Heliozoans are amoeboid protozoans. That is, they are single-celled organisms that move by the flow of their cytoplasm rather than by using oar-like cilia or whip-like flagellae. Heliozoa means sun-animals and they do resemble tiny suns for they are round with rays coming out in radially symmetrical patterns. Their closest relatives are the marine radiolaria, which also have star-like shapes. Heliozoans are mostly found in fresh water environments but there are also some marine species.
The class Heliozoa is probably not a natural group as the organisms were put together on the basis of appearance rather than actual relationships. Modern taxonomists have now separated them into three classes: the actinophyrida, the centrohelida and the desmothoracida.
Actinophryds are the most common, found in fresh water, marine and even moist soil environments. They look like spiky balls. They use vacuoles to alter their bouyancy, filling them with gases when they want to rise and emptying them in order to sink. They range from 10 to 200 millimicrons in diameter and may have one or several nuclei. Centrohelida are similar to the actinophyrda but less common. They are 30-80 millimicrons in diameter and round with spiky axopods. The desmothoracids are the smallest of the three groups and may be the most primitive as they show no geometric pattern in the arrangement of their axopods and there is no distinction between the inner and the outer cytoplasm.
The rays of the heliozoan body are called axopods and are stiffened by structures called microtubules. Heliozoans are miniature predators that capture bacteria and algal cells. Cytoplasm moving through the axopods carry the prey back to the centre of the cell where it is digested. The axopods have other uses as well. They are the sensory organelles of the organism and can be used both to sense prey items and to feel the approach of predators, probably by sensing currents.
Heliozoans can be quite common in freshwater ponds. They are usually found amongst decaying vegetation floating on the surface. This is an ideal habitat for them as it provides them with many bacterial prey while also giving them shelter from larger predators. They can attach themselves to the plant matter with their axopods and thus keep from drifting away from their hunting grounds.
Reproduction is usually by fission or cell division. Some forms of actinophrids can encyst and in this state may go through a form of sexual reproduction. Desmothoracids can reproduce by budding as well.
Like many protozoans and most amoeboids, little is known about the daily lives or the ecology of heliozoans. This is still a fertile field of study for the budding zoologist. All that is needed is a microscope and a lot of patience. Spend time looking at the life in pond water and a whole new world will open up, filled with predators and prey and tiny lives led beyond the senses of most of the world’s creatures.
for more information: http://www.micrographia.com/specbiol/protis/helioz/heli0100.htm
http://en.wikipedia.org/wiki/Heliozoa
