Search

Gnesiotrochan rotifers display a variety of life styles ranging from taxa with free-swimming larval and sessile adult stages to those with motile adult stages and colonial habits. Several explanations for the C-value enigma posit that genome size is correlated with lifestyle. To investigate this, 13 gnesiotrochan species representing nine genera were measured by flow cytometry. Genome sizes (1C) within Gnesiotrocha ranged from 0.05 pg (Hexarthra mira and Hexarthra fennica) to 0.25 pg (Sinantherina ariprepes). Genome sizes varied within genera and species; e.g., the H. fennica (El Huérfano, Mexico) genome was estimated to be 15% larger than that of H. mira and H. fennica (Keystone Wetland, TX, USA). Gnesiotrochan genome sizes are similar to those reported within Ploima, which range from 0.06 pg (Brachionus rotundiformis, B. dimidiatus) to 0.46 pg (B. asplanchnoidis). Within Gnesiotrocha, genome size was found to be significantly smaller in sessile versus motile species as well as in solitary versus colonial species. To account for phylogenetic background, linear mixed models with hierarchical taxonomic ranks showed that there is a taxonomic component underlying genome size. This study provides the first estimates of genome size within the superorder, providing a baseline for genomic and evolutionary studies within the group. [ABSTRACT FROM AUTHOR]

Rotifers are diverse and abundant aquatic micrometazoans that rely on their ciliated apical end (corona) for locomotion and feeding. In order Collothecaceae, which includes mostly sessile species, larval rotifers go through a complex metamorphosis after settlement wherein they replace their corona with an unusual cup-shaped head that functions exclusively in food capture. This new head, called the infundibulum, consists of morphological elaborations such as lobes or tentacles that develop precociously in the larval stage; in adults they function in ambush and sit-and-wait predation. Here, we provide evidence from brightfield and transmission electron microscopy that the infundibulum of collothecid rotifers is derived from the larval foregut and not the larval corona, suggesting that the adult head of collothecids is a morphological novelty and therefore not homologous with the rotifer corona as classically defined. The wide variety of morphologies of the infundibulum suggests that selection to maximize foraging may have driven the evolution of head form in these sessile species, and that future studies of collothecid rotifers should consider the infundibulum as a unique form of food-collection device that is evolutionarily distinct from the rotifer corona. [ABSTRACT FROM AUTHOR]

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]