ORGANIC-WALLED ALLOGROMIIDS: ASPECTS OF THEIR OCCURRENCE, DIVERSITY AND ECOLOGY IN MARINE HABITATS

Abstract

Foraminifera with an organic, or predominantly organic, test wall (‘allogromiids’ in the traditional sense) are an important, diverse but often overlooked component of marine benthic communities. This paper reviews some of the scattered literature on these protists. They vary from <50μm to a centimetre or more in length and are morphologically diverse, including spherical, oval, sausage-shaped and thread-like forms. Most are monothalamous and have either one aperture (e.g. Allogromia ) or two terminal apertures at either end of the test (e.g. Nemogullmia and Tinogullmia ). Some distinctive deep-sea forms ( Nodellum, Placopsilinella and Resigella ) are polythalamous; the test consists of more or less well-defined chambers and the wall is brownish in colour. Organic-walled allogromiids are reported from marine habitats ranging from supralittoral sands and intertidal mudflats to deep-sea trenches and are particularly abundant and diverse in cold, tranquil settings with fine-grained sediments (e.g. the deep sea, fjords, and some polar environments). Relatively few morphospecies have been formally described, and most of these are from coastal and intertidal settings, yet these delicate foraminifera often account for 10–20% of individuals and morphospecies in deep-sea samples. At Arctic coastal sites influenced by turbid glacial meltwater, and in some estuaries, allogromiids (including saccamminids) represent an even higher proportion of live foraminifera, in some cases >90%. In contrast, organic-walled allogromiids and other monothalamous forms are usually relatively uncommon in dysoxic settings. Experimental and field studies support the idea that, in a general sense, allogromiids are less tolerant of oxygen depletion than the calcareous taxa that usually dominate foraminiferal faunas in low-oxygen habitats. Organic-walled allogromiids occupy a variety of microhabitats. Intertidal and sublittoral species have been described from the sheltered interiors of empty polychaete tubes and large foraminiferal tests. Some species are associated with the sediment-water interface while others occur at different depths within the sediment. Very elongate, thread-like morphotypes often live several centimeters below the sediment surface in bathyal and abyssal settings. The ecological role of organic-walled allogromiids is poorly known. In general, they seem to be less responsive to inputs of fresh organic matter than calcareous foraminifera. Indirect evidence suggests that many deep-sea species, particularly forms with two terminal apertures, consume bacteria.