Dissecting a potential spandrel of adaptive radiation: Body depth and pectoral fin ecomorphology coevolve in Lake Malawi cichlid fishes

Abstract

The evolution of body shape reflects both the ecological factors structuring organismal diversity as well as an organism's underlying anatomy. For instance, body depth in fishes is thought to determine their susceptibility to predators, attractiveness to mates, as well as swimming performance. However, the internal anatomy influencing diversification of body depth has not been extensively examined, and changes in body depth could arise as a by‐product of functional changes in other anatomical structures. Using an improved phylogenetic hypothesis for a diverse set of Lake Malawi cichlid fishes, we tested the evolutionary association between body depth and the height of the pectoral girdle. To refine the functional importance of the observed substantial correlation, we also tested the coevolution of pectoral girdle height and pectoral fin area. The extensive coevolution of these traits suggests body depth in fishes like the Lake Malawi cichlids could diverge simply as a by‐product of being tightly linked to ecomorphological divergence in other functional morphological structures like the pectoral fins.

Pleiotropic jaw morphology links the evolution of mechanical modularity and functional feeding convergence in Lake Malawi cichlids

“…Our phylogenetic reconstructions highlighted that ecological convergence has occurred multiple times between the two major clades of Malawi cichlids (figure 2). As has been found previously [19,30,54,55], the primarily rock-dwelling mbuna formed a monophyletic clade. The species in the mbuna genera Genyochromis, Maylandia, Cyathochromis, Petrotilapia, Melanochromis, Tropheops and Labidochromis form a monophyletic clade with moderate support (85% bootstrap support) that is sister to the genus Labeotropheus.…”

“…We collected specimens for phylogenetic and morphological analyses from several locations in Lake Malawi. For phylogenetic analyses, we combined published ultra-conserved element (UCE) sequence data [19,29,54,55] with data produced for four additional species (GenBank BioProject: PRJNA344532). UCE sequence data were generated according to the protocols outlined in Hulsey et al ([40]; electronic supplementary material, Methods S1).…”

Weak genetic signal for phenotypic integration implicates developmental processes as major regulators of trait covariation

“…Malawi (Cooper et al, 2010;Hulsey et al, 2018). Morphological variation in the atrium and ventricle also spans a spectrum from LF-like morphologies to TRC-like morphologies, and change along these axes mimic broad differences observed in other lineages, such as cavefish (Tang et al, 2018).…”

“…The major axes of divergence among our structures reflect characteristic changes that occur across lake Malawi cichlids (Albertson & Kocher, 2006;Cooper et al, 2010;Hulsey et al, 2018). Differences in mandible shape reflect a spectrum from wide, short, and robust jaws to thin, long and slender jaws, features that are linked to differences in diet and biomechanics (Albertson et al, 2003;.…”

“…Thus, as a single bony complex, it participates in multiple functions, including lower jaw depression (via the urohyal), prey processing (via the pharyngeal jaw) and locomotion. Differences in pectoral girdle morphology among hybrids represent shape change from deep, long and thin to shallow, short and wide, shape differences that are characteristic of divergence in foraging and locomotory behaviour across cichlid species (Hulsey et al, 2018).…”