Batoids are a diverse clade of flat cartilaginous fishes that occur primarily in benthic marine habitats. The skates and rays typically use their flexible pectoral fins for feeding and propulsion via undulatory swimming. However, two groups of rays have adopted a pelagic or bentho-pelagic lifestyle and utilize oscillatory swimming-the Myliobatidae and Gymnuridae. The myliobatids have evolved cephalic lobes, anteriorly extended appendages that are optimized for feeding, while their pectoral fins exhibit several modifications that likely arose in association with functional optimization of pelagic cruising via oscillatory flight. Here, we examine variation in fin ray distribution and ontogenetic timing of fin ray development in batoid pectoral fins in an evolutionary context using the following methods: radiography, computed tomography, dissections, and cleared and stained specimens. We propose an index for characterizing variation in the distribution of pectoral fin rays. While undulatory swimmers exhibit symmetry or slight anterior bias, we found a posterior shift in the distribution of fin rays that arose in two distinct lineages in association with oscillatory swimming. Undulatory and oscillatory swimmers occupy nonoverlapping morphospace with respect to fin ray distribution illustrating significant remodeling of pectoral fins in oscillatory swimmers. Further, we describe a derived skeletal feature in anterior pectoral fins of the Myliobatidae that is likely associated with optimization of oscillatory swimming. By examining the distribution of fin rays with clearly defined articulation points, we were able to infer evolutionary trends and body plan remodeling associated with invasion of the pelagic environment. Finally, we found that the number and distribution of fin rays is set early in development in the little skate, round stingray, and cownose ray, suggesting that fin ray counts from specimens after birth or hatching are representative of adults and therefore comparable among species.

The ScanAllFish project is a large-scale effort to scan all the world's 33,100 known species of fishes. It has already generated thousands of volumetric CT scans of fish species which are available on open access platforms such as the Open Science Framework. To achieve a scanning rate required for a project of this magnitude, many specimens are grouped together into a single tube and scanned all at once. The resulting data contain many fish which are often bent and twisted to fit into the scanner. Our system, Unwind, is a novel interactive visualization and processing tool which extracts, unbends, and untwists volumetric images of fish with minimal user interaction. Our approach enables scientists to interactively unwarp these volumes to remove the undesired torque and bending using a piecewise-linear skeleton extracted by averaging isosurfaces of a harmonic function connecting the head and tail of each fish. The result is a volumetric dataset of a individual, straight fish in a canonical pose defined by the marine biologist expert user. We have developed Unwind in collaboration with a team of marine biologists: Our system has been deployed in their labs, and is presently being used for dataset construction, biomechanical analysis, and the generation of figures for scientific publication.

Holocephalans exhibit auxiliary appendages called pre‐pelvic claspers (PPCs) that are located anterior to the pelvic fins, while pelvic claspers are pelvic fin modifications located posteriorly as modified metapterygia. Articulation points of the PPCs have not previously been imaged or evaluated in a comparative context, therefore, they may represent modified pelvic fin structures if they articulate with the propterygium. Alternatively, they could represent the only example of an independent third set of paired appendages in an extant taxon, if they articulate independently from any pelvic fin basal cartilages, challenging the current paradigm that extant jawed vertebrates are constrained to two sets of paired appendages. Two extinct groups, including Placoderms and Acanthodians, exhibit variation in the number of paired appendages, suggesting this may be a plesiomorphic trait. We evaluated PPC developmental growth rates, morphology, and articulation points in spotted ratfish (Hydrolagus Colliei, Holocephali). We also compared variation in PPC morphology among representatives of the three extant holocephalan families. Both, the pre‐pelvic and pelvic claspers exhibit a dramatic surge in growth at sexual maturity, and then level off, suggesting synchronous development via shared hormonal regulation. While mature females are larger than males, pelvic fin growth and development is faster in males, suggesting a selective advantage to larger fins with faster development. Finally, microcomputed tomography scans revealed that PPCs are not modified propterygia, nor do they articulate with the propterygium. They articulate with the anterior pre‐pelvic process on the anterior puboischiadic bar (or pelvic girdle), suggesting that while they are associated with the pelvic girdle, they may indeed represent a third, independent set of paired appendages in extant holocephalans.

Batoids (skates and stingrays), exhibit swimming modes that can be characterized on a spectrum from undulation (>1 wave present along the fin) to oscillation (<0.5 wave present along the fin). The latter is a derived mode of locomotion that evolved in a group of charismatic batoids that have invaded pelagic environments (family Myliobatidae: manta rays, cownose rays, eagle rays). Oscillatory swimming is associated with changes in pectoral fin morphology, including laterally elongated pectoral fins (wings), a posterior redistribution of pectoral fin rays, and the evolution of modified anterior pectoral fin domains called cephalic lobes‐ anteriorly extended appendages used exclusively for feeding in the Myliobatidae. Variation in the number of fin rays in batoid pectoral fins has not been characterized in a comparative or phylogenetic context. To better understand how the batoid body plan was modified in association with a shift in swimming and feeding modes, we quantified fin rays that articulate with the three primary cartilages of pectoral fins and cephalic lobes in myliobatids and their relatives. We found no evidence that additional anterior fin rays evolved in association with the origin of cephalic lobes. Rather, we found that variation in the distribution of pectoral fin rays is associated with the evolution of the oscillatory swimming mode. Further, the degree of partitioning between the cephalic lobes and pectoral fins may be correlated to species‐specific ecology. We found two distinct variations in body plan morphology and skeletal arrangement in the cephalic lobe and anterior pectoral fins within the genus Myliobatis, which appear to be associated with ecology and habitat. Overall, this research has implications for morphological evolution associated with invasion of the pelagic environment, and the biomechanics of underwater flight.Support or Funding InformationGolden West Women Flyfishers (KCH) and the California State University Council on Ocean Affairs, Science and Technology (KCH), National Science Foundation IOS 1656487 (KDC), and Thematic Collections Network TCN 1701665 (APS).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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