Symbiosis in deep-water corals

Deep or cold-water corals house a rich fauna of more or less closely associated animals. This fauna has been poorly studied, and most of the records are sporadic observations of single species. In this review we compile available records of invertebrates associated with alcyonarian, antipatharian, gorgonian, and scleractinian deep-water corals, including our own previously unpublished observations. Direct observations of the location of mobile species on deep-water corals are few and samples of deep-water corals often contain a mixture of sediments and broken corals. The nature of the relationship between the associated species and the coral is therefore in most cases uncertain. We present a list of species that can be characterised as symbionts. More than 980 species have been recorded on deep-water corals, of these 112 can be characterised as symbionts of which, 30 species are obligate to various cnidarian taxa. Fifty-three percent of the obligate deep-water coral symbionts are parasites, 47% are commensals. The obligate symbionts are rarer than their hosts, which implies that reduced coral abundance and distribution may be critical to the symbionts' ecology. Most of the parasites are endoparasites (37%), whereas ectoparasites and kleptoparasites are less common (13 and 3%, respectively). There are no clear examples of mutualistic symbiotic relationships with deep-water corals, but the behaviour of some of the common associates have equally strong elements of mutualism and parasitism. Comparisons of the symbiont fauna of deep-water corals and shallow or warm-water corals revealed that the diverse decapod and gastropod fauna symbiotic with shallow water corals is not paralleled in the deep-water coral community. Possible effects on the symbionts from damage to deep-water corals by fisheries are discussed.