Micro-CT 3D imaging reveals the internal structure of three abyssal xenophyophore species (Protista, Foraminifera) from the eastern equatorial Pacific Ocean

Abstract

Xenophyophores, giant foraminifera, are distinctive members of the deep-sea megafauna that accumulate large masses of waste material (‘stercomare’) within their agglutinated tests, and organise their cells as branching strands enclosed within an organic tube (the ‘granellare’ system). Using non-destructive, three-dimensional micro-CT imaging we explored these structures in three species from the abyssal eastern Pacific Clarion-Clipperton Zone (CCZ). In Psammina spp., the low-density stercomare occupied much of the test interior, while high-density granellare strands branched throughout the structure. In Galatheammina sp. the test comprised a mixture of stercomare and test particles, with the granellare forming a web-like system of filaments. The granellare occupied 2.8–5.1%, the stercomare 72.4–82.4%, and test particles 14.7–22.5%, of the ‘body’ volume in the two Psammina species. The corresponding proportions in Galatheammina sp. were 1.7% (granellare), 39.5% (stercomare) and 58.8% (test particles). These data provide a potential basis for estimating the contribution of xenophyophores to seafloor biomass in areas like the CCZ where they dominate the megafauna. As in most xenophyophore species, the granellare hosted huge numbers of tiny barite crystals. We speculate that these help to support the extensive granellare system, as well as reducing the cell volume and lightening the metabolic burden required to maintain it.

Giant, highly diverse protists in the abyssal Pacific: vulnerability to impacts from seabed mining and potential for recovery

“…Thus, the visual dominance by xenophyophores of the megafauna in seafloor images may not be matched by their contribution to benthic biomass [39]. They probably feed at a low trophic level, either by gathering material from the sediment surface, suspension feeding, trapping particles within complex test structures, or perhaps by taking up dissolved organic compounds [39], a purpose for which the extensively branching cell body [1] would be well suited. Their role in carbon cycling is still unquantified, although grazing traces on tests and studies of metazoan gut contents show that some animals feed on xenophyophores [39][40][41], indicating that they contribute to deep-sea food webs [42].…”

“…Their tests are morphologically very diverse and reach sizes of up to 24 cm, making them among the largest known single-celled organisms. Xenophyophores have a distinctive internal structure [ 1 , 2 ] and were formerly classified as a separate group of amoeboid organisms [ 2 ] until genetic analyses revealed them to be ‘monothalamous’ (single-chambered) foraminifera [ 3 ]. They are now firmly established as a monophyletic group forming one of the terminal branches of monothalamid Clade C [ 4 ].…”

High-throughput microCT scanning of small specimens: preparation, packing, parameters and post-processing

“…Current research on museum-based µCT data encompasses an extraordinary array of material and topics, including tetrapod origins 45 , echolocation in bats 46 and whales 47 , limb reduction in lizards 48 , 49 , mammalian limb development 50 , 51 , primate neuroanatomy 52 , 53 , reproduction in insects 54 and plants 55 , paleoecology of Precambrian biota 56 , seafloor biomass estimation 57 and amber inclusions 58 , to name a few. Typically these studies involve few specimens, due to the challenges of capturing high resolution 3D data for large sample sizes.…”

Preliminary Observations of the Abyssal Megafauna of Kiribati

“…However, the pool of images in Area A H subsample was about four times smaller than that in each of the other three areas ( Table 1). Metazoan and foraminiferal data were processed separately and the latter were excluded from diversity assessments because: (i) it is not possible to determine whether foraminifera are alive in images (Hughes and Gooday, 2004); (ii) the taxonomic resolution allowed in image assessments is lower to that achieved in metazoans (see e.g., Gooday et al, 2017b); and (iii) there is a substantial mismatch between the biomass of metazoan and foraminifera specimens, since the protoplasm volume of the latter represents only 1-0.01% of their visible test (Levin and Gooday, 1992;Gooday et al, 2018).…”