Nemerteans as hosts for symbionts: a review.

Nemerteans utilize other animals as hosts in a variety of well‐studied, symbiotic relationships, but relatively little is known concerning nemerteans as hosts for other organisms. Only three haplosporidians, one microsporidian, two gregarines, three ciliates, five mesozoans, one mite and one copepod have been identified to species from nemerteans, whereas 34 nemertean species (one Archinemertea, 17 Heteronemertea, 15 Hoplonemertea, one Bdellonemertea) have been recorded as hosts. The majority of recorded nemertean symbionts are endoparasitic protozoans and orthonectids. Among the Haplosporidia, there are three species of Haplosporidium . The hyperparasite, Nosemoides vivieri , from an unidentified gregarine, is the only known microsporidian associated with a nemertean. Within the phylum Apicomplexa, besides the two species of gregarines from three species of nemerteans, there are 11 other species of nemerteans with unidentified gregarines. One species of nemertean has been found with an unidentified coccidian parasite. ‘Sporozoan‐like’ parasites, however, are known from three other species of nemerteans, two of which are pelagic polystiliferans. Two endosymbiotic and one ectosymbiotic species of ciliates have been identified from three species of nemerteans. At least five species of mesozoans (Orthonectida) infect nine species of nemerteans. Unidentified cestode, trematode and nematode juveniles, using nemerteans as intermediate hosts, have been recorded rarely and have not been linked to any life cycles. An ectosymbiotic mite and a copepod from two species of nemerteans are likely incidental associates. The nature of the host–symbiont relationships has been rarely reported. Castration of hosts by haplosporidians and orthonectids, and tissue damage by an unidentified coccidian and some gregarines, are known to occur. The suitability of nemerteans as hosts for parasites, hypersymbioses and approaches for future studies of parasite prevalence and host–parasite relationships are discussed. [ABSTRACT FROM AUTHOR]

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