There are many examples of predominantly benthic organisms in both Arctic and Antarctic pack ice, these include turbellarians, harpacticoid copepods and nematode worms. It is not known how these organisms arrive in the sea ice from the benthic zone. This is especially intriguing in the Antarctic, where the ice is formed in deep water which would make any sort of migration harder than in the Arctic, which is relatively shallow in comparison. The benthic organisms in question are generally poor swimmers and have not been sampled in the water column (Thomas, 2004). Several theories have been proposed for various different organisms but none have yet been proved.
Multiyear ice provides populations of organisms that act as inocula for newly-formed sheets of ice (Thomas, 2004).
Riemann and Sime-Ngando (1997) suggested that multi year ice could provide nematodes to colonize new areas of ice as there were no examples of the ice species under study in the benthos. They doubted this theory however as they had sampled nematodes from the Laptev Sea where there is hardly any multiyear ice. In other areas of the Arctic this could be a possibility as there are areas with substantial multiyear ice and evidence of reproduction of nematodes within the ice pack. It may not be as likely in the Antarctic, again due to the shortage of multiyear ice.
Sea ice Turbellarians and Nematodes could use migrating invertebrates as vectors (Thomas, 2004).
Janssen & Gradinger (1999) proposed that turbellarians may use crustaceans to shuttle between the water column and brine channel system. This theory was based on observations by Hyman (1951) who described a turbellarian with a unique caudal disc that lived on hermit crabs. The turbellarian found by Janssen & Gradinger possessed a globular “tail” which could act as an adhesive organ and in the lowermost layer of the ice the acoel was associated with free swimming copepods.
Tchesunov and Riemann (1995) suggested that ice nematodes could be distributed by amphipods. Riemann and Sime-Ngando (1997) sampled amphipods that could act as vehicles and found no evidence of nematodes, suggesting that they may not act as vectors, this does not rule out the use of other organisms by nematodes, similar monohysterid nematodes are found on the baleens of whales, showing that they are capable of attachment to other animals for pelagic movements (Riemann and Sime-Ngando, 1997).
Metazoans can become trapped in ice as it is formed
Although there is evidence of smaller organisms such as diatoms becoming trapped in sea ice during formation (Gradinger and Ikavalko, 1998) there is little evidence of adult metazoan incorporation into the ice during this time. This could be due to the salinity increases, space constraints of brine channels and mechanical pressure that larger animals may be unable to cope with. Gradinger and Schnack-Schiel (1998) found that two species of copepod Calanus propinquus and Metridia gerlachei which are found feeding on under ice algae but never in the ice were not tolerant of increased salinity as they died in experiments where the salinity rose above 34. They also proposed that copepods that matured from eggs to juveniles/adults in the ice would have a larger range of salinity and temperature tolerances, such a Drescheriella glacialis. I think this may be the case as seen in Paralabidocera antarctica. The naupliar stage of this copepod overwinters in the sea-ice, as the ice thickens the nauplii stop developing until the ice begins to melt (Tanimura et al. 1996) (described in Life Cycles). This could be an adaptation in life history to increased salinity/temperature tolerance or to reduce the mechanical stress, but is something that needs further investigation.
Kurbjeweit, Gradinger and Weissenberger (1993) suggested that Stephos longipes eggs (which are sticky, unlike those of many other copepods) were attached directly onto frazil ice crystals and ice platelets, therefore ensuring that they are incorporated into the newly forming ice. Reproduction of this copepod takes place from late winter to autumn and so this method may be used during autumn when the sea-ice is being formed, but cannot be the only seasonal life history strategy as ice formation does not take place through the rest of the breeding season. This may lend support to the theory that eggs and nauplier stages of some metazoans may have an increased range of salinity and temperature tolerance and being smaller, less mechanical stress in the sea ice and so can survive incorporation into newly forming sea ice.