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    AES Morphology & AES Physiology I

    2021-07-23   10:45 - 12:00

    Moderator: Kyle Newton

    1.  10:45  VIRTUAL    Older and Wiser? Ontogenetic shifts in brain size and brain organization in the Atlantic sharpnose shark, Rhizoprionodon terraevovae. Krista Laforest, University of North Carolina Wilmington; Emily Peele, University of North Carolina Wilmington; Kara Yopak*, University of North Carolina Wilmington

    Throughout an animal’s life, species may occupy different environments and exhibit distinct life stages, known as ontogenetic shifts. Life histories of most cartilaginous fishes are characterized by these ontogenetic shifts, defined by changes in habitat and diet, as well as behavioral changes at the onset of sexual maturity. In addition, cartilaginous fishes experience indeterminate growth, whereby both the brain and body grow continually throughout their lives. Despite lifelong neurogenesis in these species, little work has been done on ontogenetic changes in brain size or brain organization, which may be informative about functional shifts in sensory and behavioral specializations. This study quantified ontogenetic changes in brain-body scaling and the scaling of seven major brain regions in 35 specimens of the Atlantic sharpnose shark, Rhizoprionodon terraenovae, at all stages of maturity. Relative brain size increased significantly with body mass throughout ontogeny in this species, and the telencephalon, diencephalon, optic tectum, and medulla oblonga scaled with negative allometry against brain mass. However, notably, the olfactory bulbs and cerebellum scaled hyperallometrically to the rest of the brain, whereby these structures enlarged disproportionately as this species matured. Changes in the relative size of the olfactory bulbs throughout ontogeny may reflect an increased reliance on olfaction at later life history stages inR. terraenovae, while changes in the relative size of the cerebellum throughout ontogeny may be indicative of the ability to capture faster prey or an increase in migratory behavior as this species moves to offshore habitats, associated with the onset of sexual maturity.

    2.  11:00  VIRTUAL    Characterizing Elasmobranch Rosette Morphology Using Two-Dimensional and Three-Dimensional Methods. Aubrey Clark*, Florida Atlantic University; Tricia Meredith, Florida Atlantic University; Marianne Porter, Florida Atlantic University

    Olfaction in elasmobranchs occurs when odorants in the water are brought into contact with the sensory epithelium of the paired olfactory rosettes, or organs. Among species, these rosettes are morphologically similar but variability in size and structure have not been linked to differences in odorant sensitivity. Previous studies have characterized the morphology of these rosettes using specimens that have been dissected from a rigid cartilaginous capsule in the cranium; however, the specimens do not retain their natural positioning and shape because they lose the support of the capsule. This study aimed to characterize the shape and structure of elasmobranch rosettes using morphometric data collected from dissected andin situorgans. From each individual (N=6 individuals; 2 families), we measured fineness ratio (a representation of 2D shape) and organ volume of a dissected rosette and one that was processed using diffusible iodine-based contrast-enhanced computed tomography (diceCT). We stained fixed specimens in 5% Lugol’s iodine solution and imaged them with a Bruker SkyScan1173 microCT scanner. Using Welch’s t-tests, we found that fineness ratio and volume measurements were not significantly different between the two methods. We also found a region of unoccupied space within the capsule that may assist in modulating water flow throughout the system. These data can be used to create 3D models to examine the hydrodynamic effects of varying rosette and capsule shape, and the potential implications for odorant detection.

    3.  11:15  VIRTUAL    Description of the Inner Ear of Three Elasmobranch Genera. Jordyn Neal*, Humboldt State University; Allison Bronson, Humboldt State University

    The morphology and position of the inner ear has often been used to make inferences about animals’ ecology and behavior. However, the skeletal labyrinth of cartilaginous fishes has been relatively poorly sampled. Using high-resolution computed tomographic scanning, we isolated and described the semicircular labyrinth of three extant species of sharks:Ginglymostoma cirratum, Stegostoma fasciatum, andHemitrakis japonica, described the position and size of the labyrinth relative to the cranium and endocast, and qualitatively compared the inner ear anatomy of these taxa. The three taxa notably differ in the overall morphology of the labyrinth. In particular,Ginglymostomahas semicircular canals with a very stout, almost puffy appearance relative to the other taxa, and its semicircular canals rival in diameter the size of the saccular portion of the ear.StegostomaandHemitriakishave more separation of the canals from the sacculus and sinus superior. Among other features, inHemitriakisandGinglymostoma, the ear is elongate anteroposteriorly, relative to the more upright morphology of theStegostomaear. Unusually, inStegostoma, the posterior semicircular canal does not appear to be separated from the anterior canal in the fashion typically characteristic of elasmobranchs. Among vertebrates, sharks have a highly specialized inner ear morphology due to their detection of low frequency sound. However, among chondrichthyan taxa, much variation in the inner ear remains to be described. These differences in labyrinth anatomy may have implications for studies of ear function, and potentially yield new morphological traits for phylogenetic analyses.

    4.  11:30  VIRTUAL    Dermal denticle diversity in sharks: novel patterns on the interbranchial skin. Molly Gabler-Smith*, Harvard University; Dylan Wainwright, Yale University; Greta Wong, Harvard University; George Lauder, Harvard University

    Shark skin is covered in dermal denticles – tooth-like structures consisting of enameloid, dentine, and a central pulp cavity. Previous studies have demonstrated differences in denticle morphology both among species and across different body regions within a species, including one report of extreme morphological variation within a 1 cm distance on the skin covering the branchial pouches, a region termed “interbranchial skin”. We quantified differences in denticle morphology and surface topography of interbranchial skin denticles among 13 species of sharks to better understand the surface structure of this region. We show that 1) interbranchial skin denticles differ across shark species, and 2) denticles on the leading edge of the skin covering each gill pouch have different morphology and surface topography compared to denticles on the trailing edge. Across all species studied, there were significant differences in denticle length (P=0.01) and width (P=0.002), with shorter and wider leading edge denticles compared to trailing edge denticles. Surface skew was also higher in leading edge denticles (P=0.009), though most values were still negative. Overall, leading edge denticles were smoother-edged than trailing edge denticles in all of the species studied. These data suggest two hypotheses: 1) smoother-edged leading edge denticles protect the previous gill flap from abrasion during respiration, and 2) ridged denticle morphology at the trailing edge might alter water turbulence exiting branchial pouches after passing over the gills. Future studies will focus on determining the relationship between denticle morphology and water flow by visualizing fluid motion over interbranchial denticles during in vivo respiration.

    5.  11:45  VIRTUAL    Scalloped hammerhead sharks are metabolically poised for intensive and potentially anoxic deep dives. Mark Royer*, Hawai?i Institute of Marine Biology; Jeffery Drazen, University of Hawai?i at M?noa; Danielle Garcia, University of Hawai?i at M?noa; Kevin Weng, Virginia Institute of Marine Science; Carl Meyer, Hawai?i Institute of Marine Biology; Kim Holland, Hawai?i Institute of Marine Biology

    Scalloped hammerhead sharks (Sphryna lewini) routinely perform rapid deep dives to forage on meso and bathypelagic prey. These deep dives consist of intensive burst swimming followed by resting periods in the surface mixed layer. Swimming muscle temperature profiles suggest S. lewini suppress their gill function as a means to reduce convective heat loss. Such intensive swimming behavior coupled with supposedly reduced respiration raises questions concerning the aerobic and anaerobic metabolic capacity of their white swimming muscle. Measuring the activity rates of key enzymes used in aerobic and anaerobic metabolism provides an indirect indicator of the metabolic potential of a tissue. Here we measured the maximal activities of aerobic enzymes citrate synthase (CS), malate dehydrogenase (MDH) and anaerobic enzymes pyruvate kinase (PK) and lactate dehydrogenase (LDH) from white swimming muscle of S. lewini and were compared to those measured from other coastal, deep-dwelling, and highly active endothermic shark species. Sphyrna lewinihad significantly higher activity levels of LDH, an anaerobic enzyme, and MDH, an aerobic enzyme. Results indicate the white muscle tissue of S. lewiniis highly anaerobic dependent for burst swimming but has a high potential for lactate buildup. High activity rates of MDH indicate the potential for lactate to be rapidly reduced under aerobic conditions such as those experienced in the surface mixed layer between dives. These physiological adaptations likely enable S. lewini to remain highly active while suppressing gill function during deep dives and thereby exploit a very different ecological niche from sympatric shark species.

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