Viscidocauda

Viscidocauda repens is a biflagellate amoeboflagellate protist characterized by its oval-shaped cell body and distinctive flagellar arrangement, marking it as the only known species in the Endomyxa subphylum to possess two persistent flagella. The anterior flagellum projects forward and is typically active during swimming, while the posterior flagellum trails behind, often remaining attached to the substrate to anchor the cell during gliding movement across surfaces.^[1] The cells exhibit slow gliding motility, supported by trailing pseudopodia that stabilize the organism while it grazes on bacterial prey.^[1] Transmission electron microscopy reveals unique internal structures, including a persistent posterior flagellum unlike those in previously described endomyxans, along with membrane-bound organelles typical of eukaryotes.^[1]

Viscidocauda repens is classified under the domain Eukaryota, whose members are characterized by membrane-bound organelles, including the nuclear envelope surrounding the nucleus and the double membrane surrounding the mitochondrion.^[2] V. repens belongs to the kingdom Protista, as it is a unicellular organism that is distinct from animals, plants, and fungi. It is further placed within Rhizaria, a supergroup nesting between the domain and the phylum, characterized by temporary, extending pseudopodia used for locomotion.^[3] The phylum is Cercozoa, a grouping based on 18S rDNA phylogeny. The species is further classified within the subphylum Endomyxa, a group of protists that were originally believed not to have flagella.^[1] The genus has not been placed into a class, order, or family.

Viscidocauda repens was discovered to share a common ancestor with endomyxan groups such as Ascetosporea and Gromiidea, indicating that the species belongs to a lineage of marine parasites and amoebae.^[1] These groups were identified as part of a clade, with V. repens also showing an environmental lineage to clades identified as Endo-4 and Endo-5.^[1] The lineage has been described as a sister group to the Filosa and Skiomonadea clade.^[1] Using small subunit ribosomal RNA (SSU rRNA) gene sequences, researchers found similarities between V. repens and several environmental sequences obtained from marine samples, including seawater and the gut contents of herbivorous fish, with differences ranging from 0.3% to 2.6%.^[1] Phylogenetic analyses also indicated that a monophyly of Endomyxa was not recovered, suggesting that V. repens may represent an early-diverging lineage within the group.^[1]

Viscidocauda repens was first reported by Takashi Shiratori and Ken-ichiro Ishida of the University of Tsukuba after they examined seawater collected near Hachijojima Island, Japan, at a depth of about 5 m on September 6, 2012.^[1] The sample was filtered through a 20 µm mesh and then a 2 µm filter to concentrate smaller microorganisms before being incubated in Enriched Seawater Medium (ESM) at room temperature for about two weeks. The organism was isolated from the enrichment culture and observed using differential interference contrast light microscopy, which allowed the researchers to see its shape and gliding movement.^[1] To investigate its internal structures in more detail, they then used transmission electron microscopy, which revealed ultrastructural features of the organism.^[4]

Viscidocauda repens is a heterotrophic bacterivore that glides slowly across surfaces, anchored by its posterior flagellum while using pseudopodia to capture bacterial prey for phagocytosis.^[1] The available evidence shows that the organism is a heterotrophic protist that obtains nutrients by consuming bacteria instead of performing photosynthesis.^[5] This biflagellate motility enables both surface grazing and occasional swimming, unlike other endomyxans.^[1]

Researchers extracted total genomic DNA from cultured cells and amplified the small subunit ribosomal RNA (SSU rRNA) gene using polymerase chain reaction (PCR).^[1]The gene was then cloned and sequenced, with the resulting sequence deposited in the National Center for Biotechnology Information (NCBI) database under GenBank accession number LC847298.^[6]