“Heliozoans” are sunburst-shaped protists with long arms called axopods emerging from a spherical cell body. Despite the morphological similarity, they are not close relatives and appear to have developed this structural form independently of each other. Most freshwater heliozoans depicted here appear to be centrohelids.
The beaded appearance of some axopods is due to organelles called extrusomes, which are involved in feeding. Covering the cell body (spherical part) are spicules or scales whose full structural detail is only appreciable under electron microscopy. Here the spicules show up as a fine hairy fuzz projecting from the cell body, but shorter than the long axopod arms. These spicules may take different forms: miniature trumpets, scales, beads, etc.
Filose amoebae may sometimes be confused for heliozoans. They can be distinguished by closely examining the axopods/filopods for movement and presence of extrusomes.
The axopod arms with extrusomes are only faintly visible in this picture, however it is clear that the cell body is covered with flat (vs. spiny or trumpet-shaped) scales.
In these larger Raphidophrys (above) note the flagellates (Ochromonas) trapped on the arms, and the presence of brown-pigmented food particles, including a pennate diatom, in the heliozoan’s cytoplasm.
See a video of the heliozoan feeding on Ochromonas on the Interactions page.
This genus is distinctive in having trumpet-shaped scales (faintly visible as spines with Y-shaped forks at the end, interspersed with the axopods). Note the bead-like extrusomes on the axopods, and the small flagellates trapped in the arms.
The feeding behavior of heliozoans can be fairly quick, as the following video shows.[vimeo http://vimeo.com/28102789]
Some of these heliozoans are green, possibly retaining chloroplasts from their algal food.