I know I promised my flatworm stories a few weeks ago but better late than never.
When I first arrived to start my post-doc at the University of Windsor I thought it would be interesting to start a small project to get my feet wet, so to speak. The Hebert lab had study sites all across southern Ontario and I often went on collecting trips for Daphnia to be used in population genetics studies being conducted. I like that stuff but I was much more fascinated by the diversity of animals that came in with each haul of the plankton net. The diversity was striking to me because the sites I had used in Nebraska for my PhD were boring by comparison. Collections in any pond in Ontario were far richer in species diversity than those near Lincoln.
I was intrigued by the many animals on the bottom of our sorting trays (9” square, white, opaque plastic trays; I highly recommend them). I recognized planarians but the other flatworms were unknown to me. One of the most interesting animals I came across was a nearly transparent flatworm, 10 mm or longer, that I identified as Mesostoma ehrenbergii (these drawings doesn’t do it justice but the image of it cradling a Daphnia is fairly accurate; this photograph is better). They are about the same size and shape as a leaf of Lemna trisulca (star duckweed) and the color of a decaying leaf matches the flatworm closely. The name of the genus is descriptive of the location of the mouth being somewhere near the middle of the animal. American and European members of the species are apparently distinguished by the number of chromosomes, which speaks to the size and ease of extraction of chromosomes as this was demonstrated more than 50 years ago.
I mention all this detail because of what I noticed about the prey capturing behavior of the flatworm. One method they use is by producing sticky mucus that ensnares the prey. Even a large Daphnia (2.5 mm; remember “large” is relative) would be trapped. Their hunting method was to make their way to the surface of the tray climbing along the side, glide along the surface (perhaps using the surface tension of the water?) and then release them selves to float, and hunt on their way to the bottom. When they contacted a Daphnia they released nematocysts and their prey was instantly paralyzed. One interesting myth was that the flatworm ate Hydra and was able to store their nematocysts and employ them in capturing their own prey. I raised many, many flatworms that never saw a Hydra and all were perfectly capable of killing their prey so I have to at least partially discount that story.
Once a daphniid was captured the behavior of the flatworm was always the same. It would crawl over the animal searching for the suture between the head and body. Once found, the flatworm would extend its proboscis and puncture the suture and proceed to pump the inisdes of the Daphnia into itself. The operation took a few minutes and it was possible to see Daphnia innards inside the gut of the transparent predator. Often the Daphnia eye would remain intact and you could see it through the predator staring back at you.
I’ll explain the experiment and the results of the experiments tomorrow.
