Rheinische Friedrich‐Wilhelms‐Universität Bonn

Abstract

SummaryEchinoidea (sea urchins) are a group of benthic marine invertebrates that belong to the deuterostome taxon Echinodermata. Although extensive data have been gathered on larval and adult hard-part anatomy of sea urchins, for several reasons the knowledge of echinoid soft tissue anatomy is still fragmentary. For example, published works report differences in various taxa regarding the presence or absence of prominent internal organs such as the primary siphon, the gastric caecum, or the spongy bodies.The major aim of the present study was therefore to evaluate the potential of two non-invasive imaging techniques, namely magnetic resonance imaging (MRI) and micro-computed tomography (µCT), to visualize, in combination with advanced 3D modeling protocols, internal structures of sea urchins and to apply these techniques for large-scale comparative morphological analyses. In order to significantly extend taxon sampling, valuable specimens from museum collections were selected to be included in this study.MRI proved to be ideally suited to depict internal structures of invertebrates non-invasively, especially soft tissues, at isotropic resolutions well below 100 µm. Due to the simple specimen preparation as well as straight-forward imaging protocols, a high specimen throughput could be accomplished. Based on the digital tomographic datasets, 3D models were assembled in order to visualize selected features of echinoid soft tissue anatomy. Although imaging of soft tissues was practically impossible using conventional desktop µCT equipment due to the physical properties inherent in X-ray technology, this technique was successfully employed to visualize sea urchin hard-part anatomy at isotropic resolutions below 20 µm. MRI and µCT can therefore be seen as complementary non-invasive imaging techniques, especially for echinoderm specimens in which soft-and hard-part anatomy are equally present.To explore novel ways of communication and deposition of the acquired datasets as well as 3D models, these were made publicly available through the Internet and by integrating 3D data into PDF-based scientific publications. This approach included also the use of data from a central data repository for protein structures.The axial complex, a highly specialized organ also found in other echinoderm taxa, was then selected for an exemplary comparative study involving dozens of sea urchin species in order to highlight the possibilities offered by non-invasive imaging techniques combined with dissection, histology, and a reappraisal of information from published work. The results suggest an architectural interdependence of the axial complex with various other internal organs present in sea urchins.Finally, an attempt was made to evaluate the potential of echinoid soft tissue structures to extend the current list of morphological characters used for phylogenetic inferences. A set of selected soft tissue characters was therefore assembled and evaluated for every major sea urchin taxon. The results provide significant phylogenetic informa...