Abstract
Phenotype-environment correlations and the evolution of trait interactions in adaptive radiations have been widely studied to gain insight into the dynamics underpinning rapid species diversification. In this study we explore the phenotype-environment correlation and evolution of operculum shape in cichlid fishes using an outline-based geometric morphometric approach combined with stable isotope indicators of macrohabitat and trophic niche. We then apply our method to a sample of extinct saurichthyid fishes, a highly diverse and near globally distributed group of actinopterygians occurring throughout the Triassic, to assess the utility of extant data to inform our understanding of ecomorphological evolution in extinct species flocks. A series of comparative methods were used to analyze shape data for 54 extant species of cichlids (N = 416), and 6 extinct species of saurichthyids (N = 44). Results provide evidence for a relationship between operculum shape and feeding ecology, a concentration in shape evolution towards present along with evidence for convergence in form, and significant correlation between the major axes of shape change and measures of gut length and body elongation. The operculum is one of few features that can be compared in extant and extinct groups, enabling reconstruction of phenotype-environment interactions and modes of evolutionary diversification in deep time.
Highlights
Understanding how organismal diversity is generated and maintained, why some groups diversify when others remain relatively unchanged over geological time, and how organisms adapt to and interact with the environment are key challenges in evolutionary biology
Among the morphological traits examined in Lake Tanganyika (LT) cichlids so far, most comprise combinations of linear measurement or scored character data and, with the notable exception of body shape and size measures, few traits are directly amenable to comparison with other species flocks, such as sticklebacks and Antarctic notothenioids, which represent radiations of different geological age that have occurred in different environmental settings
Negative scores along PC2 (20.4%) reflected a widening of the operculum along the anterior-posterior axis and a shortening along the dorsal-ventral axis, whereas positive scores reflected the reverse. Some separation along this axis is evident between Lamprologini and Tropheini, overlapping with Ectodini, as for example the second most extreme positive value is represented by a member of the latter tribe (O. ventralis: ophven)
Summary
Understanding how organismal diversity is generated and maintained, why some groups diversify when others remain relatively unchanged over geological time, and how organisms adapt to and interact with the environment are key challenges in evolutionary biology. LT cichlids are the most morphologically, ecologically and behaviorally diverse of the three lake flocks, and a number of studies have explored evolutionary patterns in the group. The considerable diversity in operculum shape and size among osteichthyans has been attributed to the important role of this bone in respiration and the jaw opening mechanism of some fishes through its functional connectivity to the lower jaw[33] Owing to these properties, and further supported by insight from studies of operculum morphogenesis in zebrafish that have illuminated genetic pathways influencing its shape and size (e.g.34,35), the operculum has been the subject of several investigations, in the threespine sticklebacks[33,36,37]. The operculum is one of only few morphological features that can be studied in fossil groups because it is commonly well preserved
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