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Modern holothurian calcareous ring anatomy and stereom structure - the need for more detailed studies and research

Authors:
  • State Museum of Natural History Braunschweig
... The nervous ring lies in the buccal membrane, inside the bulb, and the nerves branch out toward the tentacles and the ambulacra; the latter leaving the bulb via the anterior notch. As a comparison to other orders, the Apodida have a perforation instead of a notch on the radial plate for the passage of nerves (they lack tube feet so there is no need for ampullae), the tentacular ampullae in the Aspidochirotida are large and located in the body cavity, and the GV in the CR of some Molpadida are prominent Reich, 2015;Reich & O'Loughlin, 2010). ...
Article
Despite descending from heavily calcified ancestors, the holothuroid skeleton is fully internal and composed of microscopic ossicles and a ring of plates bound by connective tissue, the calcareous ring. The calcareous ring exhibits a complex and poorly understood morphology; as a result, establishing unambiguous homology statements about its macrostructure has been challenging and phylogenetic studies have had to simplify this important structure. Here, we provide the first broad comparative study of Dendrochirotida calcareous rings using micro-computed tomography (μCT). A detailed description of the three-dimensional macrostructure of the calcareous ring of 10 sea cucumber species, including rare and type specimens, is presented. The structures observed were highly variable at the subfamily level, especially at the point of tissue attachment. The relationship between the calcareous ring and its associated organs, and their functional morphology are discussed. To aid future phylogenetic studies, we listed 22 characters and performed a preliminary cladistic analysis. The topology obtained supports the idea that the simple, cucumariid ring is ancestral to the mosaic-like phyllophorid ring; however, it did not support the monophyly of the cucumariids. It also did not support the family Sclerodactylidae, which was described based on the ring morphology. Differently from the dermal ossicles, which are highly homoplastic, the general homoplasy index of the calcareous ring characters was relatively low. This result highlights the importance of this structure for phy-logenetic inference. Unfortunately, time since collection, rough collection methods and fixation can damage the skeleton, and the calcareous ring is often overlooked in taxonomic descriptions. The data presented here will improve our understanding of holothuroid relationships and facilitate studies on holothuroid functional morphology and biomechanics. K E Y W O R D S comparative morphology, Holothuroidea, internal anatomy, phylogeny, skeleton, three-dimensional imaging, μCT 1 | INTRODUC TI ON Echinoderms are characterized by their complex and lightweight en-doskeleton, formed by an intricate mesh of magnesium-rich calcium carbonate-the stereom-which most likely evolved after the split between the echinoderms and the other deuterostomes (David & Mooi, 1998; Nebelsick et al., 2015; Smith, 1990). Because it is more easily preserved and identified than other characteristic echinoderm
... The nervous ring lies in the buccal membrane, inside the bulb, and the nerves branch out toward the tentacles and the ambulacra; the latter leaving the bulb via the anterior notch. As a comparison to other orders, the Apodida have a perforation instead of a notch on the radial plate for the passage of nerves (they lack tube feet so there is no need for ampullae), the tentacular ampullae in the Aspidochirotida are large and located in the body cavity, and the GV in the CR of some Molpadida are prominent Reich, 2015;Reich & O'Loughlin, 2010). ...
... With the wellsupported holothuroid higher-level phylogeny presented here, transformation analyses of other morphological features may prove rewarding. For example, some have proposed that the morphology of the calcareous ring may have high systematic value (Pawson and Fell, 1965;Reich, 2015;Reich and O'Loughlin, 2010;Smirnov, 2012). ...
Article
Sea cucumbers (Holothuroidea) are a morphologically diverse, ecologically important, and economically valued clade of echinoderms; however, the understanding of the overall systematics of the group remains controversial. Here, we present a phylogeny of extant Holothuroidea assessed with maximum parsimony, maximum likelihood, and Bayesian approaches using approximately 4.3 kb of mt- (COI, 16S, 12S) and nDNA (H3, 18S, 28S) sequences from 82 holothuroid terminals representing 23 of the 27 widely-accepted family-ranked taxa. Currently five holothuroid taxa of ordinal rank are accepted. We find that three of the five orders are non-monophyletic, and we revise the taxonomy of the groups accordingly. Apodida is sister to the rest of Holothuroidea, here considered Actinopoda. Within Actinopoda, Elasipodida in part is sister to the remaining Actinopoda. This latter clade, comprising holothuroids with respiratory trees, is now called Pneumonophora. The traditional Aspidochirotida is paraphyletic, with representatives from three orders (Molpadida, Dendrochirotida, and Elasipodida in part) nested within. Therefore, we discontinue the use of Aspidochirotida and instead erect Holothuriida as the sister group to the remaining Pneumonophora, here termed Neoholothuriida. We found four well-supported major clades in Neoholothuriida: Dendrochirotida, Synallactida, Molpadida and a new taxon, Persiculida. The mapping of traditionally-used morphological characters in holothuroid systematics onto the phylogeny revealed marked homoplasy in most characters demonstrating that further taxonomic revision of Holothuroidea is required. Two time-tree analyses, one based on calibrations for uncontroversial crown group dates for Eleutherozoa, Echinozoa and Holothuroidea and another using these calibrations plus four more from within Holothuroidea, showed major discrepancies, suggesting that fossils of Holothuroidea may need reassessment in terms of placing these forms with existing crown clades.
... With a few exceptions (Ludwig, 1889-1891: 79ff.; Hyman, 1955: 138ff.), no general overview of this structure has been undertaken since the initial descriptions and monographs of modern holothurian faunas in the late 19 th and early 20 th centuries, and the three-dimensional anatomy and organization of the calcareous ring (Fig. 1A, D, F, H) remains unclear in nearly all recent sea cucumber species (Reich and O'Loughlin, 2010). In the last few decades, this gap in knowledge has resulted in anatomical misinterpretations in modern species (e.g., Gage and Billett, 1986), as well as misinterpretations and/or incorrect reconstructions of fossil holothurian material (e.g., Haude, 1992 Haude, , 1997 Boczarowski, 1997 Boczarowski, , 2001 Jell, 2010). ...
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