURE 6.— Briarosaccus callosus: Roots. Note lack of a central lumen and the very large, peripheral nuclei 



(arrows). Feulgen. C, cuticle Bar =10 ^m. 



183 



