TEST FUNCTION FORAMINIFERA (FUNGSI TEST/CANGKANG FORAMINIFERA)

TEST FUNCTION FORAMINIFERA

(FUNGSI TEST/CANGKANG FORAMINIFERA)

Operculina ammonoides (Source: Marchel Monoarfa )

The evolutionary and ecological success of Foraminifera depends in part on the functional significance of the test. However, some agglutinated Foraminifera can live as naked organisms outside the test (Astrorhiza limicola: Schultz, 1915; Buchanan and Hedley, 1960; Christiansen, 1971; Cedhagen, 1988; Iridia: Cushman, 1922; Astrammina rara: Bowser and Delaca, 1985). Such forms may be in between growth stages for when they are supplied with detrital material, Astrammina soon makes a new test. Indeed, Cedhagen and Tendal (1989) suggest that juveniles of this species smaller than 1.5mm in size might live without a test. Individuals of Astrammina rara from which the test was experimentally removed soon formed a new one (Bowser et al.,

1995).

There are morphological features of the test that may be of functional importance (see reviews by Hallock et al., 1991 and Hottinger, 2000) but experimental evidence supporting functional interpretations of specific morphological features is still very sparse. Six possible functions for the test as a whole have so far been proposed although, as discussed below, the results areinconclusive (Marszalek et al., 1969; Murray, 1991).

1.      To provide shelter. The test gives protection against unfavourable environmental conditions and, in addition, some species close their test openings for several hours by sealing them with debris. The organic lining of calcareous tests is the ultimate defence against low pH as Bradshaw (1961) showed that Ammonia ‘beccarii’ survived pH 2.0 for 111 4 h even though dissolution of the test took place. Spirillina vivipara, which lacks an organic lining, did not survive. Marszalek et al. speculated that chambered tests, such as that of Quinqueloculina, would give protection against sudden osmotic changes but this was shown by experiment not always to be true (Murray, 1968a). There may be some protection against certain wavelengths of light in shallow waters (Haynes, 1965; Banner and Williams, 1973).

2.      To serve as a receptacle for excreted matter. There are two aspects to excretion. It is known that some foraminifera store the waste products (stercomata or xanthosomes) in their test (Tendal, 1979). However, others consider that the test itself may be a consequence of excretion (e.g., the organic lining: Banner et al., 1973; or removal of toxic Ca2þ, Brasier, 1986).

3.      To aid reproduction. There is no direct evidence that the test is of particular use during reproduction (except that some parent tests are partially dissolved to supply material for the offspring).

4.      To control buoyancy. It is likely that the density of the soft parts is similar to that of seawater. In high-energy environments tests are commonly heavy and robust and, since they are made of material much denser than the seawater they displace, they counteract any buoyancy of the soft parts. Also, in many environments individuals gather detrital particles in their reticulopodia while feeding, as camouflage and possibly as additional counter buoyancy aids.

5.      To offer protection from predators. There is no direct evidence of this. Some species are more conspicuous because of their form or colour and are therefore preferentially selected by predators.

6.      To assist growth of the cell. Foraminiferal cells are relatively large and some exceed 3 cm . The test serves as a container that not only houses the cell but also provides additional space that may be used for growth or for storage, e.g., of stercomata (Mullineaux, 1987). In active individuals the test is incompletely filled with cytoplasm but when the reticulopodia are withdrawn into the test it may become filled. In the large form, Alveolinella quoyi, on average only 43% of the test is filled (Severin and Lipps, 1989) and deep-sea forms have highly vacuolated cytoplasm filling only part of the test. In shallow-water environments rich in food, the test is normally filled with dense cytoplasm.

The six possible functions of the test are not mutually exclusive. It is clear that some of the features of the test described by taxonomists as ‘ornament’ are definitely functional. Tubercles and structures such as teeth in the aperture serve to break up aggregates of food and detritus, ribs channel the extrathalamous cytoplasm (Banner and Culver, 1978; Kitazato, 1990; Bernhard and Bowser, 1999) and spines support pseudopodia and help stabilise the test on soft substrates. Some rotaliids have a canal system that replaces primary and secondary apertures and allows communication between the chambers and the test surface for the extrusion of extrathalamous cytoplasm and reticulopodia, for removing waste products and for release of juveniles during reproduction (Ro¨ ttger et al., 1984).