Foraminifera
Introduction

Extant planktic Foraminifera occur in all pelagic environments of the World Ocean. Approximately 42 morpho-species exist today, in contrast to several hundred fossil species. Of the 42 species, 35 have so far been recorded from the South Atlantic.

Planktic Foraminifera have been recognized since the early 18th century, one of the first good figures, including the cell, having been provided by Rhumbler (1911). They are used as stratigraphic markers to determine ages of fossiliferous sediments, and since Schott's (1935, 1936) key distributional studies, have been used increasingly to trace recent and ancient current systems (Vincent and Berger, 1981).

Planktic Foraminifera form calcitic, multi-chambered shells usually coiled either plani- or trochospirally, or the chambers may be bi- or triserially arranged. The wall is multi-layered and porous when seen from the outside. However, in life these pores are closed by organic material (Hemleben et al., 1989). In all genera except one (Hastigerina), the primary wall is bilamellar (Reiss, 1957), two calcitic layers separated by a primary organic membrane. At each chamber formation event a new organic and calcite layer is secreted on top of the previous layer. Inside the test up to four calcite/organic layer couplets have been observed. Only subsequently organic layers are formed inside at each chamber formation, creating the so-called inner organic layer (Hemleben et al., 1989). In most species a rather incomplete to thick calcite layer is deposited prior to gametogenesis (gametogenic calcification), which often covers spine holes (spines are resorbed during gametogenesis leaving holes or depressions in the wall) and their bases. In cases where species migrate to deeper and/or cooler water, a secondary calcite layer or crust is deposited on top of the normal wall. The calcite crust has a coarse crystalline structure, normally with euhedral or subhedral crystals.

Following Murray (1897), one can distinguish between non-spinose and spinose species. Spines may be circular or triangular in cross-section. Both normal pores (2-4 µm) and micropores (<1 µm) occur in non-spinose species, whereas spinose species always exhibit normal pores (4-7 µm) (Bé, 1968). Pustules are present in variable numbers on and within the rather smooth shell-wall of non-spinose species, or the wall surface may be cancellate, as in some of the spinose species. Spinose species shed their spines during gametogenesis. Thus, shells settling on the sea-floor are seldom provided with spines, showing only spine holes. Occasionally some remnants can be detected under the SEM (Hemleben et al., 1989). The surface wall structure is either cancellate (honeycomb structure, (seeFo5i), or consists of irregularly distributed spine bases. Microperforate species lack a cancellate texture but may bear many small pustules. Microperforate species have the smallest pore size, the highest pore concentration, but lowest porosity (Bé, 1968).

In general, planktic Foraminifera are omnivorous. However, spinose species may prefer a carnivorous diet, whereas the non-spinose species favor algal food. Animal prey observed within the cytoplasm of spinose species includes ciliates, tintinnids, radiolarians, copepods (mostly calanoids), amphipods, pteropods, heteropods, chaetognaths, and larvae of various kinds. All kinds of algae, mainly diatoms, but also prymnesiomonads (e.g., coccolithophorids and thin-walled green algae) are observed in the cytoplasm of non-spinose species. Most spinose species host symbionts (zooxanthellae; exceptions are Globigerina bulloides, Hastigerina pelagica and H. digitata, which are devoid of symbionts), either dinoflagellates or chrysophycophytes. This implies that spinose species live in the photic zone, although they may also survive in deeper waters for a few days when expatriated from their original environment (e.g., during storms, cf. Schiebel et al., 1995). Some non-spinose species, like Neogloboquadrina dutertrei, may host green algae facultatively, especially during spring blooms. Commensals and parasites are quite common in both groups (Anderson et al., 1979; Hemleben et al., 1989).

The reproductive cycle varies from species to species, although some similarities exist within various groups. Most of the spinose, symbiont-bearing species exhibit a lunar or semi-lunar cycle, reproducing at about either full moon or new moon. Their normal life span ranges between 2 and 4 weeks (Spindler et al., 1979; Hemleben et al., 1989; Bijma et al., 1990a; Erez et al., 1991; Schiebel et al., 1997). Some of the non-spinose species follow the same reproductive scheme, while others, the deep-living non-spinose species, seem to follow a yearly cycle. All planktic Foraminifera perform ontogenetic vertical migrations. The deep-living species, like Globorotalia truncatulinoides, reproduce in surface waters during spring and then migrate downwards to layers as deep as 1000 m or even deeper (Hemleben et al., 1985). Other spinose, as well as non-spinose surface-dwelling species reproduce in the pycnocline or chlorophyll maximum layer. Thus, each species seems to have its own typical cycle and ontogenetically variable depth habitat.

So far, only sexual reproduction (gamete release) has been observed in laboratory cultures. However, a second generation has never been successfully raised, although zygotes and juveniles of up to four chambers have been observed. The ontogeny has been studied either using juveniles from plankton tows, or by back-tracking the successive growth stages (chamber sequence). Most species show a distinct development described by Brummer et al. (1986, 1987), who distinguished five different stages: prolocular, juvenile, neanic, pre-adult, and terminal stage. This sequence usually involves a drastic change during the neanic stage. The final chamber, built just prior to gametogenesis, may be morphologically different from the previous ones: either larger or smaller, or equal in size to the previous chamber but different in other aspects. In the microperforate foraminifers a bulla (see Fo5h) may be formed covering the aperture.

The distribution pattern of living planktic Foraminifera is determined chiefly by the climatic belts and their current systems. Thus, one can recognize antarctic, subantarctic, transitional, subtropical, and tropical assemblages (e.g., Bé and Tolderlund, 1971; Hemleben et al., 1989). This pattern may be overprinted under special hydrographic conditions, e.g., upwelling, shifts in boundary conditions, or seasonal events such as elevated spring blooms or a sequence of heavy storms.

Planktic Foraminifera are used to trace present-day ocean surface currents and their hydrographic properties (e.g., Boltovskoy, 1962, 1970; Cifelli and Bénier, 1976; Oberhänsli et al., 1992; Kemle-von Mücke, 1994; Ufkes et al., 1998), as well as to characterize ancient water masses over the last 100 million years. They are used as paleoceanographic proxies (e.g. in studies of stable isotopes of oxygen and carbon) to determine temperature, salinity, oxygen content, and fertility in the changing ocean environment, thus contributing to understanding ancient ocean processes (see Hemleben et al., 1989, for review).

The geologic record of foraminifers dates back to the middle Jurassic. They became more abundant, and thus important for stratigraphy, since the mid-Cretaceous. During the first half of this century they were used principally for establishing zonal stratigraphy from the upper Valanginian to the Recent, finding practical application mainly in oil exploration.