Although the majority of batoid elasmobranchs, skates and rays, are benthically associated, benthic locomotion has been largely overlooked in this group. Only skates have been previously described to perform a form of benthic locomotion termed "punting." While keeping the rest of the body motionless, the skate's pelvic fins are planted into the substrate and then retracted caudally, which thrusts the body forward. In this study, we demonstrate that this form of locomotion is not confined to the skates, but is found across a range of phylogenetically and morphologically diverse batoid species. However, only the clearnose skate, Raja eglanteria, and the lesser electric ray, Narcine brasiliensis, performed "true punting," in which only the pelvic fins were engaged. The yellow stingray, Urobatis jamaicensis, and the Atlantic stingray, Dasyatis sabina, performed "augmented punting," in which pectoral fin movement was also used to generate thrust. Despite this supplemental use of pectoral fins, the augmented punters failed to exceed the punting capabilities of the true punters. The urobatid and the true punters all punted approximately half their disc length per punt, whereas the dasyatid punted a significantly shorter distance. The skate punted significantly faster than the other species. Examination of the pelvic fin musculature revealed more specialized muscles in the true punters than in the augmented punters. This concordance of musculature with punting ability provides predictive power regarding the punting kinematics of other elasmobranchs based upon gross muscular examinations. In contrast to previous assumptions, our results suggest that benthic locomotion is widespread among batoids.

SUMMARYElasmobranchs (sharks, skates and rays) perform at the extremes of locomotion and feeding (i.e. long migrations, high-speed swimming and durophagy). However, very little is known about their cartilaginous skeletal structure and composition in response to loading regimes. In this study, we investigated a batoid (skate and ray) appendicular skeletal element, the propterygium, and its response to forces experienced during punting (benthic pelvic fin locomotion). Punting places a flexural load on this thin, rodlike element. The goals for our study were to determine: (1) the mechanical and compositional properties of the propterygium and (2) whether these properties correlate with punting ability. Using five batoid species of varying punting ability, we employed a three-point bending test and found that propterygium flexural stiffness (33.74-180.16Nm 2 ) was similar to values found in bone and could predict punting ability. Variation in flexural stiffness resulted from differences in mineral content (24.4-48.9% dry mass) and the second moment of area. Propterygia material stiffness (140-2533MPa) approached the lower limit of bone despite having less than one-third of its mineral content. This drastically lower mineral content is reflected in the radius-to-thickness ratio of the cross-section (mean ± s.e.m.5.5±0.44), which is comparatively much higher than bony vertebrates. This indicates that elasmobranchs may have evolved skeletal elements that increase buoyancy without sacrificing mechanical properties. Our results highlight the functional parallels between a cartilaginous and bony skeleton despite dramatic compositional differences, and provide insight into how environmental factors may affect cartilaginous skeletal development.

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