Turbellarians (Fig. 5), along with copepods are the dominant metazoan fauna of the Antarctic sea ice. They are present in Arctic sea ice, but are not as numerous as the rotifers and nematodes (Thomas and Dieckmann, 2003).
Adaptive features of turbellarians to the brine channel habitat include a lack of rigid morphological features, which increases their plasticity when moving through thin crevices and the ability to osmoconform with the external environment; turbellaria have the ability to change their body dimensions in response to fluctuating levels of salinity and temperature (Krembs et al. 2000). This means that when the temperature decreases and salinity increases (they can tolerate salinity levels of 75 (Thomas and Dieckmann, 2003)) the turbellarians change their body dimensions and become thinner and longer as the brine channels constrict due to increased freezing of the water.
Turbellarians are most common in the bottom 15cm of sea ice but can be found higher up. Increased abundances are found in older, thicker ice and specimens have been observed moving horizontally and vertically (Fig. 6). Feeding occurs by engulfing diatoms. The reproduction method of is not known, however individuals of different sizes have been observed which suggests that turbellarians spend a large proportion of their lives in ice (Friedrich and Hendelberg, 2001). Turbellarians are well adapted to living in such an extreme environment, evidence for this includes:
- Occurrence in both Arctic and Antarctic ice systems, especially its dominance in the Antarctic.
- Great vertical distribution in the sea ice
- Largely feed on diatoms which are abundant in brine channels
- Tolerance of salinities from 5-65% and temperatures down to -6oC ((Friedrich and Hendelberg, 2001).