Colouration is usually used to distinguish between related species of Lake Malawi mbuna, such as Petrotilapia, but body shape also plays a part in sexual differentiation.
A new study by Kassam, Mizoiri and Yamaoka of Kochi University, Japan, studied three sympatric species of Petrotilapia - P. nigra, P. genalutea and an undescribed fish to further investigate sexual dimorphism in the cichlids using landmark geometric methods.
By plotting the positions of the features of the fish and running a statistical analysis upon them, they came to the conclusion that you can't use landmark geometric methods to tell the difference between sexes, but you can differentiate between different species of Petrotilapia.
However, when comparing sexes by their centroid size using the statistical technique ANOVA (Analysis of Variance) the team showed that males were larger than females.
For more details see: Kassam, D., Mizoiri, S. and K.Yamaoka (2004) - Interspecific variation of body shape and sexual dimorphism in three coexisting species of the genus Petrotilapia (Teleostei: Cichlidae) from Lake Malawi. Ichthyological Research, Vol. 51, No. 3, pp 195 - 201.