Differences in color patterns have been the most used feature in describing cichlid
species belonging to genus Petrotilapia from Lake Malawi. In this study, we
quantified morphological variation in body shape within and among three coexisting
Petrotilapia species using landmark-based geometric morphometric methods.
Statistic analyses revealed significant body shape differences among species but not
between sexes. Post hoc multiple comparisons based on Mahalanobis distances revealed
that P. nigra was significantly different from P. genalutea and
Petrotilapia sp., whereas the latter two were not significantly different.
The splines generated showed that the most pronounced variation was in the head
region, in which P. nigra had a relatively longer and deeper head than the
other two. The most clear-cut distinction was in gape length; P. genalutea
had the longest gape, followed by Petrotilapia sp., whereas P. nigra
had the shortest gape. Body depth was shallower in P. nigra than the others.
When comparing sexes by their centroid size, ANOVA revealed that males were bigger
than females. Therefore, we conclude that color is not the only feature that can
distinguish these congeners. We discuss the observed sexual dimorphism in terms of
sexual selection and relate morphological variation among species to feeding behavior,
which may help explain their coexistence in nature.