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Aim:This study aims to evaluate the role of various factors in structuring biogeographical distribution of small-sized organisms. More specifically, we analysed the distribution of three groups of marine benthic organisms differing in body size, both unicellular and metazoans, to assess the relevance of geographical and environmental factors on their taxonomic composition. Location: Global.Taxon: Heterotrophic flagellates, Ciliophora, Harpacticoida (Copepoda). Methods:The set of predictors included species richness (a proxy for sampling effort), geographical distance, isolation by geographical or hydrological barriers and three environmental variables (sea surface temperature, its annual variation and salinity). Multiple regression on the distance matrices and null modelling were used to quantify the relative effects of these variables in taxonomic similarity among 21 regions of the Ocean.Results: Each group showed a unique biogeographical pattern. The smallest-sized organisms, the flagellates, had high regional but low global diversity and low endemicity and demonstrated worldwide, environmentally driven distribution without any dispersal limitation (only the effects of climatic variables, but neither distance nor isolation effects, were significant). Thus, flagellates basically meet the 'ubiquity model' of global distribution. The ciliates showed a moderate level of endemicity, and their distribution depended on both spatial distance and the climatic variables, in accord with the 'moderate endemicity model'. In contrast, harpacticoids demonstrated high endemicity level, and their distribution was mostly geographically driven, indicating the considerable role of dispersal limitation-the pattern typical of other multicellular taxa with 'classical biogeography'. Species richness significantly influenced the interregional similarity of each group and explained 26%-65% of the total variance, indicating the strong impact of undersampling on faunistic similarity estimations. Main conclusions:Our results support the hypothesis that body size is the 'master trait' that shapes global biogeographical patterns towards higher predictability and increasing role of dispersal limitation for larger organisms. | 1181 AZOVSKY et Al.

Nannopus palustris Brady, 1880 is a free-living widely distributed harpacticoid copepod, which has been formerly assumed to be a single, cosmopolitan but highly variable species. We compared several geographically distant N. palustris populations in terms of their morphology and genetics. Populations from the White Sea (WS), the North Sea (NS), the Black Sea (BS) and two sympatric morphs from South Carolina, USA (SC notched and SC straight morphs), were considered. The NS, BS and to a lesser extent SC notched specimens were morphologically similar and partly coincided to the 'canonical' description of the species. By contrast, WS population showed remarkable anatomical and morphometric peculiarities that correspond to some earlier descriptions. Genetic analyses of mitochondrial (cytochrome b) and nuclear (28S rDNA) genes demonstrated the significant distinctness among WS, both SC and (NS+BS) populations, the latter two being genetically indistinguishable. Concordance between mitochondrial and nuclear gene trees and morphological data supports that N. palustris is in fact composed of several pseudo-sibling species, which are genetically and morphologically divergent. Neither correlation between genetic divergence and geographical distance nor significant intrapopulation diversity was found for these species. Taxonomic status, distribution and phylogenetic relationships of the species within the Nannopus genus need to be reconsidered. A further subdivision of species complexes might have important implications for the analysis of biodiversity of benthic copepods and consequently for the interpretation of their (species-specific) ecological function.

The zoogeographic distributions of 1,747 species of shallow-sea benthic Harpacticoida from 370 genera and 51 families reported in the Northern Hemisphere (Atlantic and Arctic Oceans and the European seas) were analyzed. Faunal diversity analysis revealed that the Northeast Atlantic and the North and Mediterranean Seas contain the richest faunas, while certain Arctic regions and internal seas (Caspian, Azov and Aral Seas) are the poorest ones. The distribution of families and genera was also briefly analyzed. Most families are widely distributed (probably cosmopolitan), though others are restricted to boreo-subtropical or even tropical zones, and no purely Arctic families were discovered. Many more genera are present in (or even restricted to) the tropics/subtropics compared with the Arctic/Subarctic zone. Over 15% of species and 55% of genera can be still considered either widespread or cosmopolitan. Cluster-analysis of the regions by species composition revealed six main clusters: Arctic (central Arctic basin, Kara, Laptev, East Siberian, Chukchi and Beaufort Seas), Boreal (North Atlantic Ocean, and North, Baltic, White and Barents Seas), Mediterranean, PontoCaspian (Black, Azov, Caspian and Aral Seas), Central East Atlantic, and Central West Atlantic. The distinction between the Mediterranean and Ponto-Caspian faunas is considered in particular, as the latter is not an exact derivative of the Mediterranean one but instead has more atlanto-boreal features. The percent of regional endemics, mean species occurrence and number of families correlate with total species richness, which primarily reflects the degree of exploration in a given region. After removing this effect, the abovementioned features show significant latitudinal trends, with a few endemics present in polar waters. The distribution patterns of different life-forms also vary according to their dispersal abilities. At both global and regional scales, bentho-pelagic forms are the most widely distributed, followed by phytal species, whereas benthic species, especially interstitial, have a more restricted distribution. Overall, our results confirm and statistically validate the main features of marine harpacticoid biogeography asserted a quarter of a century ago by Abele [In: Abele LG (ed) The biology of crustacea. Academic Press, New York, pp 242-304, 1982] and Wells (Syllogeus 58:126-135, 1986).

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