The utility of geometric morphometrics to elucidate pathways of cichlid fish evolution

Abstract

Fishes of the family Cichlidae are famous for their spectacular species flocks and therefore constitute a model system for the study of the pathways of adaptive radiation. Their radiation is connected to trophic specialization, manifested in dentition, head morphology, and body shape. Geometric morphometric methods have been established as efficient tools to quantify such differences in overall body shape or in particular morphological structures and meanwhile found wide application in evolutionary biology. As a common feature, these approaches define and analyze coordinates of anatomical landmarks, rather than traditional counts or measurements. Geometric morphometric methods have several merits compared to traditional morphometrics, particularly for the distinction and analysis of closely related entities. Cichlid evolutionary research benefits from the efficiency of data acquisition, the manifold opportunities of analyses, and the potential to visualize shape changes of those landmark-based methods. This paper briefly introduces to the concepts and methods of geometric morphometrics and presents a selection of publications where those techniques have been successfully applied to various aspects of cichlid fish diversification.

Figure 1

Landmarks on (a) cichlid overall body (b) cichlid head [43] (c) a bony element (premaxilla) [44] and (d) the lower pharyngeal jaw [45].

Figure 2

Illustration of the Procrustes superimposition of the (a) original configuration (raw coordinates). (b) First, the centroid of each configuration is translated to the origin. (c) After that, configurations are scaled to a common unit size and (d) optimally rotated to minimize the squared differences between corresponding landmarks.

Figure 3

Deformation grid or Thin-plate spline showing differences (a) concerning whole body shape (19 landmarks, see Figure 1(a)) between a group of cichlid individuals living in the natural environment in Lake Tanganyika and another group of fish raised in ponds with a standardized rock architecture and artificial food. The grid accords to a canonical variate analysis carried out in an unpublished study on Lake Tanganyikan cichlids. (b) of the lower pharyngeal jaw throughout ontogeny between the smallest and the largest individual of Lepidiolamprologus elongatus (6 landmarks, see Figure 1(d)).