|Dileptus: Elephant of the Ciliate World|
Written and Photographed by Bruce J. Russell
A scum has formed on the surface of a jar of pond water that has been sitting on a shelf of several days. The stereomicroscope, at 40X magnification, shows the scum to be made up of little piles of micro-debris, and one clump appears to be animated. The impression is of a cluster of writhing snakes stirring the water. Under the microscope, the “snakes” turn out to be the whipping, trunk-like, front ends of the large ciliate, Dileptus.
I have seen this little beast many times, and have observed it capture other ciliates using the whipping trunk which is armed with toxicysts that both kill prey and hold it for swallowing. The mouth, or cytostome, is located at the base of the trunk and can be stretched around prey almost a large as Dileptus itself. But the scummy water where these ciliates are living has no large prey organisms. Instead, the scum is swarming with Chilomonas, tiny flagellated cells.
As I watched, the tip of the waving trunk made contact with a Chilomonas. Suddenly, something happened that I would never have expected. The tip of the trunk opened and engulfed the tiny cell. Although a little stunned, I continued watching.
The tip of the trunk is a long way from the mouth, and I wondered if Dileptus might transfer the morsel to its mouth the way an elephant might eat a peanut. Or, will the prey be transferred internally, down the trunk, to be digested in the main body of the cell?
But neither occurred. Instead the small cells seemed to be digesting right where they were engulfed, in the tip of the trunk. After 15 minutes they had not moved and were breaking down. Unfortunately, I lost sight of the Dileptus as it swam into the micro-bushes and so couldn’t see the final stages of this feeding process. Perhaps as the prey breaks down, smaller food vacuoles are transferred down the trunk, or perhaps digestion completes right there in the tip. More observation is needed to get the full story.
To see if this strain of Dileptus would feed using its mouth in what might be called the normal way, I found some Colpidium, a mid-sized ciliate, in another jar and introduced a few to the dish containing the Dileptus. A touch from the waving trunk caused Colpidium to literally explode and the organelles that spilled out were quickly engulfed by the predator’s extremely expandable mouth. Soon all of the Dileptus had made Colpidium kills and were bloated with food. A few hours later I observed the final stages of the feeding process, in which undigested material was shunted to the rear and eliminated, tearing away a significant amount of the tail in the process. The cell’s plasma membrane quickly sealed the wound and Dileptus went on its way.
Just when I thought I had the various Dileptus feeding strategies figured out, I observed another way they take in food. Stylonychia, is a spirotrich ciliate almost as large as Dileptus. As I watched through the microscope, a individual Stylonychia ran into the waving trunk of a Dileptus. A section of Stylonychia’s outer surface disintegrated, spilling a blob of cytoplasm into the water. The Dileptus wasted no time in sucking up the snack as the Stylonychia sped away to repair its damaged contents.