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Modern deep-sea benthic foraminifera: a brief review of their morphology-based biodiversity and trophic diversity

¹ National Oceanography Centre, Southampton, Empress Dock, European Way, Southampton SO14 3ZH, UK (e-mail: ang{at}noc.soton.ac.uk)
² Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan

Most fossil deep-sea foraminifera are multichambered and have relatively robust, calcareous or agglutinated shells. Modern assemblages, on the other hand, include many fragile monothalamous (single-chambered) forms and komokiaceans (a superfamily of protist currently placed within the foraminifera) with soft test walls. These groups are poorly known and most of the hundreds of morphospecies recognized in deep-sea samples are undescribed. The relative abundance of robust and fragile taxa varies with water depth and food supply. Calcareous and other hard-shelled species tend to predominate in relatively eutrophic areas, particularly on continental margins, but decrease as a proportion of the ‘entire’ live fauna (i.e. including soft-shelled species) with increasing water depth, even above the CCD (carbonate compensation depth). Most of the species on which the foraminiferal proxies used in palaeoceanography are based live in these bathyal regions. At abyssal depths, and particularly below the CCD, faunas are largely agglutinated and dominated by monothalamous forms. These assemblages have a much lower fossilization potential than those found on continental margins. In addition to carbonate dissolution, these patterns probably reflect adaptations to increasingly oligotrophic conditions on the ocean floor with increasing depth and distance from land. Bathyal species include herbivores and opportunistic deposit feeders (omnivores) that consume labile organic material, in addition to deep-infaunal deposit feeders, and must contribute significantly to carbon cycling. Many abyssal monothalamous foraminifera, in constrast, accumulate stercomata (waste pellets composed of fine sediment particles) and probably ingest sediment, associated bacteria and more refractory organic matter. Some monothalamous species without stercomata may be bacteriovores. Although they probably process organic carbon at a slower rate than calcareous species, the shear abundance of monothalamous taxa at abyssal depths suggests that they are important in carbon cycling on a global scale. The loss of a substantial proportion of foraminiferal biomass and biodiversity from the fossil record should be considered when using foraminifera to reconstruct palaeoproductivity, for example, by using the Benthic Foraminiferal Accummulation Rate (BFAR). Different dietary preferences among calcareous species have implications for the stable carbon isotope signal preserved in their shells.

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