Comparative analysis of nest and bower sizes and the influence of substrate-treated water on reproductive behaviours of redbreast tilapia (Boulenger, 1897) and shire tilapia (Boulenger, 1897)

Extended phenotypes, such as nests and bowers, are central to tilapiine sexual selection, yet their structural variations and responsiveness to environmental chemical cues remain poorly understood. This study evaluated architectural differences and behavioural sensitivities to soil-conditioned water in two cichlids with contrasting breeding strategies: redbreast tilapia (Coptodon rendalli) and shire tilapia (Oreochromis shiranus). In Experiment I, nest and bower sizes were compared in 1000-L tanks with sandy substrate. Redbreast tilapia constructed significantly larger and deeper nests (diameter: 23.6 ± 0.81 cm; depth: 15.5 ± 0.83 cm) than shire tilapia bowers (diameter: 19.2 ± 3.59 cm; depth: 13.2 ± 1.44 cm). In Experiment II, the influence of soil-conditioned water was tested using four dilution treatments (control, low, medium, high). Chemical cues and fish species exhibited a significant interactive effect on structure diameters (P = 0.001). Both species produced their largest structures under the medium treatment (420 mL), averaging 28.52 ± 1.16 cm for redbreast tilapia and 26.17 ± 1.16 cm for shire tilapia. Ultimately, redbreast tilapia constructs larger breeding structures than shire tilapia, and moderate organic enrichment acts as an environmental catalyst optimizing nesting behaviour. These findings offer practical applications for enhancing hatchery breeding success.