Behavioral and immunohistochemical assessment of selected cognitive functions in elasmobranch fishes (Chiloscyllium griseum)

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Sharks, rays and chimaeras comprise the class Chondrichthyes (about 1200 species), which represents the oldest extant jawed vertebrates and thereby holds a key phylogenetic position to understanding vertebrate brain evolution. While this group was formerly known as primitive fish with primitive brains, research over the last few decades has provided increasing evidence that sharks and rays show sophisticated behaviour, have a complex biology and are equipped with sensory systems perfectly adapted to life underwater. However, despite elaborate research, the extent of cognition within this group is still largely unknown. Based on previous research, our hypothesis is that elasmobranchs have (selected) cognitive abilities that closely match those of many teleosts or other vertebrates. This study aims to provide previously lacking information on selected cognitive functions regarding 1. object categorization, 2. recognition of illusionary contours, 3. aspects of spatial learning and 4. memory retention. To complement the behavioural work, immediate early gene analyses (IEGA) will be performed to determine the neuronal substrates involved in learning and higher cognitive tasks (5). The primary target site is the telencephalon. Experiments will be conducted on the grey bamboo shark (Chiloscyllium griseum), a small, benthic of about 70 cm. All proposed experiments have an ecological significance to the species tested: individuals live in complex environments such as coral reefs, lagoons and inshore areas, are probably territorial, and would therefore benefit from using cognitive spatial maps (experiment 3), spatial memory (Experiments 3, 4) as well as visual categorization abilities (experiment 1) to find and remember hiding places and food patches as well as to recognize and remember con-specifics, predators and prey. Illusionary contours are misreadings of visual information by the brain and demonstrate that the brain has certain preconceptions about the visual world that it applies to the incoming sensory information (Kandel et al. 1991). It would be extremely interesting to determine if the shark brain uses the same basic rules in this regard as other vertebrates (experiment 2). Spatiotemporal learning would provide benefits in regards to foraging efficiency or successful predator avoidance (experiment 3). Relying on global cues instead of local cues (beacons) provides more flexibility, as individual landmarks may disappear or change appearance (experiment 3). So far, no IEG studies have been performed in sharks. The study therefore aims to determine if neuronal activity induced by learning specific tasks induces up-regulation of expression of c-fos and egr-1 in the telencephalon or specific regions thereof (experiment 5).