Agonistic behaviour and biogenic amines in shore crabs Carcinus maenas

To investigate the role of certain neurohormones in agonistic behaviour, fi ghts were staged between pairs of size-matched male shore crabs Carcinus ma enas, and blood samples were taken immediately after the contests had been resolved. Samples were also taken from these crabs at rest (before and afte r fighting) and after walking on a treadmill. A control group of crabs also had samples taken on each experimental day. Concentrations of tyramine, do pamine, octopamine, serotonin (5-HT) and norepinephrine were determined in each blood sample using a gas chromatography/mass spectrometry (GC-MS) syst em. Norepinephrine was not detectable in any of the samples, but the standa rds were recovered. Tyramine values were not significantly different betwee n the control group and the fought group, so tyramine does not appear to be important in agonistic behaviour. A comparison between the control and fou ght groups shows that fighting had an effect on the concentrations of octop amine, dopamine and 5-HT, but exercise only had an effect on octopamine lev els, which showed a reduction from resting values in both winners and loser s. Resting and post-fight concentrations of octopamine, dopamine and 5-HT w ere higher in winners than in losers, 5-HT concentration increased in the b lood of fought crabs from resting values, whereas dopamine concentration de creased. In winners, octopamine concentrations decreased from resting value s, but in losers octopamine levels increased from resting concentrations. T he escalatory behaviour or intensity of fighting performed by winners and l osers was related to dopamine levels but not to those of octopamine or 5-HT , Therefore, there appears to be a link between relative concentrations of these three amines (dopamine, octopamine and 5-HT) and fighting ability; th e effects are not simply a result of activity. The better competitors have higher concentrations of these three amines at rest and after fighting.