ИСТИНА

Biodiversity of communities associated with Crinoidea is very high and present by both vertebrate and invertebrate symbionths, which may live not only on surface or in body cavity of the host, but cause the deformation of skeletal elements of the host. One of the most known symbionts of crinoids are Myzostomida, a specialized group of marine annelids. Fossilised myzostomids have been reported from the Low Jurassic and now about 200 of recent myzostomid species are described. In the aspect of host interactions, myzostomids can be divided into four ecological groups: 1) free-living forms which can move easily over the entire body of the host as adults, 2) cyst-forming, living in soft tissues of the host as adults, 3) gallicolous forms forming excrescences in skeletal structures of the host and 4) endosymbionts, living in the digestive tract or other organs of the host. Nevertheless, a new myzostomid species which cannot be attributed to the abovementioned ecological forms was found in the Nhatrang Bay (Central Vietnam). This species is associated with Clarkcomanthus albinotus Rowe, Hoggett, Birtles & Vail, 1986 and stimulates deformation of the skeletal elements of host’s arm, without the formation of full galls or cysts. Myzostoma khanhkhoaensis sp. nov lives in groups up to 40 specimens per host on distal part of the arm, and change the shape and size of the main skeletal elements, lead to it’s deformation. Morphological analysis of infected arms shows that Myzostoma khanhkhoaensis sp. nov. does not case formation of new skeletal elements, as the gallicolous species and only changes shape of present skeletal elements, make them larger, shorter and broader; the surface of skeletal elements covers by additional spines and spicules. Movability of infected arm decreases, as a result, it twists to the oral side and form a bucket-like structure inhabited by myzostomids, possible, protecting worms from predation.