Meeting Abstract
The dorsal fin is one of the most varied swimming structures in spiny-finned fishes. This fin can be present as a single contiguous structure supported by bony spines and soft rays, or it may be divided into an anterior spiny dorsal fin and a posterior soft dorsal fin. When the fin is divided, the length of the gap between the spiny and soft portions of the fin can range from almost nothing to a considerable portion of the fish’s body length. Computational models have suggested that if the spiny and soft dorsal fins are divided and the soft dorsal fin has a stiff leading edge, then the posterior dorsal fin can exploit vortices shed by the anterior fin, increasing thrust. Based on this putative role of the posterior dorsal fin, we hypothesized that gaps between the anterior and posterior dorsal fin would evolve in tandem with stiffened fin rays at the leading edge of the posterior dorsal to resist oncoming flow and to provide a more robust attachment for intrinsic fin musculature. We studied the evolution of dorsal fin stiffness and spacing across the freshwater perches of the family Percidae. Percid fishes exhibit considerable variation in dorsal fin morphology, with multiple evolutions of the split dorsal fin across their phylogeny. Using x-ray radiography of museum specimens we measured the spacing between dorsal fins (where present). From these specimens, we collected &muCT data from individual dorsal fin spines and rays. We used these data to calculate the spines’ and rays’ second moment of area, and conducted cantilever bending tests to estimate spine and ray flexural stiffness and Young’s modulus. Using phylogenetic comparative methods, we measured the strength of the association between dorsal fin spacing and the stiffness of the fin elements at the leading edge of the posterior portion of the dorsal fin.
