Walking on chains: the morphology and mechanics behind the fin ray derived limbs of sea-robins

Fish fin rays (lepidotrichia) are typically composed of paired and segmented flexible structures (hemitrichia) that help support and change the shape of the fins to affect water flow. Yet, marine ray-finned fish that are members of the family Priontinae (sea-robins) have specialized pectoral fin rays that are separated from the fin and used as limbs to walk along the seafloor. While previous kinematic studies have demonstrated the use of these specialized fin rays as walking appendages, there is little information on how the morphology of the “walking-rays” and associated musculature facilitate underwater walking. Here, we examine the musculoskeletal anatomy of the walking and pectoral fin rays in striped sea-robin, Prionotus evolans and compare the mechanical properties of the rays with those of the smaller northern sea-robin, Prionotus carolinus. We aimed to determine what structural modifications in the walking-rays allow them to function as a supportive limb. We found enlarged processes for muscle attachment, bone extensions that brace the hemitrich articulations, and reduced flexibility and increased second moment of area along the rostro-caudal bending axis in the rays used for walking. This novel limb design may have promoted the benthic foraging behavior exhibited by these species by uncoupling locomotion and feeding.