| Summary: | The present study is a first attempt to comparing the hydroid faunas of the various Zoogeographie areas of the Atlantic Ocean. We restricted ourselves to species of the orders Antho- and Leptomedusae, of which 1050 species were taken into account. The classification of zoogeographic areas used follows Ekman and Briggs, with slight modifications; thus, the Strait of Gibraltar is here considered as a sepa rate zoogeographic area due to its character of a transition zone between several areas in the North eastern Atlantic Region. Species were arranged into Zoogeographic groups to compare the faunal com position of the various Zoogeographic areas. The Dice similarity index was used for each comparison. The fauna of North-eastern Region can be split into three groups of similarity: 1) that of the Mediter ranean and Lusitanian Provinces plus the Boreal Subregion, 2) that of the Strait of Gibraltar and the Mauritanian Province, and 3) that of the Senegalian Subregion (probably due to the lack of knowl edge of its fauna). Just like in Senegalian Subregion, the South-eastern Atlantic Region takes an isolat ed position (presumably at least in part because the vast majority of data is from South Africa alone). In the North-western Region, the North American and Caribbean Provinces are closely related to each other and to the Brazilian Subregion. The Arctic and Antarctic (together with the Argentinian Subre gion) are the most isolated regions. As a general rule, widely distributed species dominate in each region. In the North-eastern Region, the number of such species increases from higher latitudes to the Equator. In the Mediterranean, their number decreases in favour of endemic species. The fauna of the Strait of Gibraltar is more related to that of the Atlantic part of the North-eastern Region than to that of the Mediterranean Province. The high number of endemics in the South-eastern Region and in the Caribbean Province is remarkable. It is also noticeable that the Brazilian Province shares more species with the North-western Region than with the South-eastern Region. The Antarctic presents a similar percentage of endemics as the Arctic. Some distribution patterns of Antho- and Leptomedusae can be explained by the general current sys tem in the Atlantic Ocean.
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