No abstract

Large sessile marine animals (sponges, corals, ascidians, and echinoderms) are always colonized by various symbionts. Their composition varies widely, from protozoans to fish [1][2][3]. In a survey of the symbionts of echinoderms, 825 species associated with different representatives of this phylum were found [4]. An individual host may be colonized by one macrosymbiont or tens or hundreds animals from one or different species simultaneously. In the latter case, multispecies symbiotic associations are formed. Both intraspecific interactions between symbionts and interspecific interactions among symbionts and hosts as well as among different symbiotic species occur in these associations.In symbiotic associations, the host serves as a buffer between symbionts and the environment, which decreases the influence of abiotic factors on the symbionts and makes biological interactions more distinct. Moreover, the symbiont-host systems have only few biotic connections and possess relatively simple organization. All these properties make symbiotic associations very suitable objects for studying of biological interactions.Shallow-water tropical holothurians and their symbionts are an example of such multispecies symbiotic associations. Holothurians possess the richest symbiotic fauna among echinoderms [4]. However, in only two papers published by now holothurians and their symbionts were considered as an integrated community [5,6], and only one of them dealt with biological (trophic) interactions between symbionts (copepods from the genus Tisbe ) and their host (holothurians from the genus Holothuria ) [5]. Recently, we studied trophic interactions between holothurians and one of their common symbionts, the polychaete Gastrolepidia clavigera Schmarda [7].Here, we describe the set of species associated with shallow-water tropical holothurians living in South Vietnam and analyze their location on (in) the hosts, distribution on holothurians, traumatism, food spectrum, co-occurrence and, on the basis of these data, intra-and interspecific interactions.The material, 984 holothurians from seven species and 1111 their symbionts, was collected in South Vietnam (Nha Trang city area) in the period from 1985 to 1990. The methods of sampling were reported earlier [7]. The location of symbionts was recorded either underwater during sampling (crabs, shrimp, and mollusks) or during the preliminary treatment of the material (fish). In addition, two series of observations in aquariums were performed to determine the location of crabs and polychetes. To analyze the distribution of symbionts on hosts, we used coefficient of variance (the ratio of the mean value to the dispersion, K δ 2 [8]). The symbionts were observed with the help of a binocular microscope to detect traumas of the body and appendages. To study the food spectrum, we dissected 15 crabs Lissocarcinus orbicularis , 12 fish Carapus homei , and 7 shrimp Periclimenes imperator. The contents of the stomachs and intestines were placed into a 1 : 1 glycerol-alcohol mixture and examin...