Genus Synamoeba Grell, 1994
Diagnosis: Synamoeba arenaria forms a meroplasmodium, i.e., a permanent association of cells connected by a reticulopodial network. The diameter of the network increases with the age of the organism. Within the network, there is an incessant streaming of ground cytoplasm and particles. While the particles in the larger reticulopodia appear to move in well-defined cytoplasmic streams, the movement of single smaller particles is saltatory with mean velocities of about 15 µm/s.
Scanning electron microscopy certifies the light microscopic observations that the plasmodial rhizopod is capturing and digesting protistan prey. Not only the whole body can catch and phagocytize the prey diatoms but also the tips of free-ending reticulopodia are used for grasping prey. The cytoplasm of S. arenaria is not subdivided into ecto- and endoplasm. An elaborated network of vacuoles (sometimes more than 10 µm in diameter) is a dominating constituent of especially the central cytoplasm, while the cortical plasma shows smaller vacuoles. Dependent on the nutritional state of the cell, the vacuoles contain several stages of digested diatoms, empty shells, and other prey remnants.
The cell membrane is more or less covered by a surface coat sometimes with extracellular deposits. As in other plasmodial rhizopods, membranosomes communicate with the cell membrane. Smooth endoplasmic reticulum, membranous cisternae, and more or less dense membranosomes as constitutive parts of other membrane systems occur. Mitochondria with vesicular cristae are distributed throughout the cell. Golgi dictyosomes appear arranged mainly around the MTOC.
Synamoeba arenaria is an uninucleated protist, whose nucleus usually has an eccentric position in the cell. Nuclei are rounded or ovally shaped with irregular surfaces. Their diameters vary from 4 to 7 µm. The nuclei are surrounded by nuclear envelopes which are interrupted by regularly arranged nuclear pore complexes. During nuclear division, the MTOC divides via typical V-shaped intermediary structures (Fig. 18). Immediately after division of the nucleus, before starting cytokinesis, two MTOCs appear in immediate proximity of the daughter nuclei.
Literature: Grell K. G. (1994b) The feeding community of Synamoeba arenaria n. gen., n. sp. Arch. Protistenkd. 144: 143-146.