Marine plankters harbor a wide variety of symbionts ranging from accidental epibionts to true parastioids. Trophic interactions in symbioses in marine plankton communities are discussed on the basis of our own studies. Some alveolates such as ciliates, dinoflagellates and their kin are parasitoids to kill hosts such as ciliates and copepods. The apostome ciliate Vampyrophrya infesting copepods is furnished with special organelles and a large cytostome to gulp host tissues. In the Seto Inland Sea, the prevalence of this ciliate on calanoid copepods was high up to 100% in warm seasons. The life cycle of Vampyrophrya is composed of two ways with or without secondary hosts such as chaetognaths, depending on the composition and abundance of the primary/secondary hosts and water temperature. Some trematodes utilize jellyfish as intermediate hosts, and finally parasitize medusivorous fish via predation. Medusivorous jellyfish also play a role as paratenic hosts. In addition to trematodes, many kinds of fish and invertebrates are associated with large-sized jellyfish. These symbionts tend to nutritionally depend on prey captured by the hosts. Both a nicothoid copepod Neomysidion and a dajid isopod Prodajus parasitize mysids and inhabit the host marsupium. These two parasites temporarily partition the habitat and possibly foods (host eggs). As a result, the parasitism is constantly found throughout a year. These symbionts have never been included in consideration of the structures and functions of marine food webs. To exactly understand the marine ecosystems, more attention should be paid to these symbionts and dynamic trophic interactions between hosts and symbionts.