Systematic distribution of birefringent bodies in Rotifera and first evidence of their ultrastructure in Acyclus inquietus (Gnesiotrocha: Collothecaceae).

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. [ABSTRACT FROM AUTHOR]