adverse environmental conditions and predation, as well as compartments in which
they can store food, protect juveniles and house symbionts. For ostracods the
development of a fully calcified carapace also played key role in their rapid
diversification. At the turn of the Cambrian and Ordovician the majority of weaklycalcified ostracodomorphs (bradoriids sensu lato, Phosphatocopida) were replaced
by well-calcified podocopomorphs (Liebau 2005).
Along the salinity gradient from a river to the sea, body size increases with
increasing salinity (Gunter 1947). Indeed, animals inhabiting estuaries are smaller
than their marine counterparts. The factor which is most likely to be responsible for
small size of both Foraminifera and ostracods is the availability of calcium. Sea
water salinity 35 ‰ contains about 400 ppm of calcium, whereas in comparison the
hardest of river waters contains almost negligible amounts (Murray 1963). Recent
experiments on Foraminifera revealed poor efficiency of calcium utilisation—only
about 30 % of available Ca is used for shell formation (Böhm et al. 2012). It seems
most likely that the barrier against massive colonisation of the freshwater realm by
Foraminifera is their inability to construct a fully calcified test in low salinity
regimes. The mechanism of foraminiferal calcification is strictly dependent on salt
water content. They use sea water not only as a source of ions to construct their
shell, but also as a biomineralisation solution, thus Foraminifera are typically
defined as marine organisms (Holzmann et al. 2003). Only few genera are occasionally represented by species in low salinity environments, and the overwhelming
majority of them are agglutinated or organic-shelled forms, which do not produce,
or produce only small amounts of calcite. The success of ostracods in fresh water
can be attributed to their development of a more effective mechanism of calcification. Less calcified ostracods are still sheltered by chitinous valves, but calcifying
Foraminifera without biomineralisation process most likely are completely
defenceless with their rhizopodial skeleton exposed. In low salinity environments
ostracods construct less calcified, thinner and poorly ornamented valves, but they
are still able to build a complete hard shell.
However, there is still a problematic question about the role of cellular organisation level. Ostracoda, as a multicellular group, seem to be better adaptable, than
Foraminifera, their distant unicellular relatives. It is possible that tissue- and organlevel organisation has a greater capacity for physiological adaptation to new salinity
regime than unicellular organisation does. That issue requires more attention in
further studies.
Acknowledgments We are very grateful to Prof. Jan Marcin Węsławski, Prof. Marek
Zajączkowski (Institute of Oceanology Polish Academy of Sciences, Sopot) and Prof. Geoffrey
Boxshall (Natural History Museum, London) for their constructive and helpful comments on the
manuscript.
The Adaptations of the Foraminifera and Ostracoda …
107
they can store food, protect juveniles and house symbionts. For ostracods the
development of a fully calcified carapace also played key role in their rapid
diversification. At the turn of the Cambrian and Ordovician the majority of weaklycalcified ostracodomorphs (bradoriids sensu lato, Phosphatocopida) were replaced
by well-calcified podocopomorphs (Liebau 2005).
Along the salinity gradient from a river to the sea, body size increases with
increasing salinity (Gunter 1947). Indeed, animals inhabiting estuaries are smaller
than their marine counterparts. The factor which is most likely to be responsible for
small size of both Foraminifera and ostracods is the availability of calcium. Sea
water salinity 35 ‰ contains about 400 ppm of calcium, whereas in comparison the
hardest of river waters contains almost negligible amounts (Murray 1963). Recent
experiments on Foraminifera revealed poor efficiency of calcium utilisation—only
about 30 % of available Ca is used for shell formation (Böhm et al. 2012). It seems
most likely that the barrier against massive colonisation of the freshwater realm by
Foraminifera is their inability to construct a fully calcified test in low salinity
regimes. The mechanism of foraminiferal calcification is strictly dependent on salt
water content. They use sea water not only as a source of ions to construct their
shell, but also as a biomineralisation solution, thus Foraminifera are typically
defined as marine organisms (Holzmann et al. 2003). Only few genera are occasionally represented by species in low salinity environments, and the overwhelming
majority of them are agglutinated or organic-shelled forms, which do not produce,
or produce only small amounts of calcite. The success of ostracods in fresh water
can be attributed to their development of a more effective mechanism of calcification. Less calcified ostracods are still sheltered by chitinous valves, but calcifying
Foraminifera without biomineralisation process most likely are completely
defenceless with their rhizopodial skeleton exposed. In low salinity environments
ostracods construct less calcified, thinner and poorly ornamented valves, but they
are still able to build a complete hard shell.
However, there is still a problematic question about the role of cellular organisation level. Ostracoda, as a multicellular group, seem to be better adaptable, than
Foraminifera, their distant unicellular relatives. It is possible that tissue- and organlevel organisation has a greater capacity for physiological adaptation to new salinity
regime than unicellular organisation does. That issue requires more attention in
further studies.
Acknowledgments We are very grateful to Prof. Jan Marcin Węsławski, Prof. Marek
Zajączkowski (Institute of Oceanology Polish Academy of Sciences, Sopot) and Prof. Geoffrey
Boxshall (Natural History Museum, London) for their constructive and helpful comments on the
manuscript.
The Adaptations of the Foraminifera and Ostracoda …
107