Small‐scale morphological differentiation in a cichlid may provide clues about rapidly diversifying systems

The diversication of Lake Malawˆ i cichlid shes has beencharacterized as the fastest speciation event within a singlevertebrate family. In an effort to understand the populationisolation mechanisms that contribute to the diversicationof these cichlids, many researchers are studying populationvariation (Markert, Danley & Arnegard, 2001) along hor-izontal (i.e. constant depth) gradients which often includedisrupted habitats. Little work, however, has been devotedto investigating population isolation along vertical scales.The vertical cline, in opposition to the horizontal, nece-ssarily introduces gradients in both water pressure and thespectral quality of light (Loew & Mcfarland, 1990). Thesecichlids, which are able to travel across depths (Hill &Ribbink, 1978), rely heavily on vision for feeding and mateselection (e.g. Jordan et al., 2003). Changes in light mayaffect behavioural and physiological changes in both feed-ing and mate selection. Given the predominance of bimodalpopulation distributions found along depth gradients inLake Malawˆ i (Ribbink et al., 1983), investigation intopopulation variation along a vertical axis is warranted.In this paper, we discuss results of an analysis of mor-phological variation in Metriaclima melabranchion, a philo-patric cichlid found along the rocky shore of Lake Malawˆ i.Tsano Rock, in southern Lake Malawˆ i (14 S and 35 E),houses a bimodal distribution of M. melabranchion (Ribbinket al., 1983) with peaks in population size separated by20 m in depth along an estimated 40-m continuousstretch of rocky habitat. Males of the species are light bluewith black barring. We quantied the light environment atthese locations and tested the null hypothesis that twopopulation subsets, one taken from 3 and one from 23 m,are morphologically similar.