Magnetic resonance imaging (MRI) has failed to distinguish between smaller gut regions and larger haemal sinuses in sea urchins (Echinodermata: Echinoidea)
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A response to Ziegler A, Faber C, Mueller S, Bartolomaeus T: Systematic comparison and reconstruction of sea urchin (Echinoidea) internal anatomy: a novel approach using magnetic resonance imaging. BMC Biol 2008, 6: 33.
KeywordsInternal Anatomy Modern Imaging Technique Marginal Sinus Comparative Morphological Analysis Magnetic Resonance Imaging Section
The sea urchin siphon is a narrow-bore tube that originates near the esophagus-stomach junction, runs parallel to the stomach, and finally rejoins the main course of the gut at the beginning of the intestine. Although a siphon is present in most sea urchins, its place is taken by a siphonal groove in the following three major clades (traditionally ranked as families): Cidaridae , Diadematidae , and Pedinidae . The presence of a siphonal groove in the Diadematidae recently became controversial when two publications [4, 5] claimed that such sea urchins actually have a siphon instead and that our previous report to the contrary  was based on a "mistaken observation."
We responded to the criticism in [4, 5] with a rebuttal  using scanning electron microscopy and histological sectioning to support our original conclusion that diadematids have a siphonal groove and not a siphon. In spite of our rebuttal, the opposing claim – that diadematids have a siphon rather than a siphonal groove – has since been perpetuated in a paper published in BMC Biology . Thus the present correspondence presents additional evidence in hope of finally laying this controversy to rest. We also consider the reasons why such strikingly discordant views of sea urchin morphology could have arisen in the first place.
The authors who disagree with us [4, 5, 6] studied the sea urchin digestive tract without considering an intimately related component of the haemal system: namely the inner marginal sinus, which runs along the adaxial side of the stomach. Moreover, those authors used either gross dissection [4, 5] or presented horizontal MRI sections , showing only the outer contours of the gut without providing any information on the internal details. Such details are indispensable for distinguishing the digestive tract, which is lined by an epithelium  from the haemal system, which is not . Thus, the data presented in [4, 5, 6] could not distinguish the inner marginal sinus from the gut.
Magnetic resonance imaging: a powerful tool in comparative morphology despite initial interpretative difficulties
Alexander Ziegler* 1, Thomas Bartolomaeus2
1Institut für Immungenetik, Charité-Universitätsmedizin Berlin, Berlin, Germany
2Institut für Evolutionsbiologie und Zooökologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany
Drs. Holland and Ghiselin have convincingly pointed out, initially as a direct response to Drs. Campos and Moura , and now in the correspondence above, that Pedinidae and Diadematidae are indeed characterized by the presence of a siphonal groove rather than a siphon. Their data effectively challenge our interpretation of these structures in Caenopedina mirabilis (Pedinidae) and Diadema savignyi (Diadematidae), as presented in our BMC Biology paper [, Fig. 3]. The images of 3D models depicted in this article were based exclusively on datasets generated using magnetic resonance imaging (MRI). We are happy to see these points of interpretation corrected and this issue of minor controversy resolved.
For some analyses, however, invasive techniques are not appropriate. Possibly, the most important attribute of modern imaging techniques is their non-invasive nature, which permits the study of rare specimens and the elucidation of the original topography of organ systems. This is of particular importance in organisms whose soft tissue structures are enclosed by skeletal elements, as is the case in sea urchins.
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