Morphology and evolution of Malacostraca

In this dissertation project comparative morphological studies on the nervous system, mandible structure and sensory equipment of Decapoda and Peracarida are presented and interpreted with regard to the evolution of the taxa. This is a cumulative dissertation and the results were obtained in several separate publications. Both larval and adult characters as well as the ontogeny of certain features were included and analysed using various sets of imaging techniques ranging from conventional light microscopy to ultrastructure research with transmission electron microscopy. Attention was focused on the description and development of previously unexplored sets of characters which can be included in a reconstruction of crustacean phylogeny. The adult nervous system in Decapoda has been extensively studied for a long time and shows very specific taxon generic adaptations. In the present thesis now also the larval nervous system was comparatively investigated in its entirety for the first time. By use of computer assisted 3D reconstruction general and species specific features were analysed and basic elements were described, including the segmental ganglia and their neuropils as well as the segmental nerves. The larval nervous system is in a transitory stage to the adult organization, already showing well differentiated basic elements. Likewise the phase-specific structure reflects adaptations to larval life. The studied species respectively represent one of the three decapod main lineages, i.e. Caridea, Anomura and Brachyura. Against this background variations in the differentiation of 3certain ganglia can best be explained with shifts in the timing of morphogenetic events, i.e. heterochrony. The studies on heterochrony as motors of evolution are another core topic of this project. Along with the latter study also the morphology and finestructure of decapod mandibles during larval development was investigated, based on two closely related species, showing different feeding modes in the zoea I. Thus, it could be tested whether the mandible structure in early larval stages only depends on feeding modes or an evolutionary ground pattern is recognizable even in species with non-feeding zoea I. In case of a comparison of mandibles, restricted only to the features of the zoea I, adaptations to food preferences may obscure taxon- specific features. In detailed inspection, however, it could be shown that even in species with non- feeding zoea I apomorph basic features of the related taxon can be recognized. This supports the hypothesis of the presence of phylogenetic relevant character sets in larval mandible morphology. The monophyly of the Mandibulata is manly based on hypotheses defending the homology of the mandibles in Myriapoda, Hexapoda and Crustacea, nevertheless, knowledge on sensory structures located on the gnathal lobe is astonishingly limited, even less is known about their ultrastructure. The development of this complex of characteristics represents a further aim of this project. For this purpose the ultrastructure of the mandibular gnathal lobe of the zoea I of a rockpool prawn was analysed. Besides external structure and location and an analysis of the modality specific structures, special attention was paid to the features of the lacinia mobilis. In total a number of seven different types of sensilla, innervated by four different types of dendrites, could be described and compared, including (1) mechanoreceptive hair-sensilla and (2) putative contact-chemo-receptors, as well as (3) sensilla without external structures and (4) sensilla associated with inflexible spines. The results reveal new insights into the functional morphology of larval decapod mandibles and constitute a significant character complex including fine- and ultrastructural features. Following-up the character complex was completed by investigations of respective features of a peracarid representative. The results also present an overview of the sensory elements of the mandible as well as a detailed analysis of the lacinia mobilis based on their ultrastructure and features related to ecdysis. By comparison it can be shown, that the lacinia mobilis on the right mandible in Peracarida and also the respective structure in Decapoda are mechanosensitive sensilla. In conclusion the hypothesis of a possible homology of the latter structures gains further support. Concerning the structures on the left mandible a differentiated consideration is necessary. No unambiguous conclusions can be made and it remains to be resolved if the lacinia mobilis on the left mandible is a derived sensillum or a compound structure equipped with multiple sensilla. With the application of many different state-of-the-art technics and the overall discussion of the results an important contribution to eumalacostracan phylogeny, maybe even crustacean phylogeny, could be made. Character sets comprising different levels of organization of the arthropod body could be established. Primarily phylogenetic relevant signal in the basic elements of the larval nervous system and the mandibles in Decapoda could be presented. Furthermore, highly complex and detailed character sets of the mandible ultrastructure were developed, revealing a comprehensive presentation of the sensory capacities of eumalacostracan mandibles and by comparison already allowed conclusions about the homology of the lacinia mobilis. Thus, also the phylogenetic position of the respective taxa can be confirmed.