Biodiversity of foraminifera and other protists in the deep sea: scales and patterns

Ocean-floor sediments harbour a variety of protistan taxa, including ciliates, flagellates, naked amoebae, testate amoebae, foraminifera and xenophyophores. Only the foraminifera and xenophyohores, however, are reasonably well studied at the species level. Despite being an important component of deep-sea communities, these protists are frequently disregarded in biodiversity studies. This is unfortunate because "live" (rose Bengal stained) foraminifera are rich in species and morphologically very diverse. Individual samples from well-oxygenated bathyal and abyssal settings may contain up to 150 and sometimes more than 200 live species (63-microm fraction). The local diversity of foraminifera seems broadly comparable to that of nematodes among the meiofauna and polychaetes among the macrofauna. Particularly at abyssal sites, many species are undescribed and belong to poorly-known, softshelled taxa. Extrapolating from local to global diversity (a popular activity in biodiversity research) is hampered by lack of information about species distribution patterns, particularly for the soft-shelled taxa. However, many deep-sea foraminiferal species in "normal" well-oxygenated deep-sea settings appear to be widely distributed, implying relatively modest levels of global diversity. Trends in foraminiferal diversity in response to regional gradients of increasing organic enrichment and decreasing oxygen concentrations are fairly well described; species richness decreases, and dominance increases. Changes in foraminiferal diversity with increasing bathymetric depth down the continental slope have also been reported, but latitudinal diversity gradients remain largely undocumented among foraminifera in modern deep-sea settings. Because of their extensive fossil record, calcareous and other hard-shelled species can be used to address the influence of historical processes on large-scale diversity patterns. For example, the establishment of an Antarctic ice sheet 35 million years ago has been linked to the development of an ancient latitudinal diversity gradient among deep-sea foraminifera in the Southern Hemisphere. Xenophyophores are much less speciose than foraminifera. It has been estimated by Tendal (1996) that only about one hundred species, described and undescribed, exist in modern oceans. Where the two groups coexist at a single locality, there may be an order of magnitude fewer xenophyophore species than foraminiferal species. The much lower number of xenophyophore species probably reflects their larger size and narrower ecological tolerance compared to foraminifera.