Large-scale comparative morphological studies of sea urchins (Echinodermata: Echinoidea) using µCT and SRµCT
A. Ziegler1*, M. Ogurreck2, and F. Beckmann2
1Institut für Immungenetik, Charité-Universitätsmedizin Berlin, Thielallee 73, 14195 Berlin, Germany 2Institut für Werkstoffforschung, Helmholtz-Zentrum Geesthacht, Notkestrasse 85, 22607 Hamburg, Germany
Background Sea urchins (Echinodermata: Echinoidea) constitute a major part of the marine benthic fauna and can be found from the tropics to the polar regions in shallow waters as well as the deep sea. They have traditionally served as model organisms for a variety of developmental, physiological, and evolutionary studies. The excellent fossil record of this group permits to calibrate molecular clock hypotheses as well as to improve statistical models used in phylogenetic inferences. In order to elucidate the evolutionary interrelationships of the various sea urchin taxa, a number of independent datasets have recently been gathered and analyzed: hard part anatomy, molecular data, pedicellariae, larvae, and soft part anatomy. The aim of the current project was (i) to gather comparative datasets of selected sea urchin species using µCT and SRµCT, (ii) to determine to what extent wet museum material could be incorporated into such studies, and (iii) to assess if large-scale scanning of hard parts could yield novel information on sea urchin systematics.
Figure 1: Initial results of micro-computed tomography scanning using the common sea urchin species Psammechinus miliaris (Gmelin, 1778). µCT-based 3D renderings of the calcite endoskeleton: aboral view (A) and virtually dissected lateral view revealing the internal masticatory apparatus in situ (B). Comparison of reconstructed image data derived from µCT (C) and SRµCT (D).
Results and Discussion For the preliminary comparison of results derived from SRµCT and µCT, the common sea urchin species Psammechinus miliaris (Gmelin, 1778) (Echinoidea: Parechinidae) was chosen. Initial µCT-based 3D renderings revealed that scanning on the cone beam apparatus would be sufficient to reveal the gross morphology of the calcite endoskeleton (Fig. 1A) as well as that of the internal masticatory apparatus (Fig. 1B) - two structures of prime interest for sea urchin hard part systematics. However, the data derived from µCT (Fig. 1C) did not match those acquired using SRµCT (Fig. 1D), in particular with regard to density information, brilliance, and voxel dimension. While the higher resolution obtainable using SRµCT is not essential for basic morphological comparisons, fine structural differences can only be resolved using SRµCT. A good example for this are sea urchin teeth (Fig. 1D): fine structural differences such as density variation within the tooth could be used to complement existing character matrices. However, as SRµCT beam time constitutes a precious resource and the specimens under study ranged from about 1 - 10 cm in diameter, the decision was made to use the cone beam µCT setup for all subsequent scans. The preliminary data on Psammechinus miliaris furthermore revealed that because of the relatively high X-ray attenuation of sea urchin hard parts, all specimens could be scanned in ethanol - thus permitting the inclusion of wet museum material with its internal hard part morphology in situ.
Figure 2: Large-scale µCT scanning using the cone beam setup of sea urchins reveals distinct shapes and forms of their endoskeleton in 3D. (A) Phyllacanthus imperialis (Lamarck, 1816), Cidaroida; (B) Salenia goesiana Lovén, 1874, Salenioida; (C) Arbacia dufresnii (Blainville, 1825), Arbacioida; (D) Echinometra mathaei (Blainville, 1825), Echinoida; (E) Rotula deciesdigitata (Leske, 1778), Clypeasteroida; (F) Nacospatangus alta (Agassiz, 1863), Spatangoida.
Clearly, the combination of µCT and SRµCT allows for a more extensive study of museum specimens. Using the cone beam source, a representative selection of sea urchin species (162 taxa in total) was scanned, allowing primarily their gross morphological comparison. The more detailed SRµCT datasets of selected species will yield additional fine structural data with higher quality. Our morphological studies reveal that a number of hard part features have so far not been recognized in sea urchin systematics and work is currently in progress to describe these novel characters [2].
References
[1] A. Ziegler, M. Ogurreck, T. Steinke, F. Beckmann, S. Prohaska, and A. Ziegler, Biol. Direct 5, 45 (2010). [2] A. Ziegler. Proceedings of the 7th European Conference on Echinoderms, submitted.