Note book entry, October 2003

24 hours ago I began experimenting with a pure culture of Paramecium which looked like this, --virtually nothing but the torpedo-shaped paramecia.

Now, after only one day, it looks like this -- virtually all the organisms to be found are a more spherical ciliate with two bands of cilia.

The organism that has wiped out all the paramecia is Didinium, a predator that feeds on only one kind of prey -- species of Paramecium. Now there are a lot of Didinium -- and they are starting to get hungry. This presents an excellent opportunity to study and record their feeding behavior.

THE CAPTURE: I add some paramecia back to the culture dish. Didinium speeds around in a random way, nose first. When it hits a Paramecium, poison darts and attachment lines fire automatically. Didinium then begins opening its nose, which becomes its mouth, and it begins to swallow the much larger cell.

THE PIG OUT: By repeating the feeding trials using a vital stain I was able to observe an interesting aspect of Didinium’s swallowing technique. With part of the Paramecium engulfed, Didinium expands, thereby sucking cytoplasm from the part still extending from its mouth (the squeeze bulb technique). With the volume in the outside portion reduced, Didinium can complete its engulfment of the prey. If the prey was hit in the middle, Didinium expands its mouth and folds the paramecium in order to cram it all in. In these feeding trials where both prey and predator are concentrated, two or three didinia may attack the same Paramecium and then compete for shares.

Engulfment begins.

Engulfment by folding prey.

Two didinia attempt to share a meal.

But not all attacks are successful. At the instant of contact with Didinium’s trichites, Paramecium’s counter measures automatically fire -- a volley trichocysts. A Didinium rebuffed in this way can not immediately spear another Paramecium.

THE "TIME OUT": Following engulfment, a stuffed Didinium swims around until it runs into something (usually the slide or coverglass) and sticks to it by the nose, spinning like a top. Didinium cells in this quiescent state show no interest in paramecia, although often rammed by them. My notion is that during this "time out" period, the Didinium is not only digesting its dinner, but that it is regenerating a new set of capturing tools. From these observations, it appears that to make a successful kill, Didinium must have a well stocked arsenal of trichites. When trichites are lost in a failed strike, new ones must be regenerated before prey can be captured. When a Didinium releases itself from its "time out" attachment, usually after 20-30 minutes, it goes off hunting for another meal, a hunt that is often successful.

THE LONG NAP: I’m looking into the culture dish in which the didinia had exhausted their food supply three days ago. Today, no swarms of dancing white specks are visible to the naked eye. The stereo microscope shows that all of the didinia have encysted. The process begins with a Didinium producing a cocoon made of clear material that hardens into a thick cyst wall. The bottom of the culture dish was strewn with Didinium cysts, so I decided to see if I could get them to excyst by simply adding paramecia, something that must occur in nature as a new prey population builds up. Four hours after flooding the dish with paramecia, a few free-swimming didinia could be seen. Scanning the cysts showed some empty cyst walls where didinia had recently excysted. Then I saw one in the act. The cell was energetically spinning and darting about, ramming the membrane enclosure with its nose cone. After about a minute the Didinium broke out and swam away at full speed.Two hours later a number of well fed didinia could be seen in various stages of division. 24 hours later there were thousands of didinia racing around and just a few paramecia hiding in the bushes. One of the questions posed by this observation is -- what exactly triggers an encysted Didinium to excyst? Was it some chemical signal given off by the paramecia, or something associated with the bacteria on which they feed? Perhaps you can devise an experiment that will reveal the excystment signal.

SEX:

A day after the didinia had finished off their second helping of paramecia, I noticed a lot of what appeared to be dividing individuals. Upon closer examination the pairs were joined nose to nose -- that’s not the way ciliates divide. These didinia were conjugating -- a sexual process in which micronuclei are exchanged. Soon about half of the thousand or so cells in the dish were also engaged in conjugation. This process is one of the ways new gene combinations are passes around the population. Virtually all ciliates exhibit this sexual process. Conjugating individuals of Didinium and Paramecium can be ordered from biological supply companies, or you can discover Didinium by accident as I did when they wiped out one of my laboratory populations of Paramecium. I have a pretty good idea about how Didinium got in there. Several days before, I had added some unfiltered lake water to the culture, and with it, maybe one little Didinium cyst.