Parvilucifera sinerae is a parasitoid of dinoflagellates, the major phytoplankton group responsible of harmful algal blooms. Here, we combine different microscope techniques to describe in detail the life cycle of P. sinerae. Infection kinetics, parasitoid mortality during the infection process, and the effect of the inoculum size on parasitoid prevalence are also presented. The life cycle of P. sinerae consists of a free-living zoospore which penetrates a host cell where it develops a trophocyte while destroying the host cytoplasm. Then, trophocyte undergoes schizogony to form hundreds of new zoospores inside a sporangium. With an external stimulus, the dormant zoospores are activated and released into the marine environment where they are able to infect a new host. P. sinerae completes its life-cycle in 3-4 days in the dinoflagellate Alexandrium minutum, being the trophocyte development the long lasting life-stage (~50h), whereas the replication occurs relatively quickly (~24h). Our results show that the prevalence of P. sinerae depends on inoculum size. Moreover, the main parasitoid losses happen during invasion. These results, together with the high number of zoospores produced from one single infection suggests that (1) this process is denso-dependent, (2) P. sinerae zoospores have a reduced time-life outside the host, and (3) the host has defense mechanisms to avoid infection

XVIII Simposio Ibérico de Estudios de Biología Marina (SIEBM), 2-5 september 2014, Gijón.-- XVIII Simposio Ibérico de Estudos de Biologia Marinha (SIEBM).-- 1 page

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