The ontogenetic scaling of form and function in the spotted ratfish, Hydrolagus colliei (Chondrichthyes: Chimaeriformes): Fins, muscles, and locomotion.

The alteration of form and function through the life of a fish can have profound impacts on the ability to move through water. Although several studies have examined morphology and function in relation to body size, there is a paucity of data for chondrichthyans, an ancient group of fishes. Ratfishes are interesting in that they utilize flapping pectoral fins to drive movement, and they diverged from elasmobranchs early in the gnathostome phylogeny. Using the spotted ratfish, Hydrolagus colliei, we quantified the scaling of traits relevant for locomotion, including median and paired fin external anatomy, the musculature of the pectoral and pelvic fins, and the kinematics of the pectoral fins. Whereas pelvic fins scaled with either positive allometry (fin span and area) or isometry (fin chord length at the base of the fin), pectoral fin measurements either scaled with negative allometry (fin span and aspect ratio) or isometry (fin area and chord length). Correspondingly, all pelvic fin muscles exhibited positive allometry, whereas pectoral muscles exhibited a mix of isometric and positively allometric growth. Caudal fin area and body frontal area both scaled with positive allometry, whereas dorsal fin area and span scale with isometry. Pectoral fin amplitude during swimming exhibited isometry, and fin beat frequency decreased with body size. Our results highlight the complex changes in form and function throughout ontogeny. Finally, we highlight that hierarchical differentiation in morphology can occur during growth, potentially leading to complex changes in performance of a functional system.