We have detected significant length variation in the gene, with homologous gene fragments ranging from 545 bp in a shallow-water scleractinian coral to 911 bp. We have sequenced a portion of the mitochondrial 16S rRNA gene from 29 species of Anthozoa, representing six orders of the subclasses Ceriantipatharia, Hexacorallia, and Octocorallia, with the focus on deep-seamount corals (> 500-m depth). Our review highlights that (1) every group of cnidarians seems to have their own assemblage of symbiotic polychaetes, (2) and some deep-sea alcyonaceans and black corals have never been reported without their often undetermined polynoidsymbionts so that its presence has been considered as a species-specific, robust taxonomic character, and (3) that we certainly expect the polychaete symbionts associated with deep-sea corals to be a hidden hot-spot of diversity, with many species of still waiting to be described. Host symbiont interrelations, host specificity, location, infestation characteristics and adaptive modifications ofsymbionts, as well as host reaction on symbionts presence, have been considered. Polynoidae include the highest number of cnidarian-associated polychaetes (almost one half of the currently known species), followed by Syllidae and Serpulidae. About 120 species of symbiotic poly-chaetes of ten families are reported from cnidarian hosts. Most polychaete-hosting cnidarians belong to skeleton-bearing taxa, particularly Scleractinia (125 species or 44.48 % of the total cnidarian hosts), Alcyonaria (73 species or 25.97 %) and Hydrozoa (60 species or 21.35 %). In the current review 281 species of cnidarian hosts involved in 324 relationships with symbiotic polychaetes were reported. The objective of the present paper is to summarize data on species composition and ecology of polychaetes associated with cnidarians. At the moment, actual mechanisms of most of mutual relations between host and symbiont in such associations are generally unknown. Close association and interrelation of highly host-specific symbionts and cnidarian hosts often lead to dramatic changes in the host morphology. Some populations of scleractinians, sea fans and black corals show up to 100 % infestation by symbiotic polychaetes. ![]() However, this number is considerably underestimated. 36:217–340, 1998) reckoned about 60 species of symbiotic polychaetes associated with more than 100 species of cnidarian hosts. The last revision (Martin and Britayev, Oceanogr Mar Biol. Members of the class Polychaeta are between the most diverse and perhaps the least studied taxa of coral symbionts. Cnidarians, especially skeleton-bearing anthozoans and hydrocorals, are known to host abundant and diverse symbiotic fauna encompassing members of the majority of metazoan taxa ranging from sponges and flat worms to fishes.
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