Soft-walled, monothalamous benthic foraminiferans in the Pacific, Indian and Atlantic Oceans: Aspects of biodiversity and biogeography.

Delicate foraminiferans with test walls that are either agglutinated (Order Astrorhizida, Families Saccamminidae and Psammosphaeridae) or composed of organic material (Order Allogromiida) are often an important element of deep-sea benthic assemblages. Many species have soft, flexible walls and, except for a few allogromiids, are monothalamous (single-chambered). We examined quantitative multicorer samples from abyssal (4263–5570 m) water depths in the North and western Equatorial Pacific and a qualitative sample from a hadal site (7800 m water depth) in the Atacama Trench, SE Pacific. Soft-walled foraminiferans made up 29–42% of all complete, ‘live’ (rose Bengal stained) foraminiferans in the >32 μm fractions (0–1 cm sediment layer) of the North Pacific samples and 63% in the western Equatorial Pacific sample. They included the following morphotypes: (i) organic-walled allogromiids with one or occasionally two apertures; (ii) Nodellum- and Resigella-like forms in which the organic wall is generally brownish in colour and the test sometimes divided into more or less well-defined chambers by constrictions or partitions; (iii) flask-like agglutinated saccamminids with one aperture; (iv) oval saccamminids with two apertures; (v) agglutinated spheres which lack obvious apertures (psammosphaerids). In addition, numerous isolated spherical chambers, believed to be fragments derived from much larger komokiacean foraminiferans, were present in some samples and easily confused with saccamminids. At each Pacific station, we recognised 26–55 morphologically distinct forms (morphospecies) in addition to numerous ‘indeterminate’ specimens that could not be categorised on the basis of morphological criteria. A comparison of the Pacific assemblages with existing data on monothalamous assemblages from abyssal and bathyal depths in the Atlantic and Indian Oceans (>63 μm fraction) suggests that some morphospecies have wide geographical and bathymetric distributions. These wide-ranging morphospecies could provide good targets for future molecular-level studies addressing species differentiation among soft-shelled, monothalamous deep-sea foraminiferans.