Biology of symbiotic apostome ciliates: Their diversity and importance in the aquatic ecosystems

Abstract

Recently apostome ciliates have been paid more attention in the aquatic ecosystems, since the host population dynamics are more highly influenced by these protists than previously expected. Apostomes are all symbiotic, associated mainly with a wide variety of planktonic and benthic crustaceans and other invertebrates such as cnidarians and chaetognaths . They can also be found within cysts of other apostomes . Some taxa require two distinct hosts. The life cycles of apostomes are complicated, essentially consisting of four morphologically and functionally different stages: quiescent, encysted phoront s ; feeding trophonts ; divisional, encysted tomont s ; and infective tomite s , with several substages in some taxa. Each metamorphosis accompanies reformation of kineties and organelles. Excystation from phoronts to trophonts is triggered by cues such as molting, injury and predation of hosts. A cell within a phoront is furnished with cilia ready to hatch, and with specialized, membranous organelles related to rapid expansion of food vacuole s. Trophonts with or without a cytostome take nutrients through phagocytosis or pinocytosis, respectively. Some taxa such as Gymnodinoides are exuviotrophic and harmless to the host, while genera such as Vampyrophrya are regarded as parasitoids rather than parasites . Proliferation is mainly due to palintomy to produce numerous tomites within a tomont , whose duration seems to be most greatly influenced by water temperature in the life cycle of apostomes. Tomites actively search for a new host and then transform into phoronts on it. The present paper briefly reviews previous studies concerning apostomes , and our original data on the histotrophic species Vampyrophrya pelagica infecting copepod s.