Sofie Katrine Dam Nielsen:
Cnidarians have long been recognized as occupying a unique position in the nervous system evolution, and consequently, have attracted considerable attention from neurobiologist throughout the years. Currently researchers have focused on the box jellyfish, Tripedalia cystophora to serve as an invertebrate model organism in the field of neurobiology. This Caribbean cubomedusa have a simple nervous system but still performs a range of behaviours including proper mating, obstacle avoidance, habitat recognition and navigation. Most of these behaviours are visually guided and they possess 24 eyes, some which are similar in structure to vertebrate eyes. The eyes are situated on four sensory structures, the rhopalia, also holding a major part of the central nervous system (CNS) of the animal.
The rhopalial nervous system (RNS) holds approximately 1000 neurons in the adult animal and putatively performs most of the visual processing. Despite their visual processing centre, only holds relatively few neurons, behavioural pilot studies indicated that the box jellyfish Tripedalia cystophora are able to learn. One important aim of this thesis is thus to thoroughly test if T. cystophora is able to learn indicating that this important quality was already present in one of the first nervous systems in evolution (Paper I). This thesis also looks into the dilemma of how the small cubozoan nervous system can control their complex behaviours. This we did by examining the transcriptomics of T. cystophora with a focus on neuronal elements and by examining the expression pattern of the discovered neuropeptides (Paper II-III), which led to a greater insight in the visual processing and neuronal subsystems involved. Finally, we used state of the art transcriptomic methods to compare the nervous system complexity across the three medusozoan classes Cubozoa, Scyphozoa and Hydrozoa (Paper IV-V). These comparative studies further link neuropeptides to visual processing and points to a co-evolution of the peptidergic neurons involved in visual processing, along with the development of complex eyes in Cubozoa.