Actinocoma ramosa
Actinocoma ramosa

Actinocoma ramosa Penard, 1903

Diagnosis: Body naked, grayish, spherical or star-shaped, without a clear distinction between ectoplasm and endoplasm; surface with one or several large contractile vacuoles; cytoplasm containing small vacuoles and colorless brilliant grains. In the center of the plasm is a nucleus of unusually large size, very clearly drawn, with a very thick membrane, and filled with a nuclear juice with usually one nucleolus; sometimes two nucleoli are present.Nucleolus more or less spherical, normally with one or more small lacunae.
Pseudopodia radiating in all directions, rarely less numerous, most often in considerable numbers, straight and simple, or each branched into a certain number of filaments that deviate little from each other, thus taking together the appearance of a broom. These filaments bear mobile granules, very small and few in number (Original diagnosis, translated from French).

Dimensions: Penard (1903), central body 14-26 µm, my measurements: 22-31 µm. Nucleus 12-16 µm.

Ecology: Fresh water. In oligotrophic and mesotrophic watertypes, e.g. the Spiegelplas, a deep and large lake, and in the IJsselmeer near Kornwerderzand and in Crailoo, a mesotrophic pond, both in the Netherlands. It multiplied in wet mounts kept in humidity chambers. This species has also been found in the USA by Yana Eglit (2012, pers. comm.) and in Germany in a fresh water aquarium by Eckhard Voelcker (2015, pers. comm).

Remarks: The granules are very hard to detect, only with phase contrast they become more distinct. They are more numerous than Penard describes and they move in a bidirectional way. I have found this species several times. They came up in so called wet-chamber-mounts. The pseudopodia of my specimens differ from the ones described by Penard, but their nucleus fits well to his description. It is not unusual that amoeboids develop completely different pseudopodia when settled or attached to the slide. I have seen this phenomenon also with specimens of Clathrella foreli and Leptophrys-species.
Actinocoma ramosa is very sensitive to light. It reacts immediately when it comes within the light path of the microscope. It retracts its long pseudopodia and broad veils. What remains is a more or less spherical body with rod-like short pseudopodia moving and waving restlessly.
Penard (1903): “This curious body has the appearance of a heliozoaire, and would remind somewhat Actinophrys sol; by its branched pseudopodia  and devoid of any axial filaments, it is an amoeboid rhizopod. It is still rare, and I found only a few individuals, at Point à la Bise, along the shore of the lake.”

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