Body shape transformations by alternate anatomical adaptive peak shifts in blenniiform fishes

Extreme body elongation has occurred repeatedly in the evolutionary history of ray-finned fishes. Lengthening of the anterior-posterior body axis relative to depth and width can involve changes in the cranial skeleton and vertebral column, but to what extent is anatomical evolution determined by selective factors and intrinsic constraints that are shared broadly among closely related lineages? In this study, we fit adaptive (Ornstein-Uhlenbeck) evolutionary models to body shape and its anatomical determinants and identified two instances of extreme elongation by divergent anatomical peak shifts in the Blenniiformes, a radiation of small-bodied substrate-associated marine teleost fishes. Species in the genus Xiphasia (hairtail blennies) evolved toward a peak defined by a highly elongated caudal vertebral region but ancestral cranial and precaudal vertebral morphology. In contrast, a clade that includes the genera Chaenopsis and Lucayablennius (pike and arrow blennies) evolved toward a peak with a long slender skull but ancestral axial skeletal anatomy. Neither set of anatomical peak shifts aligns closely with the major axis of anatomical diversification in other blenniiform fishes. These results provide little evidence that ancestral constraints have affected body shape transformation, and instead suggest that extreme elongation arose with distinct shifts in selective factors and development.在辐鳍鱼（ray-finned fishes）的演化历程中，躯体极端延长的现象曾多次独立发生。相较于体高与体宽，躯体前后轴的延长可伴随颅骨骼与脊柱的形态改变，但在亲缘关系较近的支系中广泛共享的选择因子与内在约束，能在多大程度上决定解剖结构的演化？本研究将自适应奥恩斯坦-乌伦贝克（Ornstein-Uhlenbeck）演化模型应用于躯体形态及其解剖决定因子，通过分析鳚形目（Blenniiformes）——一类小型底质关联海洋真骨鱼（teleost fishes）的辐射演化类群——的趋异解剖特征峰值漂移，鉴定出两例极端躯体延长的案例。剑齿鳚属（Xiphasia，带鱼鳚）的物种朝着以极度延长的尾椎区域为特征的演化峰值演化，但保留了祖先的颅骨骼与尾前椎形态；与之相反，包含Chaenopsis属与Lucayablennius属（梭鳚与箭鳚）的演化支，朝着以修长颅部为特征的演化峰值演化，但保留了祖先的轴向骨骼解剖结构。上述两类趋异解剖特征峰值漂移，均未与其他鳚形目鱼类的解剖结构多样化主轴线高度吻合。本研究结果几乎未提供证据表明祖先约束会影响躯体形态的演化转变，反而提示极端躯体延长的产生，伴随着选择因子与发育机制的显著改变。