Rotifers (Fig. 4) are well adapted to extreme environments; they are amongst the most dominant groups in Arctic sea ice, flourish in meltwater pools and can tolerate a wide range of salinities (Friedrich and De Smet, 2000). There are seven fully identified species, collected from ice samples in the Barents, Laptev and Greenland seas. Synchaeta hyperborea is the most abundant species, followed by S.tamara (Thomas and Dieckmann, 2003). Rotifers are most abundant in the lowermost centimetres of the ice, but have been found further up ice floes, towards the top; this distribution may be due to their diet of algae, small diatoms, flagellates and bacteria in the sea ice which are primarily found in the bottom layer of the ice pack. The presence of rotifers further up the ice pack shows that they are well adapted for movement up the ever decreasing diameter of the brine channels that are inaccessible to other metazoans such as turbellaria (Krembs et al. 2000). This allows them to escape predation by larger organisms and avoid competition for food (Friedrich and De Smet, 2000). Rotifers can penetrate long, narrow, cylindrical passages (57% of their body diameter and several times their body length).
The presence of males and amictic eggs (those that cannot be fertilized) suggests that rotifers can reproduce both parthenogenetically (when an amictic egg develops into a new individual) and sexually. It is thought that egg production in the sea ice may help colonize the under ice pelagic zone and brine channel system each spring (Friedrich and De Smet, 2000).