Cryptic anatomical adaptive peak shifts and transitions along the body elongation continuum in zoarcoid fishes

Body elongation is a major feature of ray-finned fish evolution. Elongate body shapes have arisen repeatedly across lineages and led to a phylogenetically diverse array of eel-like fishes. Despite apparent convergence in overall shape, independent elongation events have occurred through a diversity of skeletal anatomical changes, suggesting variability in underlying developmental, functional, and ecological shifts. It is less clear, however, whether such diversity characterizes evolution in lineages following elongation. In this study, we show that, even within a predominantly elongate radiation of teleost fishes, the Zoarcoidei (eelpouts, pricklebacks, and others), a variety of anatomical shifts have occurred, and they exhibit a complex relationship with body shape. We measured dimensions of the cranial and axial skeleton as well as overall body shape from 71 species representing 12 of 14 recognized zoarcoid families, and we fit multi-optimum adaptive evolutionary (Ornstein-Uhlenbeck) models to these data. We estimated a moderately elongate ancestral optimum for Zoarcoidei that was retained in most lineages. Still, we identified three peak shifts to highly elongate body shapes, which differed in associated anatomical changes. In addition, we detected multiple cryptic anatomical peak shifts in which transitions in cranial and axial skeletal anatomy occurred in lineages that retained the ancestral body shape optimum. Altogether, our results reveal a hidden level of morphological evolution in elongate fishes, suggesting that similarity in body shape belies diverse ecological demands and varied functional capacities.