"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
A common perception is that sharks are small-brained animals with a limited behavioral repertoire. Recent research has been dispelling these myths and has found that sharks possess brains that are of comparable size to birds and mammals. One brain structure of particular interest is the cerebellum, largely due to its extensive evolutionary history as well as the ongoing debate surrounding its function. This structure originally appeared in early sharks and has been carried through vertebrate evolution from the earliest cartilaginous fishes to the base of the human brain. However, the cerebellum and its function, from fish through to humans, has been an area of debate among neurobiologists. A large body of research asserts that the cerebellum controls and orchestrates movement while other research suggests that the primary function of the cerebellum is not movement, but is the acquisition and discrimination of sensory information. What can elasmobranchs (sharks, skates, and rays) contribute to our understanding of the human cerebellum and the origin of its function?
Large variation exists in the size and convolution (or foliation) of the cerebellum across cartilaginous fishes, including sharks. Until recently, the variation in cerebellar complexity has only been qualitatively assessed by scoring surface structure on the basis of length and depth of the folds using the visual grading index. The output of these visual assessments has shown that the highest levels of foliation are found in agile predators that lived in open ocean (3D) environments, such as Isurus oxyrinchus (shortfin mako shark), Alopias vulpinus (thintail thresher shark), and Sphyrna mokarran (great hammerhead shark) and the lowest levels of foliation occur in slow-moving, passive predators such as Orectolobus maculatus (wobbegong shark), Squalus acanthias (spiny dogfish), Squatina angelus (angel shark). These preliminary ecological correlations with brain development suggest a functional basis for this characteristic. However, visual classification is limited, as it does not provide a quantitative method for characterization and comparison of foliation. To confirm these trends, which show that cerebellar complexity is correlated with ecological factors (primary habitats, with varying locomotory styles, reproductive modes, and prey capture strategies), a more quantitative approach is necessary.
Magnetic resonance imaging (MRI) is unique in its ability to non-invasively acquire high-resolution data from soft tissue structure and Diffusion Tensor Imaging (DTI) is an MRI method that provides information on the orientation and coherence of white matter fiber tracts, thereby facilitating the reconstruction of neural fiber pathways. This project aims to utilize microscopic anatomical MRI and microscopic DTI in conjunction with novel analysis methods (shape analysis and fiber tract mapping) to assess the degree of folding in the cerebellum and how this relates to diversification and proliferation of fiber tract pathways from this brain structure to other neural processing centers. Through these methods, the team will inform the debate on cerebellar function, provide rationale for the evolution of cerebellar foliation in cartilaginous fishes, and explore the extent to which adaptive, developmental, and phylogenetic processes are driving neural evolution.
Publications, results, databases, and tools will be disseminated to the research community and general public via the existing Digital Fish Library website (www.digitalfishlibrary.org).
