Systematic distribution of birefringent bodies in Rotifera and first evidence of their ultrastructure in Acyclus inquietus (Gnesiotrocha: Collothecaceae)
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Small birefringent concretions have been reported in rotifers for over a century and often hypothesized as energy sources. Here, we provide an update on their distribution in superorder Gnesiotrocha and the first data on their ultrastructure. Within Gnesiotrocha, these birefringent bodies (BRB) are known from at least ten species of Collothecaceae and 14 species of Flosculariaceae, both of which include planktonic and sessile species. Among sessile species, the predator Acyclus inquietus contains a single BRB that has been described as starch-like. We examined larvae of A. inquietus with transmission electron microscopy and revealed the BRB to have an irregular, electron-dense margin that surrounds a speckled core. The core appears mostly amorphous, but contains numerous, very small electron-dense spots and thin electron-dense fibers; there is no evidence of any crystalline lattice. The intestinal lumen contains smaller concretions that are probably the result of BRB metabolism. The thin epithelium contains abundant electron-dense granules but relatively few organelles. We hypothesize that the BRB is a unique form of extracellular glycogen that functions as an energy source in larvae for their dispersal and metamorphosis. In adult A. inquietus, the BRB may provide energy permitting reproduction when prey are no longer available.
KeywordsAnisotropic Glycogen Indirect life cycle Lecithotrophic Planktonic Sessile
We thank the editor and two anonymous reviewers for critical comments that dramatically improved this manuscript. We also thank the participants of the Rotifera XV Symposium for their ideas on the topic of birefringent bodies. We acknowledge the staff at the Core Electron Microscope Facility (UMM) for their assistance in the use of their TEM. Collections at Hueco Tanks State Park and Historic Site were made under Texas Parks and Wildlife permit TPWD 2016-03 (E.J. Walsh). Kevin Bixby facilitated sampling at La Mancha Wetland. Funding was provided from the National Science Foundation (NSF), DEB Grants 1257110 (R. Hochberg), 1257068 (E.J. Walsh), and 1257116 (R.L. Wallace), and by Grant 5G12MD007592 (E.J. Walsh) from the National Institutes on Minority Health and Health Disparities (NIMHD), a component of the National Institutes of Health (NIH). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF or NIH.
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Conflict of interest
The authors declare that they have no conflicts of interest.
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