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Updated: Feb 11, 2026

Physiological Experimentation with the Crayfish Hindgut: A Student Laboratory Exercise
Published on: January 18, 2011
Alteration of size perception: serotonin has opposite effects on the aggressiveness of crayfish confronting either a
Julien Bacqué-Cazenave1, Daniel Cattaert1, Jean Paul Delbecque1
1Health and Life Science Department, Université de Bordeaux, CNRS, UMR5287, INCIA (Institut des Neurosciences Cognitives et Intégratives d'Aquitaine), 146 Rue Leo Saignat, 33076 Bordeaux, France.
Abstract:
We injected serotonin (5-HT) into adult male crayfish before pairing them with size-matched non-injected competitors, and observed dyadic agonistic interactions. Paradoxically, 5-HT elicited opposite behavioral responses if the injected animal was opposed by a smaller or larger rival: the level of aggressiveness of the injected crayfish was higher when facing a larger rival but lower when facing a smaller rival. Our results indicate that the effects of 5-HT on aggressiveness are dependent on the perception of the relative size difference of the opponent. In both cases, however, 5-HT significantly delayed the decision to retreat. We conclude that 5-HT does not primarily act on aggressiveness but rather on the brain centers that integrate risk assessment and/or decision making, which then modulate the aggressive response. Our findings support a reinterpretation of the role of 5-HT in crustacean agonistic behavior that may be of interest for studies of other animals.
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