Heliozoans eat by catching prey on their long, needle-like projections called axopodia, then pulling the food inward and digesting it inside a specialized pocket within the cell. Often called “sun animalcules” because of their radial, sun-like appearance, these single-celled predators are surprisingly effective hunters that can capture organisms even larger than themselves.
What Heliozoans Actually Eat
Heliozoans are active predators that feed on a wide variety of microscopic life: other protists, flagellates, ciliates, and even small multicellular animals like rotifers. In acidic lake ecosystems, the heliozoan Actinophrys sol functions as a top predator in the planktonic food web, exerting clear predation pressure on every prey species tested in laboratory studies.
What makes heliozoan feeding remarkable is the size range of their prey. Ciliates they consume are roughly 10% of their own cell volume, which is a manageable meal. But rotifers, another common food source, can be two to three times the heliozoan’s volume, ranging from 50,000 to 100,000 cubic micrometers compared to the heliozoan’s average of about 34,000. Catching and engulfing something bigger than yourself is no small feat for a single cell. The heliozoan’s body size itself fluctuates significantly depending on how recently and how well it has eaten.
The Axopodia: Sticky, Stiff Fishing Lines
The key to heliozoan feeding is the axopodia, dozens of thin, rigid arms that radiate outward from the cell body in every direction. Each axopodium is stiffened internally by a bundle of microtubules, protein filaments that act like a scaffolding rod. This rigidity is critical because it lets the arms extend far into the surrounding water, dramatically increasing the area where the heliozoan can intercept passing organisms.
The surface of each axopodium is lined with tiny membrane-bound packages called extrusomes, positioned just beneath the outer membrane. When a prey organism brushes against an axopodium, these extrusomes burst open and release a sticky, filamentous substance that anchors the prey to the arm’s surface. The adhesive is remarkably tough. A 40 kDa protein isolated from these extrusomes plays a central role in the gluing process, and the adhesive resists breakdown even when treated with protein-digesting enzymes like trypsin. Experiments have shown that even dead heliozoan cells, killed by freezing or extreme heat, retain enough of this adhesive to cause prey flagellates to clump together in bouquet-like clusters, their flagella and cell bodies stuck fast.
Two Ways to Reel In a Meal
Once prey is stuck, the heliozoan has two distinct methods for bringing it to the cell body.
The first is rapid axopodial contraction. The entire arm collapses inward, yanking the prey directly toward the cell surface. This happens fast, at roughly 1 millimeter per second, which is explosive speed at the microscopic scale. The contraction works because the microtubule core inside the axopodium rapidly breaks down, causing the stiff arm to buckle and shorten. After contraction, the original microtubules are replaced by C-shaped microtubules, a structural change that reflects how dramatically the arm has been remodeled during the strike.
The second method is axopodial flow. Instead of the arm collapsing, the prey is transported along the surface of the axopodium toward the cell body, carried by a current of material flowing along the outside of the arm. The axopodium itself stays intact during this process, with no breakdown of its internal microtubule structure. This slower, gentler method works well for smaller or less mobile prey that don’t require the violent snap of full contraction.
Engulfing the Prey
When the prey reaches the cell body, the heliozoan wraps its outer membrane around the food organism to form a food vacuole, a sealed internal compartment. The membrane for this pocket comes partly from the extrusomes themselves, which expand and fuse with each other and with the cell’s outer membrane during engulfment. As this pocket forms, a chemical substance is already being secreted into it, beginning the breakdown process before the vacuole is even fully sealed.
Digestion Inside the Food Vacuole
Once the food vacuole closes, digestion proceeds through a series of well-defined stages. First, a second wave of internal vesicles fuses with the vacuole, delivering additional chemicals that lyse the prey, essentially dissolving its cellular structure. In the case of captured ciliates, the prey cell bursts open and its contents coagulate within the vacuole.
Next, the cell removes excess fluid from the vacuole, shrinking it down around the coagulated food. This fluid removal is accompanied by intense activity at the vacuole’s outer surface, with many small vesicles budding on and off the membrane. Then the cell’s internal packaging system kicks in. Structures near the nucleus produce lysosomes, compartments loaded with digestive enzymes, which fuse with the food vacuole and break down the remaining material. Within about four hours of feeding, the food vacuole has condensed tightly around its contents.
As digestion continues, small vesicles filled with absorbed nutrients pinch off from the shrinking food vacuole and distribute throughout the cell. Meanwhile, the leftover mass inside the vacuole grows increasingly concentrated as usable material is extracted. Throughout this process, clear vacuoles in the surrounding cytoplasm gradually fill with a filamentous material and migrate toward the cell center, a sign that nutrients are being processed and stored. Once digestion is complete, these storage vacuoles shrink and empty.
Getting Rid of Waste
What remains after digestion is a small, dense clump of indigestible material still sitting inside the remnant of the food vacuole. The heliozoan expels this waste by moving the vacuole to the cell surface and releasing its contents into the surrounding water, a process called egestion. The whole cycle, from capture to waste expulsion, transforms a living prey organism into absorbed nutrients and a tiny packet of refuse, all managed by a single cell with no mouth, stomach, or intestine.