The nervous system of box jellyfish: A surprisingly complex system in a simple animal

Sammanfattning: The four surprisingly complex visual sensory organs of cubomedusae, the rhopalia, contain each six eyes of four morphologically different types, two of which are camera-type eyes. This means that the evolutionary basal nervous system of cubomedusae must process the visual signals received from a total of 24 rhopalial eyes. Since the neuronal organization of the rhopalia is obscure, the role of the rhopalial nervous system in the processing of visual information is unknown. The aim of this thesis is to investigate the organization of the rhopalial nervous system in order to better understand its role in the visual processing of cubomedusae (Paper I-III). Prior to this thesis, there existed only fragmentary knowledge concerning the neuronal organization of the umbrella ("body") of cubomedusae. Moreover, the neurons of the manubrium were unknown and those of the tentacles were only superficially described. Arg-Phe-amide (RFamide) peptides are putative neurotransmitter substances in all cnidarians. The distribution of RFamide-immunoreactive (ir) neurons was examined in a cubomedusan species to investigate firstly whether RFamide-ir neurons may represent the swim motor system of cubomedusae, and secondly to determine the general neuronal organization of the manubrium and tentacles (Paper IV). This thesis provides the first detailed description of the nervous system of cubomedusae, based on immunohistochemical staining techniques. It clearly shows that the cubomedusa nervous system is more extensive and more highly organized than previously thought. Strong indications are that the rhopalial nervous system not only plays a prominent role in the processing of visual information but also in the integration of signals from other senses. In addition, the rhopalia seem to contain multiple neuronal populations that might serve different functions. The extensive RFamide-immunoreactive neural population could represent the swim motor system of the medusa, but may also serve other functions, including the conduction of impulses that coordinate prey capture and feeding. It is therefore likely that in cubomedusae the rhopalial nervous system, as well as that of the umbrella, contain multiple conduction systems.