1 Introduction
This is a review paper focused on the invasion of non-marine habitats by Foraminifera and Ostracoda. The paper reviews the comprehensive paleontological and
modern data and integrates these with the knowledge of physiological adaptations,
which have enabled them to colonise of freshwater habitats. Both these groups have
undergone spectacular adaptive radiations and have invaded a wide variety of
marine habitats. In the Early Carboniferous ostracods and Foraminifera thrived, and
both groups faced an opportunity to colonise freshwater habitats. While the Ostracoda succeeded, the Foraminifera have remained an almost exclusively marine
group. The aim of this study was to elucidate what could be the crucial ability that
limited the Foraminifera from successfully colonising freshwater habitats, and why
ostracods were more efficient in the invasion of low salinity environments.
The earliest fossil Foraminifera appeared in the Early Cambrian, but molecular
data indicate a much earlier, Neoproterozoic, origin (Pawlowski et al. 2003). The
origin of the Ostracoda may be slightly younger. Evidence from molecular studies
suggests that ostracods diverged from near the base of the Pancrustacea during the
late Proterozoic (Regier et al. 2005; Siveter 2008; Williams et al. 2008), although the
oldest unequivocal fossil record is from the mid-late Cambrian (Harvey et al. 2012).
The Foraminifera seems to be an enormously successful invader of new environments. During the Phanerozoic they colonised most shallow, marginal to deep
water, fully marine habitats, and diversified to exploit a wide variety of life modes
(Hottinger 1982; Goldstein 2003). Some attained relatively gigantic size, such as
the extinct Lepidocyclina elephantina at 14 cm (Grell 1973). The mean size of
modern Foraminifera ranges from 0.1 to 0.5 mm, however, some species may reach
up to several centimetres (Pawlowski 2009). Despite their unicellular level of
organization foraminiferans perform the same range of fundamental functions as
metazoans (Goldstein 2003). There are two characteristic features that help to
distinguish the Foraminifera from other protists. First, all possess granuloreticulopodia (pseudopodia) which are used for motion, feeding, constructing a test, protection and for some aspects of reproduction. Second, almost all Foraminifera have
a test which encases the body, separating it from the surrounding environment.
There are three different types of tests: organic, agglutinated (constructed from
cemented particles) and mineralised, composed of calcium carbonate or, in rare
cases, of silica (Goldstein 2003; Pawlowski et al. 2013).
Ostracods are also efficient colonisers of new habitats. The wide geographical
distribution and their almost simultaneous appearance on several palaeocontinents
suggest rapid dispersal and wide environmental tolerance (Williams et al. 2008). By
the mid Silurian originally benthic myodocopes had started colonising the pelagic
(Siveter et al. 1991; Vannier and Abe 1992; Perrier et al. 2011) and by the turn of
the Devonian and Carboniferous they had invaded inland freshwater habitats and
the deep ocean (Williams et al. 2006; Bennet 2008). Martens et al. (2008) estimated
that ostracods had undergone between 9 and 12 independent incursions into fresh
water from the marine environment.
92
A. Iglikowska and J. Pawłowska
This is a review paper focused on the invasion of non-marine habitats by Foraminifera and Ostracoda. The paper reviews the comprehensive paleontological and
modern data and integrates these with the knowledge of physiological adaptations,
which have enabled them to colonise of freshwater habitats. Both these groups have
undergone spectacular adaptive radiations and have invaded a wide variety of
marine habitats. In the Early Carboniferous ostracods and Foraminifera thrived, and
both groups faced an opportunity to colonise freshwater habitats. While the Ostracoda succeeded, the Foraminifera have remained an almost exclusively marine
group. The aim of this study was to elucidate what could be the crucial ability that
limited the Foraminifera from successfully colonising freshwater habitats, and why
ostracods were more efficient in the invasion of low salinity environments.
The earliest fossil Foraminifera appeared in the Early Cambrian, but molecular
data indicate a much earlier, Neoproterozoic, origin (Pawlowski et al. 2003). The
origin of the Ostracoda may be slightly younger. Evidence from molecular studies
suggests that ostracods diverged from near the base of the Pancrustacea during the
late Proterozoic (Regier et al. 2005; Siveter 2008; Williams et al. 2008), although the
oldest unequivocal fossil record is from the mid-late Cambrian (Harvey et al. 2012).
The Foraminifera seems to be an enormously successful invader of new environments. During the Phanerozoic they colonised most shallow, marginal to deep
water, fully marine habitats, and diversified to exploit a wide variety of life modes
(Hottinger 1982; Goldstein 2003). Some attained relatively gigantic size, such as
the extinct Lepidocyclina elephantina at 14 cm (Grell 1973). The mean size of
modern Foraminifera ranges from 0.1 to 0.5 mm, however, some species may reach
up to several centimetres (Pawlowski 2009). Despite their unicellular level of
organization foraminiferans perform the same range of fundamental functions as
metazoans (Goldstein 2003). There are two characteristic features that help to
distinguish the Foraminifera from other protists. First, all possess granuloreticulopodia (pseudopodia) which are used for motion, feeding, constructing a test, protection and for some aspects of reproduction. Second, almost all Foraminifera have
a test which encases the body, separating it from the surrounding environment.
There are three different types of tests: organic, agglutinated (constructed from
cemented particles) and mineralised, composed of calcium carbonate or, in rare
cases, of silica (Goldstein 2003; Pawlowski et al. 2013).
Ostracods are also efficient colonisers of new habitats. The wide geographical
distribution and their almost simultaneous appearance on several palaeocontinents
suggest rapid dispersal and wide environmental tolerance (Williams et al. 2008). By
the mid Silurian originally benthic myodocopes had started colonising the pelagic
(Siveter et al. 1991; Vannier and Abe 1992; Perrier et al. 2011) and by the turn of
the Devonian and Carboniferous they had invaded inland freshwater habitats and
the deep ocean (Williams et al. 2006; Bennet 2008). Martens et al. (2008) estimated
that ostracods had undergone between 9 and 12 independent incursions into fresh
water from the marine environment.
92
A. Iglikowska and J. Pawłowska