Actinosphaerium eichhorni

The microcosm of the aquarium is home not only to worms and microorganisms that eat waste, but also to ferocious predators. Today we will talk about a member of a group of unicellular microorganisms that have learned to build trapping nets to catch their victims: Actinosphaerium eichhorni.

What are Actinosphaerium eichhorni?

The class of Heliozoa – sun-animalcule – unites several evolutionary lines of protists that are not related to each other, but have similarly organized cell features and structures. Actinosphaerium eichhorni is a representative of these microorganisms living in aquariums and ponds. It is a rather large protist, reaching up to 1 millimeter in diameter, which can be seen with the naked eye. Across the entire surface of the Actinosphaerium eichhorni cell, straight outgrowths called axopodia, protrude. These spines make the cell look like the sun radiating beams of light.


Axopodia can be used for several interactions with the environment such as sensation, capturing food, attachment and locomotion. When moving, Heliozoa are rather slow and are not able to chase prey like fast swimming infusoria. Therefore, to catch dinner, the sun-animalcule uses the ambush predator strategy, spreading axopods around the perimeter of the cell. Hovering in the water column, Heliozoa simply wait for their prey to touch its axopodia. It is almost impossible to get out of these “clutches”. Gradually, the prey is enveloped by the predator and dragged inside the cell for digestion.

Actinosphaerium eichhorni’s diet

The Heliozoan diet includes various planktonic microorganisms such as infusoria, flagellates, algae, and even multicellular organisms such as rotifers. Rotifers are stronger creatures than infusoria and many other protists. Once trapped, rotifers have a chance to escape from the tenacious clutches of their hunters. But not always… Heliozoa won’t let their prey escape so easily!

Benefits of Actinosphaerium eichhorni

Actinosphaerium eichhorni‘s predatory lifestyle can also be a benefit for your aquarium. Just imagine a swarm of hundreds of white spot disease-causing baby ciliates, freshly emerged from the mother cyst, looking for fish to infect and parasitize on. Theronts ending up in the paws of a Heliozoan, will certainly be eaten. Thus, Heliozoa are natural predators that can reduce the number of pathogenic microorganisms in the aquarium. So, this ferocious predator is actually a beneficial inhabitant!


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