Associations between living benthic foraminifera and dead tests of Syringammina fragilissima (Xenophyophorea) in the Darwin Mounds region (NE Atlantic)

The xenophyophore Syringammina fragilissima is abundant in a bathyal region of the NE Atlantic (950 m water depth) characterised by large numbers of 100 m wide sandy mounds (the Darwin Mounds). We examined “live” (i.e., rose Bengal-stained) benthic foraminifera associated with fragments of four dead S. fragilissima tests from four box-cores collected in this region. Many of the abundant species associated with the xenophyophore tests were common bathyal species that also occurred in the surrounding sediment (e.g., Nonionella iridea and Eponides pusillus). Foraminifera associated with the xenophyophore tests were generally more abundant than those inhabiting the surrounding sediments, although species richness and diversity were not significantly different. Foraminiferal abundances were lower than those previously reported from comparable sites in the Northeast Atlantic. Some species had patchy distributions. In particular, the organic-walled Cylindrogullmia sp., the overall top-ranked species, was abundant in one test, where it constituted 36% of the foraminiferal assemblage, and rare or absent in others.Four distinct microhabitats, inhabited by particular foraminiferal species, were present within the complex structure of the xenophyophore tests: (1) Within the tests, in the remains of the organic tubes (granellare) that enclose the protoplasm in live xenophyophores (various monothalamous foraminifera, e.g., the elongate organic-walled Nemogullmia sp.). (2) Within the branches of the test, but outside the granellare tubes; sometimes associated with dark-grey sediment presumed to be the decayed remains of xenophyophore waste products (e.g., the allogromiid Cylindrogullmia sp. and the calcareous species Chilostomella elongata). (3) The outer surface of the test (numerous small calcareous foraminifera). (4) In mud trapped between the test branches (common bathyal species, many of which also occurred in the surrounding sediments). A number of metazoans, mainly nematodes, were also found occupying specific microhabitats within the tests, but these were generally less abundant than the foraminifera