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.