Large, deep-sea agglutinated Foraminifera; two differing kinds of organization and their possible ecological significance

Southampton Oceanography Centre, Southampton, United Kingdom

We consider two groups of large deep-sea agglutinated foraminifera: 1) robust, tubular astrorhizaceans (Bathysiphon, Rhabdammina) and hippocrepinaceans (Hyperammina, Saccorhiza) which reach several centimeters or more in length and contain a substantial protoplasmic mass (sarcode); and 2) delicate komokiaceans with tests typically 1-5 mm in size and forming a branching tubule system containing accumulations of stercomata and sparse protoplasm, sometimes with fine sediment incorporated between the tubules to form a mudball structure. The tubular taxa occur in eutrophic settings, usually on continental margins, where the food supply originates from strong surface primary production, currents, or downslope transport. We speculate that they are opportunists, able to grow fairly quickly when food is available. The large sarcode may provide a buffer against periods of food shortage, enabling these taxa to take advantage of a relatively high but fluctuating food input. Komokiaceans, on the other hand, have a "large volume/low biomass" organization and are most important in oligotrophic, central oceanic regions. We speculate that the combination of relatively large body size and low protoplasmic content may reflect slow growth and longevity; it may also serve to reduce the food content of komokiaceans and hence their attractiveness to predators. In order to illustrate these contrasting groups, we briefly consider faunas from the Oman margin (3400 m depth) and NE Atlantic abyssal plains (4850-4950 m depth), distinguishing between: 1) oligotrophic, central oceanic regions rich in komokiaceans (Madeira Abyssal Plain); 2) central oceanic regions with seasonal phytodetritus inputs where komokiaceans coexist with small (<150 mu m) opportunistic foraminifera (Porcupine Abyssal Plain); and 3) continental margin areas where the organic matter flux is sufficient to support large tubular agglutinated foraminifera (Oman margin). Such a scheme provides a basis for using large agglutinated foraminifera to interpret the paleoceanographic record.

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