Pseudocollinia histophagous ciliates infect krill in the Pacific and Atlantic Oceans and possibly worldwide

The genus Pseudocollinia (Apostomatida, Pseudocolliniidae) currently includes four species (P. beringensis, P. oregonensis, P. brintoni, and P. similis) that infect adults of seven numerically dominant krill species (Order Euphausiacea) in the northeastern Pacific (Bering Sea-to-Gulf of California). In this review, we found four reports in other parts of the world of misidentified or unidentified protists infecting krill. Based on their morphology, cell size and infection of the hemocoel’s host, we infer they are histophagous Pseudocollinia ciliates. We thus conclude that previous reports of protists (identified as microsporidians) infecting Thysanoessa inermis in the northwestern Atlantic Ocean (Bay of Fundy) and unidentified endoparasite ciliates infecting the Euphausia superba in the Southern Ocean (Admiralty Bay, King George Island), Euphausia pacifica in Sanriku, Japan and Euphausia similis var. armata in Tasmania, Australia are actually Pseudocollinia parasitoid ciliates that await to be morphologically and genetically described. This review provides strong evidence that apostome Pseudocollinia ciliates are widespread distributed in the Pacific and Atlantic Oceans, infecting at least nine krill species from Arctic-to-Antarctic zoogeographic regions, having perhaps worldwide distribution as several of their krill hosts. Future studies should focus on discovering parasitoid ciliates in other krill species, as well as in other phytoplankton and zooplankton taxonomic groups.