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Brief Summary of Xenophyophora
Xenophyophores, are giant single-celled, eukaryotic organisms that can be found living in remote regions of the deep sea such as submarine canyons, seamounts, and oceanic trenches. Xenophyophores frequently occur in high densities (2000 individuals per 100 m2) on the seafloor beneath highly productive surface waters, such as the upwelling zone off Mauritania, West Africa (McClain 2008). An untethered “dropcam” deployed on a research cruise in the Pacific Ocean, recorded xenophyophores living as deep as10,641 m in the Mariana Trench (Aguilera 2011).
While some xenophyophore species, such as Syringammina fragillissima were originally classified as foraminiferans (Brady 1884); other xenophyophores, such as Stannophyllum setosum, were initially thought to be deep-sea sponges (Haeckel 1889). Once ranked as a phylum, Xenophyophora is currently classified as a subgroup within Foraminifera based on evidence from molecular genetic studies (Gooday et al. 2011, Pawlowski et al. 2003).
Syringammina fragillissima, and other xenophyophore species, construct complex homes of interconnecting tubes, the entire tangled mass of which can measure from 10-20 cm in diameter (Schaechter 2013). This mass of tubes, more commonly referred to as a “test,” may represent the largest structure built by a single cell (Marshall 2010). The cell body of the living organism contains 100s to 1000s of nuclei, and is encased in a membrane-bound network of branching tubes. The cytoplasm of many xenophyophore contains dense accumulations of barite (barium sulfate) crystals lying free in the cytoplasm (Tendal 1972). The function of these crystals is presently unknown; however, scientists speculate that they play a role in weighting down the test of the giant cell. Alternatively, the formation of barite crystals may function to inactivate barium, an element that is highly toxic to living cells. Xenophyophores also package their waste products into egg-shaped pellets (or “stercomes”) that are stuffed into membrane-like “sausage casings,” and stored within the confines of their tubular tests.