The comparative morphology of the musculature controlling the pectoral free rays in scorpaenoid fishes.

Free rays are ventral pectoral fin rays (lepidotrichia) that are free of the pectoral fin webbing. They are some of the most striking adaptations of benthic fishes. Free rays are used for specialized behaviors such as digging, walking or crawling along the sea bottom. Studies of pectoral free rays have focused on a small number of species, most notably the searobins (Family Triglidae). Previous research on the morphology of the free rays has emphasized their functional novelty. We hypothesize that the more extreme specializations of the pectoral free rays in searobins are not precisely novel, but are part of a broader range of morphological specializations that are associated with the pectoral free rays in suborder Scorpaenoidei. We perform a detailed comparative description of the intrinsic musculature and osteology of the pectoral free rays in three families of scorpaenoid fishes: Hoplichthyidae, Triglidae, and Synanceiidae. These families vary in the number of pectoral free rays and the degree of morphological specialization of those rays. As part of our comparative analysis, we propose significant revisions to earlier descriptions of both the identity and function of the musculature associated with the pectoral free rays. We focus particularly on the specialized adductors that are important for walking behaviors. Our emphasis on the homology of these features provides important morphological and evolutionary context for understanding the evolution and function of free rays within Scorpaenoidei and other groups.