Reticulomyxa hannae Völcker and Clauß, 2020
Diagnosis: Trophozoites surrounded by a very thin mucous envelope, with slowly moving plasmodia, variable and reticulate in shape. Cell body 0.5-3.0 mm in diameter, with granuloreticulopodia up to 10 cm long. Plasmodia multinucleate, with about 10,000 ovular coarse-grained nuclei, 4.3-5.9 μm in diameter. Cytoplasm coarse-to-fine-grained, yellowish, with numerous rod-shaped crystals. Locomotion only via migration as unidirectional plasma movement. Cell division during migration phase. Cysts 100-350 μm in diameter, formed from the plasmodia in the central area within the mucus-like envelope.
Type locality: Among submerged Sphagnum, fen in high moorland near Hormersdorf, Germany (50°39’21.61″N 12°52’42.74″E), October 2017.
Description: Cells are large and naked, surrounded by a very thin mucous envelope, with very slowly moving plasmodia, variable and reticulate in shape. The thickness of the individual plasmodium strands in the central area is on average about 30 to 50 μm, reaching a maximum extent of up to 3 cm. Many thin, filiform and anastomosing granuloreticulopodia emerge from the central area. The relatively thick granuloreticulopodia in the peripheral zone (10 μm) narrow to fine offshoots with a thickness of 1 μm. Overall, they can reach a length ten times the diameter of the central region. Cytoplasm in constant bidirectional plasma movement, with many food vacuoles and numerous small rod-shaped refractive crystals. Locomotion only via migration as unidirectional plasma movement. Plasmatomy during the migration phase. Cysts are formed from the plasmodia in the central area within the mucus-like envelope which partially constricts like a string of pearls. The constricted areas each form a cyst with a very thick, multi-layered wall. The cytoplasm of the cysts has a yellow-brown to red color. The size of the cysts varies between 100 and 350 μm (usually around 200 μm). Before encysting, the peripheral area is resorbed. Feeds on bacteria, cyanobacteria, small green algae, diatoms, rotifers, ciliates, small lobose amoebae and yeast. Excrements are excreted through the surfaces of the main strands by exocytosis. Local depletion of food supplies causes migration.
Remarks: This species differs from R. filosa by its significantly smaller and thinner central plasmodial area, 0.5-3 mm vs. up to 6 mm for R. filosa when fully extended. These plasmodia are never flap-shaped in contrary to R. filosa.
The Reticulomyxidae comprise three species based on morphological and genetical evidence, Reticulomyxa filosa, Haplomyxa saranae and Dracomyxa pallida (Wylezich et al. 2014). With R. hannae, a fourth species is added now to this family that is described morphologically and genetically. Reticulomyxidae are characterized by the presence of a large number of nuclei. Dellinger et al. (2014) mention 1800 (2-4 µm) nuclei for a medium-sized cell of H. saranae, Wylezich et al. (2014) note hundreds or thousands of nuclei (3-4 µm) in D. pallida, while R. hannae cells have about 10,000 nuclei. These are slightly larger, 4.3-5.9 μm, than those in H. saranae and D. pallida.
Reticulomyxa hannae cells can be stationary for hours or even days, showing no locomotion of (streaming phenomena are present) the main cell body. At this stage the cells have an extensive bidirectional network to catch and transport food. During the migration phase, or when they multiply, the protoplasm flows unidirectionally through a fairly thick pseudopodium. This unidirectional streaming was also noted by Wylezich et al. (2014) in the emended diagnosis of the Reticulomyxidae: propulsion occurs through migration via one or several giant pseudopodia with predominantly unidirectional centrifugal flow of protoplasm.
The main difference between Reticulomyxa and Haplomyxa is the extended and reticulated cell body mass of Reticulomyxa versus the massive central cell body of Haplomyxa, although the cell body of the latter can sometimes be branched or with holes (Dellinger et al. 2014). An important difference between Haplomyxa and Reticulomyxa on the one hand and Dracomyxa and Wobo on the other is the presence of a tectum in the latter two taxa (Wylezich et al. 2014). This membranous cover might have a protective function and is absent in Haplomyxa and Reticulomyxa where cells are covered with exocytotic products (Wylezich et al. 2014). A similar protective function is also present in the Velamentofexidae, which are surrounded by a cloak of debris. Wylezich et al. (2014) also describe another form of protective retreat, namely that cells from D. pallida, R. filosa and W. gigas hide in cavities of, for example, small gastropods shells. Occasionally, we could observe this phenomenon, where a fragment of an insect’s leg and an empty cell of an alga (Pleurotaenium truncatum) served as protective retreat for the delicate bodies of two unidentified freshwater monothalamids (Siemensma 2019, 2020).