How to tuna fish: constraint, convergence and integration in the neurocranium of pelagiarian fishes

Abstract

Morphological evolution of the vertebrate skull has been explored across a wide range of tetrapod clades, but teleost fishes, accounting for roughly half of all vertebrate species, have largely been overlooked. Here we present the results of a study investigating three-dimensional morphological evolution across 114 species of Pelagiaria, a morphologically diverse clade of open-ocean teleost fishes that includes tuna and mackerel. Despite showing high shape disparity, the majority of taxa are concentrated in fairly restricted regions of morphospace, with taxa from all families falling into three distinct clusters. Phylogenetic signal in shape data is significant but low (Kmult = 0.27, p = 0.001) and a single-rate Ornstein-Uhlenbeck model of evolution is supported, revealing convergence in shape within and between families. Shape is significantly correlated with body elongation (R2 = 0.35, p =0.001), but correlation with size, diet, and habitat depth is weak. Integration of the neurocranium is high, supporting the hypothesis that high integration may promote the evolution of more extreme morphologies. Taken together, these results suggest that shape evolution in the pelagiarian neurocranium is constrained by a number of factors, resulting in the repeated evolution of a restricted range of morphologies.