Frontiers in Zoology BioMed Central Research Open Access Tiny but complex - interactive 3D visualization of the interstitial acochlidian gastropod Pseudunela cornuta (Challis, 1970) Timea P Neusser*1,2, Martin Heß2 and Michael Schrödl1,2 Address: 1Zoologische Staatssammlung München, Münchhausenstr. 21, 81247 München, Germany and 2Department Biology I, Ludwig- Maximilians-Universität München, Grosshadernerstr. 2, 82152 Planegg-Martinsried, Germany Email: Timea P Neusser* - [email protected]; Martin Heß - [email protected]; Michael Schrödl - [email protected] * Corresponding author Published: 11 September 2009 Received: 12 March 2009 Accepted: 11 September 2009 Frontiers in Zoology 2009, 6:20 doi:10.1186/1742-9994-6-20 This article is available from: http://www.frontiersinzoology.com/content/6/1/20 © 2009 Neusser et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: Mesopsammic acochlidians are small, and organ complexity may be strongly reduced (regressive evolution by progenesis), especially in microhedylacean species. The marine interstitial hedylopsacean Pseudunela cornuta (Challis, 1970), however, was suggested as having a complex reproductive system resembling that of much larger, limnic and benthic species. The present study aims to reconstruct the detailed anatomy and true complexity of P. cornuta from serial, semithin histological sections by using modern computer-based 3D visualization with Amira software, and to explain it in an evolutionary context. Results: Our results demonstrate considerable discordance with the original species description, which was based solely on paraffin sections. Here, we show that the nervous system of P. cornuta has paired rhinophoral, optic and gastro-oesophageal ganglia, three distinct ganglia on the visceral nerve cord, and a putative osphradial ganglion, while anterior accessory ganglia are absent. The presence of an anal genital cloaca is clearly rejected and the anus, nephropore and gonopore open separately to the exterior; the circulatory and excretory systems are well-differentiated, including a two-chambered heart and a complex kidney with a long, looped nephroduct; the special androdiaulic reproductive system shows two allosperm receptacles, three nidamental glands, a cavity with unknown function, as well as highly complex anterior copulatory organs with two separate glandular and impregnatory systems including a penial stylet that measures approximately a third of the whole length of the preserved specimen. Conclusion: In spite of its small body size, the interstitial hermaphroditic P. cornuta shows high complexity regarding all major organ systems; the excretory system is as differentiated as in species of the sister clade, the limnic and much larger Acochlidiidae, and the reproductive system is by far the most elaborated one ever observed in a mesopsammic gastropod, though functionally not yet fully understood. Such organ complexity as shown herein by interactive 3D visualization is not plesiomorphically maintained from a larger, benthic ancestor, but newly evolved within small marine hedylopsacean ancestors of P. cornuta. The common picture of general organ regression within mesopsammic acochlidians thus is valid for microhedylacean species only. Page 1 of 17 (page number not for citation purposes) Frontiers in Zoology 2009, 6:20 http://www.frontiersinzoology.com/content/6/1/20 Background highly complex copulatory systems that culminated in a The meiofauna of marine sands includes species of nearly giant, trap-like "rapto-penis". all taxa of invertebrates, many of which show regressive characteristics in their anatomy or specialized features in Methods their organ systems [1]. Compared to their supposed basal Sampling and specimen preparation opisthobranch relatives [2,3], mesopsammic acochlidian During an expedition to Guadalcanal, Solomon Islands in sea slugs display many of such reductions, e.g., they have October 2007, two specimens of Pseudunela cornuta were a small and worm-like body, lack a shell, are unpig- collected at the beach of Komimbo Bay near Tambea Vil- mented, cephalic tentacles and eyes are reduced in several lage (09°15.843'S, 159°40.097'E). They were extracted lineages, many species are aphallic, and in general, the from sand samples (fine sand of the lower intertidal) reproductive, excretory and circulatory systems have a according to Schrödl [13] and relaxed using 7% MgCl2 very simple organization. Due to such reductions, which solution. Both specimens were preserved in 75% ethanol. are especially pronounced in one subclade, the Micro- hedylacea, the Acochlidia were hypothesized to have Later in the laboratory, the visceral sac of one specimen undergone "regressive evolution" [4], as a result of pro- was removed for further molecular analysis. The remain- genesis [5]. However, several recent studies [6,7] show ing anterior body and the other entire specimen were that original, macroscopic or paraffin-based histological decalcified with Bouin's solution overnight. For better vis- descriptions of small acochlidian species could hardly ibility of the translucent specimens and an appropriate give a reliable picture even of simple organs. In contrast, orientation during the embedding procedure, the material computer-based 3D-reconstruction of serial semithin his- was stained with Safranin (0.5% Safranin in 80% ethanol) tological slices is highly efficient to obtain detailed and for a few minutes and rinsed with 80% ethanol. Finally, reliable knowledge even on tiny and complex structures, the two specimens (in one case only anterior part) were such as the considerably differentiated acochlidian central dehydrated in a graded series of acetone in distilled water nervous system [8-10]. (80, 90 and 100%) and embedded in Spurr's low viscosity resin [14]. Two series of ribboned serial semithin sections Species of the second acochlidian subclade, the Hedylop- of 1.5 m thickness were prepared using a diamond knife sacea, may show fewer tendencies for reductions; in con- (Histo Jumbo, Diatome, Biel, Switzerland) and contact trast to the microhedylaceans, the circulatory and cement at the lower cutting edge [15], and finally stained excretory systems, and reproductive and copulatory with methylene blue-azure II according to Richardson et organs may be highly complex and are derived especially al. [16]. The sections were deposited at the Zoologische in members of the Acochlidiidae s.l., a clade of larger- Staatssammlung München, Mollusca Section (entire spec- sized, benthic, limnic members [3]. According to a phylo- imen: ZSM N° 20071911 and anterior body: ZSM N° genetic analysis [3], the genus Pseudunela is the sistergroup 20071809). to such derived acochlidians, despite species of Pseudunela being small, marine, interstitial forms. Only two Pseu- 3D reconstruction dunela species are known, P. eirene Wawra, 1988 [11] and Digital photographs of every slice (420 images in total) P. cornuta [12]. The description of P. eirene is brief and were taken with a CCD microscope camera (Spot Insight, based on a single specimen with ganglia of the nervous Diagnostic Instruments, Sterling Heights, USA) mounted system and stylets of copulatory organs studied on a on a DMB-RBE microscope (Leica Microsystems, Wetzlar, whole-mount by light microscopy only. No histological Germany). The image resolution was reduced to 1120 × sections were made, and the radula was studied light- 840 pixels (resulting pixel size: 0.8 m) and images were microscopically after dissolving the soft parts and stylets. contrast enhanced, unsharp masked and converted to 8bit Information on other organ systems is absent, and no fur- greyscale format with standard image editing software. A ther specimens are available for study. In contrast, the detailed computer-based 3D-reconstruction of all major original description of P. cornuta, the type species, is based organ systems was conducted with the software AMIRA on paraffin sections, and quite detailed data about the 4.1 and 5.2 (TGS Europe, Mercury Computer Systems, central nervous and the digestive systems is included. Merignac Cedex, France) following basically the proce- However, information about the excretory system is frag- dure explained by Ruthensteiner [15]. The interactive 3D mentary and improper, and data about the reproductive model for the electronic 3D PDF version were prepared system is confusing. Well-preserved specimens of P. cor- using the 3D tools of Adobe Acrobat Professional nuta were made available for detailed 3D-reconstruction. Extended 9.0 (Adobe Systems Incorporated) according to The present study thus explores the complex anatomy and Ruthensteiner & Heß [17]. The 3D model (accessible by potential role of a member of the stemgroup of a radiation clicking onto Fig. 1 in the 3D PDF version of this article; that accounted for major evolutionary changes, i.e. a hab- see also additional files 1 and 2) permits standard opera- itat switch to freshwater systems and an evolution towards tions as zoom and rotation, the selection of the recon- Page 2 of 17 (page number not for citation purposes) Frontiers in Zoology 2009, 6:20 http://www.frontiersinzoology.com/content/6/1/20 3DFigure reconstruction 1 of the general anatomy, the CNS and the digestive system of