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Why does a freshwater snail jerk its shell when touched by a leech?
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Avoidance behaviour
Cat fleeing by upward climbing.
A freshwater snail, such as those in the genus Physa, jerks its shell when touched by a leech as a specific avoidance behaviour in response to a major predator.1
This shell-jerking action is an innate reflex triggered by the physical contact of the leech, which serves as a defensive mechanism to deter the predator. By rapidly moving its shell, the snail attempts to disrupt the leech's ability to attach or feed, thereby increasing its chances of survival. This response is a clear example of how invertebrates have evolved specialized, stimulus-specific reflexes to protect themselves from threats in their environment.1