Delineating species along shifting shorelines: Tropheus (Teleostei, Cichlidae) from the southern subbasin of Lake Tanganyika

Abstract

Background: Species delineation is particularly challenging in taxa with substantial intra-specific variation. In systematic studies of fishes, meristics and linear measurements that describe shape are often used to delineate species. Yet, little is known about the taxonomic value of these two types of morphological characteristics. Here, we used Tropheus (Teleostei, Cichlidae) from the southern subbasin of Lake Tanganyika to test which of these types of characters best matched genetic lineages that could represent species in this group of stenotypic rock-dwelling cichlids. We further investigated intra-population variation in morphology. By linking this to a proxy of a population's age, we could assess the evolutionary stability of different kinds of morphological markers.

Results: Morphological data was collected from 570 specimens originating from 86 localities. An AFLP approach revealed the presence of five lineages in the southern subbasin: T. moorii, T. brichardi, T. sp. 'maculatus', T. sp. 'Mpimbwe' and T. sp. 'red', which we consider to represent distinct species. Although both types of morphological data supported this classification, a comparison of P_ST-values that describe inter-population morphological differentiation, revealed a better correspondence between the taxon delineation based on AFLP data and the patterns revealed by an analysis of meristics than between the AFLP-based taxon delineation and the patterns revealed by an analysis of shape. However, classifying southern populations of Tropheus was inherently difficult as they contained a large amount of clinal variation, both in genetic and in morphological data, and both within and among species. A scenario is put forward to explain the current-day distribution of the species and colour varieties and the observed clinal variation across the subbasin's shoreline. Additionally, we observed that variation in shape was larger in populations from shallow shores whereas populations from steep shores were more variable in meristics. This difference is explained in terms of the different timescales at which small and large scale lake level fluctuations affected populations of littoral cichlids at steep and shallow shores.

Conclusions: Our results showed meristics to be more evolutionary stable, and of higher taxonomic value for species delimitation in Tropheus, than linear measurements that describe shape. These results should be taken into account when interpreting morphological differences between populations of highly stenotypic species, such as littoral cichlids from the Great East African Lakes.

Keywords: AFLP; Africa; Body shape; Clinal variation; Evolution; Meristics; Morphology; PST; Population differentiation; Species delimitation.

Fig. 1

Map of Lake Tanganyika. Indicating the 94 collection localities of the 570 specimens of Tropheus studied for morphology and 108 specimens studied genetically. The dark blue area denotes the zone deeper than 600 m below the current lake level, corresponding to the southern paleolake

Fig. 2

PC2 vs. PC1 of the PCA calculated on 108 AFLP fingerprints. The northernmost specimen of T. sp. ‘red’ (Kikoti, loc. 4) is visualised separately as it is not included in the convex hull with the other specimens of T. sp. ‘red’, PCA was calculated on the correlation matrix

Fig. 3

PC1 (a) and PC2 (b) of the PCA calculated on 108 AFLP fingerprints vs. geographical distance. Distance was taken along the shoreline (in km) starting from the northwesternmost locality, LOESS curves were calculated with a fixed smoothening parameter of 0.5, PCA was calculated on the correlation matrix

Fig. 4

Heat map of genetic similarity and AFLP-based NJ tree of 108 southern specimens of Tropheus. Genetic similarity was calculated using Mahalanobis distances between AFLP fingerprints, and visualised on the heat map via the Pearson correlation matrix, using the red-blue colour scale. Specimens are grouped according to their origin along the lakeshore, with specimens from the northwesternmost locality bottom-left. The NJ tree was constructed using three specimens of T. duboisi as the outgroup. Colours next to the heat map and around the tree denote group membership. The T. sp. ‘red’ specimen that is visualised separately in the NJ tree and in the heatmap is the sole specimen from Kikoti (loc. 4). A more detailed version of the NJ tree is presented in Additional file 3

Fig. 5

Morphological variation vs. geographical distance along the shoreline. Variation in meristics was descibed by SR-PC1 (a), SR-PC2 (b), SR-PC3 (c) calculated on 15 meristics, variation in shape was described by PC2 (d), PC3 (e) and PC4 (f) calculated on 23 log-transformed measurements for 570 specimens from 86 locations. The distance was taken along the shoreline (in km) starting from the northwesternmost locality. LOESS curves were calculated with a fixed smoothening parameter of 0.5

Fig. 6

Comparison of intra- and inter-specific morphological differentiation between populations, described by P_ST values. Values were calculated on SR-PC1 (a), SR-PC2 (b), SR-PC3 (c) of the 15 meristics and on PC2 (d), PC3 (e) and PC4 (f) of the 23 measurements taken on 549 Tropheus specimens from 67 locations. With inter: inter-specific, d intra: distant intra-specific and t intra: true intra-specific comparisons (see text)