Wiring a Periscope – Ocelli, Retinula Axons, Visual Neuropils and the Ancestrality of Sea Spiders
Tobias Lehmann 1,2*, Martin Heß 2, Roland R. Melzer 1
1 Bavarian State Collection of Zoology, Munich, Germany, 2Department Biology I, Ludwig-Maximilians-Universität München, Planegg-Martinsried, Germany
Abstract
The Pycnogonida or sea spiders are cryptic, eight-legged arthropods with four median ocelli in a ‘periscope’ or eye tubercle. In older attempts at reconstructing phylogeny they were Arthropoda incertae sedis, but recent molecular trees placed them as the sister group either to all other euchelicerates or even to all euarthropods. Thus, pycnogonids are among the oldest extant arthropods and hold a key position for the understanding of arthropod evolution. This has stimulated studies of new sets of characters conductive to cladistic analyses, e.g. of the chelifores and of the hox gene expression pattern. In contrast knowledge of the architecture of the visual system is cursory. A few studies have analysed the ocelli and the uncommon “pseudoinverted” retinula cells. Moreover, analyses of visual neuropils are still at the stage of Hanström's early comprehensive works. We have therefore used various techniques to analyse the visual fibre pathways and the structure of their interrelated neuropils in several species. We found that pycnogonid ocelli are innervated to first and second visual neuropils in close vicinity to an unpaired midline neuropil, i.e. possibly the arcuate body, in a way very similar to ancestral euarthropods like Euperipatoides rowelli(Onychophora) and Limulus polyphemus (Xiphosura). This supports the ancestrality of pycnogonids and sheds light on what eyes in the pycnogonid ground plan might have ‘looked’ like. Recently it was suggested that arthropod eyes originated from simple ocelli similar to larval eyes. Hence, pycnogonid eyes would be one of the early offshoots among the wealth of more sophisticated arthropod eyes.
Citation: Lehmann T, Heß M, Melzer RR (2012) Wiring a Periscope – Ocelli, Retinula Axons, Visual Neuropils and the Ancestrality of Sea Spiders. PLoS ONE 7(1): e30474. doi:10.1371/journal.pone.0030474
Editor: Eric James Warrant, Lund University, Sweden
Received: October 17, 2011; Accepted: December 19, 2011; Published: January 18, 2012
Copyright: © 2012 Lehmann et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was supported by a Graduiertenstipendium (BayEFG) from the Universität Bayern e.V. to T. Lehmann, and by the Deutsche Forschungsgemeinschaft (DFG Me 2683/6-1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: lehmann@zsm.mwn.de
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