Resumen del producto

Histophagous ciliates of the genus Pseudocollinia cause epizootic events that kill adult female krill (Euphausiacea), but their mode of transmission is unknown. We compared 16S rRNA sequences of bacterial strains isolated from stomachs of healthy krill Nyctiphanes simplex specimens with sequences of bacterial isolates and sequences of natural bacterial communities from the hemocoel of N. simplex specimens infected with P. brintoni to determine possible trans- mission pathways. All P. brintoni endoparasitic life stages and the transmission tomite stage (out- side the host) were associated with bacterial assemblages. 16S rRNA sequences from isolated bac- terial strains showed that Photobacterium spp. and Pseudoalteromonas spp. were dominant members of the bacterial assemblages during all life phases of P. brintoni and potential patho- bionts. They were apparently unaffected by the krill’s immune system or the histophagous activity of P. brintoni. However, other bacterial strains were found only in certain P. brintoni life phases, indicating that as the infection progressed, microhabitat conditions and microbial interactions may have become unfavorable for some strains of bacteria. Trophic infection is the most parsimo- nious explanation for how P. brintoni infects krill. We estimated N. simplex vulnerability to P. brin- toni infection during more than three-fourths of their life span, infecting mostly adult females. The ciliates have relatively high prevalence levels (albeit at <10% of sampled stations) and a short life cycle (estimated < 7 d). Histophagous ciliate?krill interactions may occur in other krill species, par- ticularly those that form dense swarms and attain high population densities that potentially enhance trophic transmission and allow completion of the Pseudocollinia spp. life cycle.

Palabras clave: Euphausiacea; Collinia; parasite; Gulf of California; Histophagous; Apostome ciliates; host; association

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