As a further confirmation of the occurrence of the serotonin-cHH-glycaemia physiological axis, immunoreactive and ultrastructural studies have demonstrated serotonergic synaptic structures on the axonal ramification of the cHH-producing cells of the X-organ of crayfish, P. clarkii included. The involvement of the serotonin-cHH-glycemia physiological axis could explain both the mechanisms through which cHH controls agonism and the expression and timing of dominant behaviours triggered by either cHH or serotonin injections. The availability of an adequate amount of cHH by synthesising it with the correct post-translational modifications conferring a full biological activity will allow further validation or rejection of this hypothesis. Consistent with the study on the serotonin effects on P. clarkii, also the cHH did not lead to a permanent inversion of the dominance hierarchy. Cheating seems not to be sufficient to maintain the role of dominant in prolonged fights against stronger opponents. Intrinsic properties of crayfish other than body size, weight, chelar dimensions or circulating neuropeptides may likely determine the structure of dominance hierarchies in decapods. For instance, in the American lobster, H. americanus, the outcome of contests between size-matched individuals was predicted from hidden cues such as plasma protein level and exoskeleton calcium concentration. These variables are not clearly visible to the rivals, but fighting lobsters may indirectly assess them by claw contraction forces, the resistance of the exoskeleton to pressure, and general fighting vigour. Notwithstanding the neuropeptides injected, betas have neither the physical characteristics nor the experience of a dominant, and prolonged fights could result in both losing time/energy and increasing the risks of injury that eventually may lead to their death. The original rank is thus quickly re-established since it allows betas to minimize the costs and risks of fighting with a superior individual. As a consequence, the relevance of both intrinsic physical characteristics and experience cannot be excluded in the dynamics of dominance hierarchies. Undoubtedly, behavioural physiology opens new avenues for our understanding of the functioning of cHH and is expected to unravel its role in modulating invertebrate agonistic Perifosine inhibitor behaviour. Future researches are obviously needed to answer the exciting questions of how physiology and environment interact in regulating the neural systems underlying the formation and maintenance of social hierarchies across species. Homooligomeric proteins have large interface areas between the subunits resulting in stable complexes. Because the molecular functions of homooligomers often require their complete oligomeric forms, the overall structure of a homooligomer may help understand its molecular function. It is known that the complex structure of a homooligomer often assumes a symmetric structure, with the subunits arranged in either a ‘close-packed’ form or a ‘ring’ form. The close-packed form has n/2-fold rotational symmetry around one rotational axis and 2-fold rotational symmetry around the other rotational axes perpendicular to the first rotational axis. Oligomers with this form contain an even number of subunits. In a statistical analysis of the Protein Data Bank , we found that homooligomers composed of even numbers of subunits are dominant because of the abundance of the closepacked oligomers. In the close-packed form, the subunit interfaces are arranged in a face-to-face manner, and every structural feature or interaction is repeated twice. It was pointed out by Monod et al. that the effect of a single mutation in complexes with the close-packed form may be much greater than in complexes without dihedral symmetry.