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Aquatic flatworms use chemical cues from injured conspecifics to assess predation risk and to associate risk with novel cues

A growing number of aquatic organisms have been shown to display antipredator behaviour in response to injury-released chemical cues from conspecifics. Here, we demonstrate a clear antipredator response in the form of avoidance behaviour by a free-living flatworm Dugesia dorotocephala to chemical cues from injured conspecifics. This is the first demonstration of a chemical alarm cue in a platyhelminth. In a second experiment, we exposed planaria to combined cues of sunfish odour and planaria alarm cue, or sunfish odour alone. Planaria avoided the sunfish+alarm cue but did not avoid the sunfish odour, indicating no prior aversion to sunfish odour. When these same planaria were subsequently retested 2 days later with sunfish odour only, planaria that had previously received sunfish odour+alarm cue avoided the cue but planaria that had previously received sunfish odour alone did not. These data indicate that planaria learned to recognize sunfish odour as an indicator of danger based on a single simultaneous exposure to conspecific alarm cue and the novel cue. This is the first demonstration of this phenomenon in a platyhelminth and the simplest nervous system known to be capable of learned risk association.