Lankesteria ascidiae is a protozoan (Kingdom Chromista; Phylum Ciliophora; Infaphylum Sporozoa; subclass Gregararasina) parasite of tunicates, including Ciona intestinalis, Dendrodoa grossularia, Polycarpa pomaria, Styela rustica (Levine 1977). It belongs to a group known as gregarines, which hatch from an oocyst, and penetrate host cells, growing and feeding as trophozooites. Two trophozooites pair up, and become enclosed by a wall as a gametocyst, The gametocyst breaks up into hundreds of gametes. Two gametes can to fuse form an oocyst, which is enclosed by a wall. Meiosis occurs within the oocysts, releasing sporozoites, and repeating the cycle.
Lankesteria ascidiae was described from the Northeast Atlantic (England) from Ciona intestinalis, by Lankester in 1872. It was later found in France, and Italy) and in Ciona robusta in Japan. It was found in the Northeast Pacific, at the Hopkins Marine Station on Monterey Bay, in 1980 (when this tunicate was identified as C. intestinalis). This tunicate was presumably introduced with its host Ciona robusta, in ship fouling. Lankesteria ascidiae has also been found in Japan, where C. robusta is the native form, once previously identified as C. intestinalis, there, too. In cultured C. robusta, in Japan, L. ascidiae is associated with a 'long feces syndrome', in which the intestine become clogged by digested material. Growth of the diseased animals is inhibited, and the intestine becomes pale, and the tunicate dies within several weeks. This is a problem in culturing C. robusta for research purposes, but could be seen as beneficial for fouling control.