14Traditionally, many small-sized copepod species are considered to be widespread, bipolar or 15 cosmopolitan. However, these large-scale distribution patterns need to be re-examined in 16 view of increasing evidence of cryptic and pseudo-cryptic speciation in pelagic copepods. 17Here, we present a phylogeographic study of Oithona similis s.l. populations from the Arctic Introduction 34The existence of widespread or cosmopolitan species is widely accepted in marine plankton 35 due to the lack of apparent geographic barriers and high dispersal potentials. Thus, it has 36 been assumed that allopatric speciation may be less important in pelagic species than 37 sympatric or parapatric speciation (Norris, 2000). During the past years molecular studies 38 have revealed that many supposedly widespread species are mosaics of several cryptic or 39 pseudocryptic species (Andrews et al., 2014; Burridge et al., 2015; Cornils and Held, 2014; 40 Darling et al., 2007;Halbert et al., 2013; Hunt et al., 2010;Miyamoto et al., 2012). The mode 49The cyclopoid copepod Oithona similis s.l. Claus, 1866 is accepted as a cosmopolitan 50 species, presumably occurring in all oceanic zones (Razouls et al., 2016) and presumably 51 the most abundant copepod species worldwide, although its importance has long been 52 overlooked due to the use of too coarse mesh sizes for plankton hauls (Gallienne and 53 Robins, 2001;Turner, 2004). Previous investigations have stressed that O. similis s.l. is a 54 prominent link between the microbial loop and higher trophic levels (Atkinson, 1995; 55 Castellani et al., 2005; Zamora-Terol et al., 2013). Its preferred habitats are polar and 56 temperate epipelagic marine waters where it is often more abundant than calanoid copepods 57 (Atkinson, 1998; Hopcroft et al., 2010; Pinkerton et al., 2010;Sabatini and Kiørboe, 1994; 58 Zamora-Terol et al., 2014). Furthermore, it has also been found in subtropical and tropical 59 waters (Cepeda et al., 2012; Nishida, 1985), although some of the reported findings seem 60 questionable (Nishida, 1985). Morphological differences have not been detected between O. 61 similis s.l. populations across latitudes, except for morphometric differences in the Northern 62 hemisphere,where prosome length and cephalon shape differed significantly among 63 populations from North Atlantic and Arctic water masses (Shuvalov, 1972). He suspected 64 that O. similis s.l. is "a polytypic species which comprises various subspecies or intraspecific 65 forms and groups with distinct ecological requirements regarding temperature throughout the 66 life cycle." A first attempt to investigate the population structure of O. similis s.l., using the 67 nuclear molecular marker 28S, revealed low levels of intraspecific variety between North and 68 South Atlantic populations (Cepeda et al., 2012). However, the polar habitats of O. similis s.l. 69were not included in that study. 751838, Pseudocalanus spp. or Triconia borealis Sars, 1918 (Ashjian et al., 2003 Hunt et al., 76 2014; Kos...

In October 2010, specimens of Oithona were taken from the List Tidal Basin in the northern Wadden Sea (North Sea) for a biogeographic study on Oithona similis.

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