Statistical characterization of social interactions and collective behavior in medicinal leeches

In the present study we analyzed the behavior and interactions among leeches in the same observation tank. Colored beads were glued onto their skin so that their behavior could be followed and quantified. When two or three leeches were present in the observation tank, they searched around for a maximum of 2 h and their motion and behavior were independent from those of their conspecifics. When the number of leeches in the tank was increased to 10, leeches were attracted to each other and exhibited episodes of highly correlated behavior. Solitary leeches injected with serotonin or dopamine increased the portion of time spent pseudoswimming and crawling, respectively. The behavior of three to five leeches injected with serotonin was not statistically independent, and leeches were attracted to their conspecifics and exhibited episodes of correlated behavior. Therefore, serotonin not only induces pseudoswimming in leeches but also promotes social interactions, characterized by a mutual attraction and by episodes of correlated/collective behavior.