Extant Foraminifera are known to exhibit numerous variations on this general
cycle, however life cycles of only about 30 of over 10,000 modern species have
been studied (Goldstein 2003). The variation includes trimorphism, apogamic life
cycle, binary fission, various forms of budding, the occurrence of test and nuclear
dimorphism. The alternation of generations in Foraminifera may by facultative or
obligatory (Goldstein 2003).
Some species are apogamic and they have reduced the complexity of their life
cycle by omitting the sexual generation. In Fissurina marginata and Spiroloculina
hyalina only the asexual phase is observed (Arnold 1964), whereas some planktonic
species reproduce exclusively sexually, and no asexual generation has been
observed (Goldestein 2003).
The life cycle of allogromiids and astrorhizids seems to be more variable than in
other Foraminifera. Binary fission has been observed in Allogromia laticollaris,
budding, serial and multiple budding in Saccammina sphaerica, S. alba, and
A. laticollaris, fragmentation occurs in the miliolid Calcituba polymorpha, and
Floresina is capable of producing multiple broods (Arnold 1954, 1964, 1967;
Goldstein 1988, 2003).
Most groups of unicellular organisms (including most Foraminifera) adopt an
opportunistic r-strategy, where the population grows quickly by frequent cell
divisions. However, there is some evidence that larger-sized benthic species of
Foraminifera conform to a K-selected mode of life over long periods. The production of a complex, large-sized test which is slow growing, houses storage
products and symbionts, and generates the permanent body shape can be viewed as
advanced adaptations to their particular mode of life (Hottinger 1982).
For benthic Foraminifera there are four methods of dispersal: (1) Release
gametes, zygotes, or embryonic agamonts or gamonts into the water column, (2)
Meroplanktonic juvenile stages with subsequent passive spread by currents, (3)
Self-locomotion along sea floor, and (4) Passive dispersal by means of a physical or
biological vector (Alve 1999). Sexual generation appears to be the most efficient
method, because released gametes are advected by the bottom water currents
(Kitazato and Matsushita 1996). Dispersal by gametes may be efficient over short
distance, whereas zygotes and embryonic juveniles, with their density comparable
to sea water, are more prone to disperse over larger distance (Alve 1999).
3.4 Other Adaptations
Foraminifera show additional adaptations against unfavourable environmental
conditions. They are traditionally considered to be obligate aerobes, and most
species become dormant during exposure to adverse conditions such as oxygen
depletion. However, some benthic allogromiid species living in low-oxic habitats
are capable of storing and respiring nitrate through complete denitrification to N 2
(Kuhnt et al. 2013). Foraminifera have evolved at least two ways to carry out this
process: one involving symbionts and the other by the foraminifer itself. Bernhard
The Adaptations of the Foraminifera and Ostracoda …
101
cycle, however life cycles of only about 30 of over 10,000 modern species have
been studied (Goldstein 2003). The variation includes trimorphism, apogamic life
cycle, binary fission, various forms of budding, the occurrence of test and nuclear
dimorphism. The alternation of generations in Foraminifera may by facultative or
obligatory (Goldstein 2003).
Some species are apogamic and they have reduced the complexity of their life
cycle by omitting the sexual generation. In Fissurina marginata and Spiroloculina
hyalina only the asexual phase is observed (Arnold 1964), whereas some planktonic
species reproduce exclusively sexually, and no asexual generation has been
observed (Goldestein 2003).
The life cycle of allogromiids and astrorhizids seems to be more variable than in
other Foraminifera. Binary fission has been observed in Allogromia laticollaris,
budding, serial and multiple budding in Saccammina sphaerica, S. alba, and
A. laticollaris, fragmentation occurs in the miliolid Calcituba polymorpha, and
Floresina is capable of producing multiple broods (Arnold 1954, 1964, 1967;
Goldstein 1988, 2003).
Most groups of unicellular organisms (including most Foraminifera) adopt an
opportunistic r-strategy, where the population grows quickly by frequent cell
divisions. However, there is some evidence that larger-sized benthic species of
Foraminifera conform to a K-selected mode of life over long periods. The production of a complex, large-sized test which is slow growing, houses storage
products and symbionts, and generates the permanent body shape can be viewed as
advanced adaptations to their particular mode of life (Hottinger 1982).
For benthic Foraminifera there are four methods of dispersal: (1) Release
gametes, zygotes, or embryonic agamonts or gamonts into the water column, (2)
Meroplanktonic juvenile stages with subsequent passive spread by currents, (3)
Self-locomotion along sea floor, and (4) Passive dispersal by means of a physical or
biological vector (Alve 1999). Sexual generation appears to be the most efficient
method, because released gametes are advected by the bottom water currents
(Kitazato and Matsushita 1996). Dispersal by gametes may be efficient over short
distance, whereas zygotes and embryonic juveniles, with their density comparable
to sea water, are more prone to disperse over larger distance (Alve 1999).
3.4 Other Adaptations
Foraminifera show additional adaptations against unfavourable environmental
conditions. They are traditionally considered to be obligate aerobes, and most
species become dormant during exposure to adverse conditions such as oxygen
depletion. However, some benthic allogromiid species living in low-oxic habitats
are capable of storing and respiring nitrate through complete denitrification to N 2
(Kuhnt et al. 2013). Foraminifera have evolved at least two ways to carry out this
process: one involving symbionts and the other by the foraminifer itself. Bernhard
The Adaptations of the Foraminifera and Ostracoda …
101