Marine Biology Research, 2006; 2: 200Á241 ORIGINAL ARTICLE A review of bipolarity concepts: History and examples from Radiolaria and Medusozoa (Cnidaria) S. D. STEPANJANTS1, G. CORTESE2, S. B. KRUGLIKOVA3 & K. R. BJØRKLUND4 1Zoological Institute, Russian Academy of Sciences, St Petersburg, Russia, 2Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, 3P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia & 4Natural History Museum, Department of Geology, University of Oslo, Norway Abstract Bipolarity, its history and general interpretation are investigated and discussed herein. Apart from the classical view, namely that a bipolar distribution is a peculiar biogeographical phenomenon, we propose that it is ecologically controlled too. This approach was used for bipolarity assessment within the following groups: Phaeodaria, Nassellaria, Spumellaria (Radiolaria) and Medusozoa (Cnidaria). We recognize 46 bipolar radiolarian species and three radiolarian genera. However, although species concepts in radiolarians are relatively stable and well known, the high-rank taxonomy of radiolarians is still not well defined. Caution should therefore be taken in the interpretation of distribution data at a taxonomic level higher than the species. In the Medusozoa, bipolarity is observed for 23 species and 32 genera. The different ways in which bipolarity can develop are discussed under the different groups, but preference has been given to the recent and most probable routes of migration. In our investigation of the bipolarity phenomenon, we reviewed more than 400 articles dealing with taxonomy, ecology and biogeography of the modern fauna in both groups. Key words: Biogeography, bipolarity, Medusozoa, Radiolaria Introduction Bipolarity is an interrupted distribution of iden- tical or closely related species (or higher taxa level) The bipolarity phenomenon is herein investigated of flora or fauna in polar, temperate or subtropical using examples from the exclusively pelagic classes zones of both hemispheres, characterized by their Euradiolaria and Medusozoa (phylum Cnidaria), the absence in the tropics (Bergh 1947; Stepanjants latter having both pelagic and benthic stages. In both et al. 1996, 1997a). groups, the phenomenon of bipolarity is very clearly represented: as Radiolaria (Bjørklund, Cortese, Many publications have been dedicated to different Kruglikova) and Cnidaria (Stepanjants) specialists aspects of the bipolarity phenomenon, including we concentrated on these two groups of organisms, some (Stepanjants et al. 1996, 1997a) concentrating and therefore hope that this paper will be of interest on the bipolar distribution of Medusozoa (Cnidar- to colleagues studying other living taxa, and thus ia). The representatives of this group display, during stimulate debate on the bipolarity problem within their ontogeny, both benthic and pelagic stages, other taxonomic groups. inhabit all depth horizons, and are distributed in As the term ‘‘bipolarity’’ has different interpreta- all oceanic provinces. The study of bipolarity in tions and meanings, we will first cite the classical Cnidaria is limited to their Recent representatives, as definition and then, after a short and critical analysis relict forms and fossils are extremely rare. Stepan- of the history of this problem, we will offer our own jants et al. (1996, 1997a) reported 38 bipolar species views on this phenomenon. and 30 bipolar genera of Hydrozoa and Siphono- Correspondence: G. Cortese, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany. E-mail: gcortese@ awi-bremerhaven.de Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark (Accepted 18 April 2006; Printed 24 July 2006) ISSN 1745-1000 print/ISSN 1745-1019 online # 2006 Taylor & Francis DOI: 10.1080/17451000600781767 Bipolarity, examples from Radiolaria and Medusozoa 201 phora, but the number of bipolar species is adjusted (‘‘convergent appearance’’) species in areas with to 23 herein. similar ecological conditions. Our interest in analysing the bipolar distribution We emphasize that all pioneering investigations of of Radiolaria is connected with Petrushevskaya bipolarity analysed the same species, or closely (1986), who was the first to question the bipolar related species, from the temperate or subtropical distribution of some radiolarian species, and with zones of both hemispheres. These species had an our own knowledge of the geographical distribution interruption in their distribution pattern in the of cold-water radiolarian species in the northern and tropics. This, without doubt, allowed earlier biogeo- southern hemispheres. Radiolarian skeletal remains graphers to equate the meaning of the notions are preserved in sediments and, for some species, we ‘‘bipolar’’ and ‘‘antitropical’’ distribution, as also are able to provide evidence about the time of their suggested by Kafanov & Kudrjashov (2000). origin and, accordingly, outline how and where their Bergh (1920, 1936, 1947) devoted three research bipolar distribution developed. New data, however, papers to this question and paid special attention to persuaded us that earlier estimates of the bipolarity On the Origin of Species (Darwin 1859). Darwin phenomenon are different from that proposed by showed that the same plant species (or subspecies) classical authors. were found in Europe, southern Australia and New Zealand as in high-mountain areas of the tropics. These plants were typical for the temperate zones of The bipolarity concept and its history: From both hemispheres, and were absent in the lowlands Captain J. C. Ross until today of the tropics. This conclusion of Darwin’s was James Clark Ross participated on many expeditions adopted as a fact by Bergh. in the Arctic and the Antarctic. He was by no means Bergh (1947: 128, 129) concluded that it should a biologist, but he personally collected and investi- be possible to characterize the presence and dis- gated several zoological collections, which made it tribution of ‘‘temperate forms’’ in the deep cold possible for him to compare faunistic peculiarities water of the tropical and subtropical zones as bipolar from northern and southern polar regions. The species. If it is possible to conclude that, from a biogeographer’s point of view, bipolar and antitro- results from these observations, based on the collec- pical species distributions are quite different, then tions taken during the H.M.S. Erebus and H.M.S. the bipolar distribution probably has an ecologicalÁ Terror expeditions from 1839 to 1843, led by biogeographical significance, and the two concepts Captain J. C. Ross, have been summarized and have different meanings. Bergh analysed all data published in two volumes. In the first of these known at his time about closely related species of the volumes (Ross 1847), the idea of a bipolar distribu- same genus that were known from the cold waters off tion of some marine organisms was expressed for the New Zealand, South Africa and South America from first time. Captain Ross traced the deep-water the southern hemisphere (and from England, Nor- connection between populations of the same, cold- way and Kamchatka from the northern hemisphere), water species from the northern to the southern but which were absent in the shallow, warm-water hemispheres. This means that he was the first author areas of the tropical zone. He wrote: ‘‘this distinctive to present the hypothesis that the bipolar distribu- and enigmatic peculiarity of the geographic distribu- tion of some taxa of marine organisms was due to tion was named bipolarity’’ (Bergh 1947: 128). This deep-water migration (Andriashev 1978). opinion by Bergh (1947: 131) applies mostly to J. Hooker (1847), a member of Ross’s expedition, ‘‘identical forms or subspecies, or close species’’ of investigated the plants that were collected during water mammals or marine fishes, common for ‘‘the this expedition to the Antarctic, and he supported temperate latitudes of the North and South’’. Ross’s hypothesis by discovering some plant species Derjugin (1915), in his Fauna of the Kola Bay and in Patagonia, which were also known from Great Conditions of its Life, included a chapter on ‘‘The Britain, but unknown in the tropics. Candolle theory of cosmopolitism and bipolarity’’, where the (1855), in his Botanical Geography, also touched faunistic similarities of the ‘‘ArcticÁSubarcticÁbor- upon ‘‘des espe`ces disjointes’’ (species with disjunct eal’’ and ‘‘AntarcticÁSubantarcticÁnotal’’ zones distribution), referring to 32 species that were were analysed. Here ‘‘the same or closely related known from the temperate zones of both hemi- species’’ were also found, but these were absent in spheres. Dana (1854) pointed out the similarity, the ‘‘tropicalÁsubtropical waters’’. This chapter is between New Zealand and Great Britain, in genera rich in examples, many of which are without composition of Decapoda (Crustacea). In his opi- supporting data. However, with respect to some nion, the closeness (propinquity) effect of faunas in species, e.g. some hydroids, their status as ‘‘bipolar’’ the temperate zones creates morphologically similar is still valid (see below). 202 S. D. Stepanjants et al. The existence of bipolar species distributions is cal species of undetermined/independent origin practically accepted by all biogeographers. Never- (Stepanjants et al. 1996, 1997a). theless, after more