Understanding species diversity in the deepsea is a fundamental problem in marine biology and biological oceanography. Knowledge of species diversity is critical also for management and conservation of deep-sea ecosystems in view of increasing impacts of human activities on deep-sea habitats and communities. Although the deep ocean remains the least explored and least understood ecosystem on the planet, facts accumulate demonstrating that the deep sea supports one of the highest levels of

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... ersity on Earth (for recent review see Ramirez-Llodra et al., 2010) . We will never know the exact number of species in the deep ocean, because its floor is too large and heterogeneous, too difficult to access, and the number of apparently rare species is very high. Apart from difficulties caused by the nature of the abyss, there are various problems related to technology and methodology of deep-sea research. Among such problems is the taxonomy of deep-sea species. The number of deep-sea species waiting for description has been growing at an ever-increasing rate over the last decades and this number already is overwhelming (Ebbe et al., 2010). At the same time the number of taxonomic experts is continuously and dramatically declining worldwide. Today the lack of taxonomists has been recognised on a worldwide scale. For many taxa few specialists exist, and already this problem presents a serious threat to biodiversity research (Kim, Byrne, 2006; Pearson et al., 2011) . This situation makes it crucial to concentrate efforts