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Reticulomyxa filosa is a foraminiferan that lives in a diversity of terrestrial freshwater habitats, ranging from damp leaf litter in close proximity to tidal wetlands, to fresh water lakes. Unlike the majority of other foraminiferans, Reticulomyxa filosa does not possess an external test (or shell), but is instead characterized by a “naked” globose cell body that is covered by a glycocalyx and surrounded by an extensive network of anastomosing pseudopodia (or reticulopodia). The pseudopodial array can extend for several cm out from and around the periphery of the central mass of the cell.

The reticulopodia of Reticulomyxa filosa exhibit bidirectional streaming, a phenomenon in which organelles and cytoplasmic vesicles are simultaneously transported away from and in towards the central cell body. Particles, such as food item, can also be transported on the surface of the reticulopodia. The reticulopodia are underlain by a complex network of branched and anastomosed microtubules.

The multinucleate cytoplasm of Reticulomyxa filosa contains hundreds to thousands of haploid nuclei that are 5 µm in diameter. In laboratory cultures, only asexual mitotic multiple fission (plasmotomy) of the multinucleate cytoplasm has been observed; however, numerous genes that code for meiosis and the potential flagellar apparatus of gametes are present in the genome. In response to deteriorating environmental conditions, vegetative individuals may undergo multiple fission and form spherical-to-ovoid resting stages that can temporarily survive unfavorable conditions for a few weeks at a time. When conditions improve, the resting stages recover and reassemble their pseudopodial networks. During the summer months, walled cysts may form in response to adverse conditions, such as extreme temperatures and extended periods of desiccation.

The genome of Reticulomyxa filosa was the first foraminiferal genome to be sequenced. The genome is approximately 320 Mb in size, highly repetitive, and contains 40,443 predicted protein-coding genes (with mean of length of 957 bases). Of these genes, 29,352 are unique to Reticulomyxa filosa, and without homologous sequences in other species. Genes introduced by lateral gene transfer from bacteria appear to be relatively rare.