There are two major divisions in the Rotifera, the Monogonata and the Bdelloidae. Of the two major divisions of Rotifera, the Bdelloidea have been largely neglected because of difficulties with identification. Their diversity is underestimated since most studies use animals recovered from rehydrated moss samples, precluding recovery of species lacking, or with poor, capacity to form dormant anhydrobiotic stages. Moreover, as is likely to be the case in many invertebrate groups, recent molecular biological studies have demonstrated that cryptic diversity is high in bdelloids (Fontaneto et al., 2007).
The known Svalbard bdelloid fauna comprises 67 morphospecies. All morphospecies recorded occur in limno-terrestrial habitats (mosses, lichens) with 15 also reported from freshwater habitats (permanently submerged vegetation, cryoconite holes).
In this group, older reports are biased in favour of the loricates, a group that includes species with a rigid body wall that fix well and are amenable to microscopic study. Species with a soft integument, the illoricates, contract on fixation and become unrecognizable. Furthermore, re-examination of historical samples has shown that loricate diversity per sample was on average 2-4 times higher than in the original publication (De Smet unpubl.). Interpretation of older data may also be compromised due to taxonomic inconsistencies. For example, several monogononts show large phenotypic plasticity, while some taxa originally considered to exhibit wide morphological variation are now recognized to consist of several species. Given these reservations it is impossible to differentiate, for instance, the currently recognised species Keratella hiemalis Carlin, 1943, K. quadrata (Miiller, 1786) and K. testudo (Ehrenberg, 1832) in earlier reports of ‘Anuraea (Keratella) aculeata’ and its forms in the absence of preserved material.
The second division, the Monogonata, consists to date of 134 limno-terrestrial and aquatic morphospecies. The global diversity of non-marine Monogonata totals approximately 1,500 species of which 11% occur in the Barents Sea archipelagoes. In the Arctic region as a whole 327 species are known (De Smet unpubl.) of which 50% have been reported from these archipelagoes. Only 16 species occur occasionally in aerophytic moss with the most frequently found being Encentrum incisum Wulfert, 1936, Lecane arcuata (Bryce, 1891) and Lepadella patella (Müller, 1786). As with the bdelloids, the majority of these species are cosmopolitan or widespread, although a small proportion show more restricted distributions: the Arctic endemic Notholca latistyla (Olofsson, 1918) occurs in all three archipelagoes; Trichocerca longistyla (Olofsson, 1918), described from Spitsbergen, is also known from Novaya Zemlya and Swedish Lapland; Encentrum boreale Harring and Myers, 1928, E. dieteri (De Smet, 1995), E. murrayi Bryce, 1922 are currently thought to be endemic to Spitsbergen, and the sub-species Synchaeta lakowitziana arctica De Smet, 1988 is restricted to Bjørnøya.