Flatworm predator (Mesostoma cf. lingua) releases a toxin to catch planktonic prey (Daphnia magna)1

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Abstract
In aquaria, Daphnia magna decreased in numbers as the population density of the substrate‐bound predaceous flatworm Mesostoma cf. lingua increased. Above a threshold worm density, Daphnia disappeared completely. It was shown experimentally that this was due to the presence in the environment of a neurotoxin diffusing from webs of mucus produced by the worm. The toxin decomposed rapidly, and its inactivation rate was strongly temperature‐dependent. It is concluded that Mesostoma is an ambush predator with a strategy that comes closest to that of a web‐building spider, but with the complication that both the worm and the web are poisonous.