Abstract
Orthonectida is an enigmatic group of parasitic invertebrates with an unclear taxonomic position. Recent molecular studies demonstrated that Orthonectida belongs to Annelida; however, the lack of morphological data does not allow to follow the evolutionary pathway from free-living annelids to parasitic orthonectids. Here, we studied the nervous and the muscular systems in the male and female orthonectid Rhopalura litoralis using confocal laser scanning microscopy and immunohistochemistry. The muscular system is formed by four outer longitudinal muscular bundles and several inner transversal muscles. The nervous system of females is represented by a well-developed cerebral ganglion and a nerve plexus in the body. In males, the cerebral ganglion is significantly smaller, and the body plexus is absent. Instead, a pair of nerves with three pairs of serially organized nerve cells runs posteriorly from the ganglion along the lateral sides of the body. Analyses of the structure of all the orthonectids studied so far suggest that reduction and simplification of the free-living males and females are the dominant mode of evolution in orthonectids.
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The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Acknowledgements
We are thankful to Dr. M. Makarov for providing accommodation and facilities for field work at the Barents Sea and to the staff of the Marine Biological Station of the Saint Petersburg State University for the assistance in collecting the material. The study was performed at the Core Facility Centers for Microscopy and Microanalysis, Center for Molecular and Cell Technologies, center “CHROMAS,” and center “Culture Collection of Microorganisms” of St. Petersburg State University. The research was partially supported by the budget program AAAA-A19-119020690076-7 to VVS. The main financial support for this study was provided by the Russian Foundation for Basic Research (RFBR) grant № 19-04-00218 and Russian Science Foundation grant 19-74-10013 to N.I.B.
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13127_2021_519_MOESM1_ESM.png
Supplementary file1 (PNG 505 KB). Partial z-projection of a lateral side of a female Rhopalura littoralis. The arrowheads label places where the striation of the muscular cells is clearly seen.
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Slyusarev, G.S., Bondarenko, N.I., Skalon, E.K. et al. The structure of the muscular and nervous systems of the orthonectid Rhopalura litoralis (Orthonectida) or what parasitism can do to an annelid. Org Divers Evol 22, 35–45 (2022). https://doi.org/10.1007/s13127-021-00519-7
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DOI: https://doi.org/10.1007/s13127-021-00519-7