Collaborative Research: RUI: Testing the link between skeletal labyrinth morphology and habitat in sharks

Cal Poly Humboldt Sponsored Programs Foundation, Arcata CA

Investigators

Abstract

The inner ear is used both for detecting sound and for the sense of equilibrium (sensing the body’s orientation in space). The shape of an animal’s ear often correlates with locomotion or primary habitat. For example, ear shape differs between burrowing, terrestrial, and tree-dwelling mammals. This work will use Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) to evaluate the degree to which ear shape differs between chondrichthyans (sharks, rays, and ratfishes) living in different habitats. If correlations with habitat exist, then ear shape characterization in representative extant species can be used to make inferences about the lives of extinct sharks and rays. If ear shape is related more to evolutionary history, rather than habitat, then ear shape can be used to learn more about the ancestor-descendant relationships between living and fossil groups of sharks. The project will produce 3D renderings of the inner ears of all living families of sharks and rays, assess structural similarity between soft tissue and skeletal parts of shark inner ears, predict locomotory mode and habitat for fossil sharks, and develop a tree diagram that depicts evolutionary relationships of sharks and their relatives. Funding will support two graduate students, as well as undergraduates, all of whom will present their research at conferences and contribute to publishing papers. 3D anatomical models will be publicly available online and used as educational tools at both the K-12 and college level, as well as for public displays at university campuses and public natural history museums. Elasmobranchs represent an excellent but understudied non-tetrapod group for testing the link between inner ear geometry and ecomorphology: their labyrinths are fully enclosed in cartilage, the inner ear displays both symplesiomorphies and specializations for low-frequency sound detection, and the group occupies a wide range of habitats including pelagic taxa, reef-associated species, and sandy-bottom benthic forms. The membranous canals of the inner ear are housed in the skeletal labyrinth, which can be virtually infilled to create an endocast used for comparative analyses. To assess the relationship between ear shape and locomotor ecology and habitat, the project will first use micro-computed tomography to document skeletal labyrinth morphology for all living shark and batoid families. A combination of Diffusible Iodine-based Contrast-Enhanced CT and MRI will be used to determine whether the skeletal labyrinth is an accurate reflection of the variation in shape of the soft-tissue membranous labyrinth. Then, a regression analysis will test for correlation between lifestyle and labyrinth geometry, and discriminant function analysis and cross-validation analysis will test the predictive power of shape variables. If a strong ecomorphological correlation is present, inferences can then be made about locomotory ecology of extinct taxa. Recent phylogenetic analyses of chondrichthyans have revealed clade instability and paraphyly that may be resolved with additional morphological characters. Therefore, the dataset will be implemented toward resolving phylogenetic conflicts by adding inner ear characters to a total evidence analysis. Datasets resulting from the work will also be publicly accessible and therefore helpful for downstream analyses by researchers around the world. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.