HELIOZOA – SUN ANIMALCULES
Heliozoa (and one other actinopod)
The word heliozoa means ‘sun animals’. They are single celled protists, such that the name was adjusted to ‘animalcule’ to indicate a small guy. They have little to do with animals except in that they engulf food. And while we are being correct: one heliozoon, more than one heliozoa, and heliozoan is to be heliozoon-like.
There are a handful of different kinds of protists that have converged on a body with the shape of a sphere and with thin radiating arms extending from the body. They use the arms a bit like a spider uses its web, hoping that some other small organism will bumble into the arm. Then the heliozoon arms release materials that hold on to the food and or kill it.
The first kind are actinophryids. There are only a couple of genera. Upper left is Actinophrys. The arms have internal skeletons that extend through the cell and end on the surface of the large central nucleus. The second picture is of an Actinophrys eating a ciliate (Colpidium). Third is a cyst, in which the cell is enclosed by glassy plates. The upper right image is of Ciliophrys, a small flagellate from which the actinophryids appears to have evolved. It too has thin arms with which it captures bacterial prey, and a single flagellum. The two pictures with a black background in the second row of images are of a single and multiple examples of a large multinucleate actinophryid – Actinosphaerium.
To the right of each Actinosphaerium picture is a centrohelid heliozoon. Bottom left is a more detailed image of the body of a centrohelid heliozoon and shows how the axes that support the arms end on a tiny central organelle. Many centrohelids have siliceous spines and scales around the body. The black on white images are examples of the fancy and cute structures that they produce.
On the third row is a heliozoon that lives inside a stalked cage. It is Clathrulina, a kind of desmothoracid. As with the other heliozoa, it has stiff arms that project from holes in the wall of its cage.
Finally, and we cheat slightly here, is one more sun-like organism. It is Astrolithium, a member of the Acantharea. Just like heliozoa, it can produce stiff projecting arms to help it catch food. The strongly refractile structures are skeletal elements that are made of strontrium sulphate. The acantharea, heliozoa, and radiolaria used to be grouped as the ‘Actinopoda’ – a group which is no longer in use because the groups that were included are not closely related – rather they converged on the same kind of architecture.