WunderkanoneExploring the Microscopic World

Actinospaerium eichhornie

Actinospaerium lurking in the dark

If you ever wondered where heliozonans got their name from ...

When prey comes in contact with the axopodia (the rays) it is immediately immobilized. A food vacuole is build around the captured prey.

When the food vacuole is complete it is drawn into the body of the heliozoan.

Actinospaerium eichhornie, polarized light

This is not Elefant skin but an optical cut through A. eichhornie (DIC image).

Details of A. eichhornie. In the middle is the central food vacuole with some older food remains. If you look carefully you can see some of the many nuclei.

The two most prominent types of Heliozoans are Actinophyrda (Actinospaerium, Actinophyrs) and Centrohelida. They have been put in the phylum Heliozoa because of their morphological similarities, though are not phylogenetically related. Heliozoans possess a ball-like body with axopodia (rays, spikes) and no internal skeleton. The axopodia are supported by bundles of microbubules (axoneme). The axopodia are coated with plasma, in which extrusomes (kinetocyts and/or mucocysts) travel up and down.

Actinosphaerium eichhornie (Ehrenberg, 1840) is the largest of the Heliozoans, reaching the diameter of a pinhead (200-300µm). There is a variation that can even reach 1 mm. Actinosphaerium is the only Heliozoan with multiple nuclei (the actual number depends on the size of the individual species) and is a planktic species living between aquatic plants, where it floats and waits patiently for prey. Actinosphaerium is a fierce predator that can catch and engulf algae, larger cilates and even rotifers.

When prey comes in contact with the axopodia (the rays) it is immediately immobilized. A food vacuole is constructed around the captured prey. When the food vacuole is complete, the axopodia are retracted and the prey is drawn into the body of the Heliozoan. Actinospaerium has a big central food vacuole which seems to be the preferred place for phagocytosis. Building the food vacuole takes up to 10 minutes, the entire phagocytosis around 30 minutes. When living conditions get bad, Actinospaerium can build a cyst to wait for better times.