My experiments with flatworms were pretty simple. I fed the flatworms until they were all satiated (none were hungry) then I starved them for 24 hours so they were all equally hungry. Then, I put them into cups each with a different food. The results were clear. They preferred small Daphnia (1.2 mm) but not the much smaller Ceriodaphnia (0.65 mm). Larger prey, like adult Daphnia and Simocephalus (each around 2.5 mm), were also not consumed as much as the mid-sized animals. I didn’t try feeding them copepods because they were just too fast. A flatworm could consume daphniids a day with ease, so I concluded that Mesostoma could be substantial predators in ponds and was a selective feeder.
My prediction proved prophetic with a few months. One of the main interests of the lab were the ponds around Churchill, Manitoba. These ponds are ephemeral but really aren’t vernal because the ice doesn’t melt until well into the summer. There are two general types of ponds in this region, those in the granite rocks surrounding Hudson Bay and those inland in the tundra. They have completely different zooplankton even though they are in some places a few meters apart.
One of the grad students was doing an experiment where he took a large tube made of plankton netting, a few feet across and a few feet long and suspended it in a pond. To the best of my recollection (and I could be wrong), the idea was to suspend the tube in one pond type and place Daphnia from the other in the tube. It would then be possible to determine if the two could co-exist and the separation was due to factors other than chemistry. I was in Windsor when the call came from the student that the experiment had started successfully but within a few days every animal in the tube was gone. The mesh was too small for them to escape and preliminary experiments had shown that they could survive in the water. The student was perplexed and worried because much of his thesis depended on these experiments.
I had been reading about flatworms in the subarctic and had come across a report of Mesostoma lingua (I couldn’t find any pictures on the web; it looks like a tiny, black tube tapered on both ends) in these ponds. I suggested that he have a look inside the tube to see if perhaps there were any of these in the tube. I wouldn’t be telling the story if I was wrong, so when the student looked in the tubes they were filled with hundreds of worms. They were small enough to crawl through the mesh and then had a trapped meal. Every time a daphniid swam against the tube it was a meal for a flatworm.
While the student was distraught over these results I was excited. I was in my predator-prey mode and was anxious to travel to Churchill and do some experiments on these flatworms. Later that summer I did just that. And, as it turned out, one of the determining factors for distribution of Daphnia species in Churchill was an invertebrate predator, but not a flatworm.
More soon.
