Timing of breeding to an optimal season is a requirement for a successful reproductive outcome in seasonally
breeding species. Photoperiodic signals are capable of modifying the reproductive behaviour and
reproductive systems in several vertebrate species. The cichlid fish Cichlasoma dimerus shows highly
organized breeding activities and different social status. The aim of this study is to test whether C. dimerus
reproductive behaviour (male aggressive behaviour and female choice) and reproductive physiology (GnRH3
morphometric parameters, pituitary hormones content and organ-somatic indexes) are modulated by
photoperiod. Before spawning, dominant pairs were isolated and kept in opposite tanks of 20 l for one week,
so they could see each other but not physically interact. Afterwards, a group was exposed for four weeks to a
short photoperiod (8 h light:16 h dark) (short photoperiod exposed animals: SP) while another group was
exposed to a long photoperiod (14 h light:10 h dark) (long photoperiod exposed animals: LP). Temperature
was maintained constant. Behavioural experiments showed that male aggression related to territory
selection and its defence is reduced in SP males. Further, SP females were never chosen. At the brain level we
demonstrated that GnRH3 neuronal optical density of staining was reduced. Finally, at the pituitary level we
showed that SP males showed low levels of β-LH, PRL and GH in the pituitary, and that SP females showed no
significant differences in the pituitary content of any hormone. Taken all together these results suggest that
in C. dimerus the photoperiod is a relevant environmental cue related to reproductive behaviour and
physiology