Abstract
Representatives of Eunicida have the most complex jaw apparatus among the Annelida group. The general morphology of their jaws is well studied and is important both for the classification of recent species and for the evolutionary interpretation of extant–extinct eunicidan relationships. The fine structure of the jaws can be useful as an additional trait to external morphology that allows to clear up the evolutionary relationships within the group. However, the ultrastructural data remain absent for the Lumbrineridae and Hartmaniellidae, which are also the only families with symmetrognathous jaws. In the present study, we describe the fine structure of the jaws of Scoletoma fragilis from the Lumbrineridae family. More than 40 S. fragilis specimens, from juvenile to adult, were studied with different morphological approaches, concentrating on electron microscopy. We have distinguished three stages of jaw structure, depending on the worm size: juvenile, subadult, and adult jaws. The juvenile jaws had the simplest structure, consisting of only scleroproteins. The adult ones had a more complex and multilayered structure that varied in different areas of the jaw apparatus. The subadult jaws exhibited the intermediate fine structure between juvenile and adult ones that raises the question of continuous growth possibility, which is also discussed in the article. Whereas juvenile and massive jaws share the same pattern as the other eunicidan families studied to date, the most interesting finding was the new, undescribed type of jaw structure found in the elastic elements of adult maxillary apparatus. It is characterized by heterogeneous sclerotization with a lack of mineralization.
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All data supporting the findings of this study are available within the paper and its Supplementary Information. Full series of semi-thin sections available from corresponding author upon request.
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Acknowledgements
We thank the staff of the Laboratory of Electron Microscopy at Lomonosov Moscow State University. The material collection would have been impossible without the research vessel crew “Prof. Zenkevitch” and “Student MSU” from WSBS MSU. Additionally, we want to thank Elena Vortsepneva, Glafira Kolbasova, Ekaterina Nikitenko, and Feodor Plandin (Lomonosov Moscow State University) for their help with TEM and microCT. Finally, we are very grateful for the comments of two anonymous reviewers, which helped to improve the manuscript. This work was supported by the Russian Science Foundation Grant No 21-14-00042.
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This work was supported by the Russian Science Foundation Grant No 21-14-00042.
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A.T. proposed the main concept of the study. A.K. and A.T. collected and prepared material for the study. A.K. prepaired figures and wrote the draft of manuscript. Both authors discussed, reviewed, and corrected the manuscript.
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Koroleva, A., Tzetlin, A. Jaw apparatus of Scoletoma fragilis (Lumbrineridae, Annelida): fine structure and growth. Zoomorphology (2024). https://doi.org/10.1007/s00435-024-00651-w
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DOI: https://doi.org/10.1007/s00435-024-00651-w