Cryoconite holes (water-filled reservoirs) are considered ecologically simple ecosystems but represent biological hotspots of biodiversity on glaciers. In order to check for fine-scale spatial distribution of metazoans on the bottom of the holes, in this study, we analysed three groups of grazing invertebrates as a model: tardigrades, rotifers, and mites. We addressed differences within cryoconite holes comparing the distribution of invertebrates within and between separate holes and between glaciers at a worldwide scale. We divided each cryoconite hole into three sampling zones (established in relation to water flow on a glacier) and collected nine subsamples within cryoconite holes on glaciers in the Arctic (Longyearbreen), Norway (Blåisen), the Alps (Forni) and maritime Antarctic (Ecology Glacier). Generally, we found no consistent difference in sampling zones within cryoconite holes, which suggests homogeneity on the hole floors. However, we did find strong differences and high heterogeneity between subsamples, even within the same zone. Invertebrate densities ranged between 52 and 426 individuals per ml in subsamples collected from the same hole. We found from zero to four trdigrade species in the cryoconite hole on Longyearbreen. Our results show that benthic animals in cryoconite holes Handling Editor: Télesphore Sime-Ngando.

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This is a temperate-boreal northern hemisphere species, in tropical regions known only from the East African mountains, and extending down to South Africa (Blockeel et al., 2006c). In East Africa it was first reported [as Neckera complanata (Hedw.) Huebener var. maxima Dixon] from Kenya (Mt Kenya, at 3000 m) by Dixon (1918), and it also occurs in the Taita Hills (Mbololo, at 1650 m) in the SE part of the country (Bytebier & Chuah-Petiot, 2002). It was reported from Tanzania (Mt Meru, at 2375 m) by Blockeel et al. (2003). The South African collections came from much lower altitudes (400 and 450 m). The African specimens reported by Bytebier & Chuah-Petiot (2002), Blockeel et al. (2003, 2006c) were all epiphytic. The present Ugandan specimens were collected on rocks. Altitude (4100 m) is given on only one of the two specimens, but as both were collected the same day, the other one probably comes from an approximately similar altitude. They do not represent Dixon's (1918) var. maxima, as the shoots are less than 5 cm long; in var. maxima they were up to 15 cm long. The leaves in the Ugandan specimens are slightly narrower than is typical for the species, but otherwise the specimens agree well with the rather variable A. complanata.

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