The morphology and host-specificity of the histophagous apostome ciliate Vampyrophrya pelagica infecting pelagic copepods in the Seto Inland Sea, Japan, were intensively investigated. Four stages were reconfirmed in the life cycle of the ciliate. A mature cell within the phoront bears cilia ready for quick excystation, and unique lamellar structures in the cytoplasm appear to be precursors of food vacuole membranes. These lamellar structures completely disappear in the fully grown trophont. The phoronts were attached to the ventral surface of the copepod prosome or legs, but were almost totally absent on the urosome. The number of phoronts per copepod was up to 43 for the adult female of Paracalanus parvus s.l. Phoront attachment was found irrespective of developmental stage and sex of P. parvus s.l., although the early copepodid stages were less frequently infected than the later stages, and the adult female was more intensively infected than the adult male. There was a marked seasonal change in prevalence and host-specificity of the phoronts. From middle summer to early winter, P. parvus s.l., Acartia pacifica, Tortanus forcipatus, Euterpina acutifrons, and Corycaeus affinis were frequently infected, while Oithona spp. and Microsetella norvegica were rarely infected, whereas from late winter to early summer, phoronts were detected only on the large-sized calanoids, Calanus sinicus and Euchaeta plana. This may be explained by a combination of longevity and molting of copepods, turnover time of the apostome life cycle which depends on water temperature, and seasonal changes in the abundance and food selectivity of predatory chaetognaths. Considering the high prevalence of apostome ciliates on not only copepods but also other crustaceans in the world oceans, the ecological influence of these ciliates on marine ecosystems should be re-evaluated. KEY WORDS: Apostome ciliate· Vampyrophrya pelagica · Copepod · Parasite · Host · Histophagy · Trophodynamics Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 282: [129][130][131][132][133][134][135][136][137][138][139][140][141][142] 2004 are epibionts which live exclusively on the copepod body surface (Steuer 1932, Sewell 1951, Hiromi et al. 1985, Nagasawa 1986, Fernandez-Leborans & TatoPorto 2000.Apostome ciliates are known to infect a wide variety of marine and freshwater crustaceans including shallow-and deep-water copepods (Chatton & Lwoff, 1935, Sewell 1951, Kudo 1966, Lindley 1978, Grimes & Bradbury 1992, Ohtsuka et al. 2003. The complex, unique life cycle of the apostome ciliate Vampyrophrya pelagica (Chatton and Lwoff) on coastal copepods was elucidated by Grimes & Bradbury (1992). According to them, 4 functionally different stages are recognized in this apostome: phoront (resting stage), trophont (feeding stage), tomont (division stage), and tomite (infective stage). Excystation of the trophont is triggered either by injury to the host (single-host cycle) or by predation by invertebrate predators such as...