Continuous plankton records: The occurrence of apostome ciliates (Protozoa) on Euphausiacea in the North Atlantic Ocean and North Sea

The phoronts (resting cysts) of apostome ciliates have been recorded on Euphausia hemigibba, E. krohni, Meganyctiphanes norvegica, Nematoscelis megalops, Nyctiphanes couchi, Thysanoessa gregaria, T. inermis, T. longicaudata and T. raschi taken in the Continuous Plankton Recorder Survey of the North Atlantic Ocean and the North Sea. Variations in the percentage of host euphausiids infested by apostomes have been studied in relation to species, geography, body size and season. The percentage infestation was found to be related mainly to the body size of the hosts. The apostomes are believed to be species that develop either in the exuviae discarded exoskeletons or corpses of the euphausiids, or in the digestive systems of predators of the euphausiids.