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An apostome ciliate, Collinia oregonensis n. sp., is reported inhabiting the cephalothorax and abdomen of 3 euphausiid species from the Oregon-Washington coast: Euphausia pacifica Hansen, 1911, Thysanoessa spinifera Holmes, 1900, and Thysanoessa gregaria G.O. Sars, 1883. This ciliate is the 7th species described for the genus Collinia and the 2nd species known to infect euphausiids. Disease progression and ciliate morphology are described using (1) modified protargol stain, (2) hematoxylin counterstained with Fast Green, and (3) Scanning Electron Microscopy (SEM). All endoparasitic developmental stages (trophont, tomont, tomitogenesis, protomite, and tomite) of C. oregonensis are astomatous and possess between 14 and 22 kineties. C. oregonensis is smaller than C. beringensis Capriulo &amp; Small, 1986, which infects the euphausiid Thysanoessa inermis Krøyer, 1846 in the Bering Sea and which possesses between 24 and 80 kineties. The ciliate is a parasitoid because it must kill the host to complete its life cycle. Infections and mortalities in multiple host species likely reflect the virulent nature of the ciliate. Adult euphausiids infected with this parasitoid possess a swollen and bright orange cephalothorax. C. oregonensis feeds and proliferates inside euphausiids, producing fulminating infections that rupture the cephalothorax and release large numbers of tomites into the surrounding water. After several hours in the free swimming stage under shipboard conditions in the present study, the tomites adhered to each other, forming filaments. Infection rates ranged between 3 and 20% within individual euphausiid aggregations, but infected aggregations were randomly and sparingly distributed. Infected euphausiids were found at 6.7% of 316 stations sampled during 3 summer cruises. No infected euphausiids were collected in winter. Because E. pacifica and T. spinifera account for about 90% of the euphausiid standing stock in the northern California Current System, this parasitoid ciliate may have a significant impact on euphausiid population abundance, distribution and secondary productivity.

Discovery of a ciliate parasitoid of euphausiids off Oregon, USA: Collinia oregonensis n. sp. (Apostomatida: Colliniidae)