Search for: All records

Genus Keratella comprises 55 species of rotifers, 53 of which possess a dome-shaped, dorsal plate divided by slightly raised ridges into a network of polyhedral units (facets). Thus, most species resemble an elongate, truncated, geodome, with facets defining their facet pattern (FP). Traditionally, FP morphospace has been categorized into groups based on their archetypical model: K. cochlearis (cFP); K. quadrata (qFP); K. serrulata (sFP). A previous study posited that Keratella FP provide functional significance of resisting physical stress, e.g., that may be incurred when caught in the brachial chambers of Cladocera. Using finite elemental analysis as applied by the architectural program (SkyCiv©), they tested whether a geodome resembling K. cochlearis could meet architectural code, i.e., a proxy for structural stability. Here we extended that study by testing the ability of 20 Keratella-like geodomes to meet architectural code. Of these, 14 met code at all stress levels, three failed at level 3, two failed at 4, and one failed at all levels. We posit that the three FP groups represent adaptive peaks that can resist physical stress that reside within the fitness landscape of Keratella FP morphospace. Morphological FP variants and the two species without FPs represent locally adapted sub-optima. 

Abstract Diapausing embryos of sessile rotifers remain poorly characterized, limiting comparative analyses of diapause strategies across Monogononta. We examined the diapausing embryos of four sessile species—Collotheca monastica,Ptygura crystallina,Ptygura ctenoida,andPtygura thalenoiensis—using light microscopy, electron-based imaging, and atomic force measurements to characterize eggshell morphology, layering, and mechanical properties. These approaches reveal either a three-layered, single-piece eggshell or a two-layered bivalved configuration, together with marked differences in stiffness, adhesion, and deformation modes. Comparisons with planktonic rotifers indicate that sessile species generally produce more elongated diapausing embryos with thinner eggshell layers, whereas planktonic taxa tend to retain thicker and more ornamented eggshells. These differences may reflect the distinct protective contexts in which embryos develop. Our observations also suggest a possible correspondence between the eggshell layers of sessile and planktonic lineages, although confirming such homology would require broader taxonomic sampling and ultrastructural comparisons. Variation in tube geometry and firmness probably parallels differences in embryo shape. Ornamentation is presumably less developed in species whose tubes or substrates provide stronger protection. Together, these results provide a comparative framework linking eggshell structure, mechanical performance, and protective environment across sessile and planktonic lineages. 

Abstract Rotifers are aquatic micrometazoans with a variety of feeding styles and dietary preferences. The morphology of their digestive tracts is well known, but there are few details on the structure of any organs. Here we use transmission electron microscopy and fluorescence microscopy to investigate the stomach of Epiphanes clavulata, which is unusual in its possession of 6 diverticula that are absent in other taxa. We focus on this region to determine whether these diverticula are exocrine and potentially homologous to gastric glands, which are the main digestive organs of rotifers, but absent in this species. Results reveal that the stomach is cellular and that all cells are ciliated, nucleated, and contain abundant phagosomes and lipid droplets. The stomach appears to play 2 major roles: intracellular digestion as evidenced by the presence of caveolae and phagosomes and lipid biogenesis and storage as evidenced by accumulation of large lipid droplets. The 6 diverticula are syncytial extensions of individual stomach cells and contain 1 to several nuclei, many mitochondria, autophagic vesicles, ribosomes, and inclusion bodies. The rough endoplasmic reticulum is abundant and associated with small electron-dense droplets enclosed in a phospholipid monolayer, suggesting they contain lipids; however, the absence of discernable staining with a neutral lipid dye confounds a more specific identification of their contents. There is no evidence to suggest the diverticula are homologous to gastric glands nor do they play an obvious role in digestion; instead, they may function as a secondary site of lipid biogenesis and storage. 

Understanding the general biology, biodiversity, ecology, and evolutionary history of organisms necessitates correct identification. Found worldwide in fresh, brackish, and some marine waters, rotifers can be difficult to identify due to their small size, complex characteristics, and dearth of keys to their identification. Moreover, many species lack a hard body wall (i.e., illoricate species), thus they are nearly impossible to identify when preserved. As a result detailed study of many illoricate rotifers is wanting. This is especially acute for the sessile rotifers where quality illustrations, either as line art or light or scanning electron photomicrographs, of adults and trophi is deficient. This leads to a serious impediment in providing a comprehensive accounting for some species. Lacinularia and Sinantherina (Monogononta; Gnesiotrocha; Flosculariidae) are two sessile genera in which the literature provides inadequate treatment. In this contribution we (1) provide simple, dichotomous keys for the identification of all valid species of both genera and (2) present collated information on their morphology thereby detailing where additional research is needed. Both keys focus on easily observable characters of adult female morphology, including features of their coronae, antennae, colony formation behaviors, and presence/absence of eyespots in the adults. We hope that our effort promotes additional research on these two genera, including better documentation of their trophi and general body morphology.   

Abstract Genome size is an important correlate of many biological features including body size, metabolic rate, and developmental rate, and can vary due to a variety of mechanisms, including incorporation of repetitive elements, duplication events, or reduction due to selective constraints. Our ability to understand the causes of genome size variation are hampered by limited sampling of many non-model taxa, including monogonont rotifers. Here we used high throughput Nanopore sequencing and flow cytometry to estimate genome sizes of nine species of monogonont rotifers representing seven families, including three representatives of Superorder Gnesiotrocha. We annotated the genomes and classified the repetitive elements. We also compared genome size with two biological features: body size and metabolic rate. Body sizes were obtained from the literature and our estimates. Oxygen consumption was used as a proxy for metabolic rate and was determined using a respirometer. We obtained similar genome size estimates from genome assemblies and flow cytometry, which were positively correlated with body size and size-specific respiration rate. Importantly, we determined that genome size variation is not due to increased numbers of repetitive elements or large regions of duplication. Instead, we observed higher numbers of predicted proteins as genome size increased, but currently many have no known function. Our results substantially expand the taxonomic scope of available genomes for Rotifera and provide opportunities for addressing genetic mechanisms underlying evolutionary and ecological processes in the phylum. 

Abstract Rotifers possess complex morphologies despite their microscopic size and simple appearance. Part of this complexity is hidden in the structure of their organs, which may be cellular or syncytial. Surprisingly, organs that are cellular in one taxon can be syncytial in another. Pedal glands are widespread across Rotifera and function in substrate attachment and/or egg brooding. These glands are normally absent inAsplanchna, which lack feet and toes that function as outlets for pedal glandular secretions in other rotifers. Here, we describe the ultrastructure of a pedal gland that is singular and syncytial inAsplanchnaaff.herricki, but is normally paired and cellular in all other rotifers.Asplanchnaaff.herrickihas a single large pedal gland that is active and secretory; it has a bipartite, binucleate, syncytial body and a cytosol filled with rough endoplasmic reticulum, Golgi, and several types of secretory vesicles. The most abundant vesicle type is large and contains a spherical electron‐dense secretion that appears to be produced through homotypic fusion of condensing vesicles produced by the Golgi. The vesicles appear to undergo a phase transition from condensed to decondensed along their pathway toward the gland lumen. Decondensation changes the contents to a mucin‐like matrix that is eventually exocytosed in a “kiss‐and‐run” fashion with the plasma membrane of the gland lumen. Exocytosed mucus enters the gland lumen and exits through an epithelial duct that is an extension of the syncytial integument. This results in mucus that extends from the rotifer as a long string as the animal swims through the water. The function of this mucus is unknown, but we speculate it may function in temporary attachment, prey capture, or floatation. 

Genus Pompholyx Gosse, 1851 (Rotifera; Monogononta; Testudinellidae) comprises three species described from freshwater plankton around the globe. Here we describe a new species of Pompholyx collected from a freshwater pond in Massachusetts, USA. The new species resembles its congeners with respect to the following characters: paired eyespots; a dorsally arched lorica with a dorsal occipital convexity behind the corona; lateral flared and rounded lorica surfaces; a ventral surface bearing an occipital concavity posterior of the mouth; a unique egg-gland system; and the absence of a foot. However, P. faciemlarva sp. n. differs from its congeners in possessing a transverse furrow on both the dorsal and ventral surfaces of the lorica. While the trophi of P. faciemlarva sp. n. generally resemble those of other species of Testudinellidae, they do have a symmetrical pattern of unci teeth (17/17) that differs from Pompholyx sulcata (17–20/18–21, right/left), the only other species in the genus with well-described trophi. The description of this new species enhances the floristic richness of freshwater in North America. 

Correct identification of species is necessary if we are to understand their biology, ecology, and evolutionary history, as well as to catalog their global biodiversity. This is acutely critical for many micrometazoans like rotifers, which are often difficult to identify because of their small size and complicated morphologies. Rotifers are ubiquitous micrometazoans that are found worldwide in fresh, brackish, and some marine waters. However, their study is hindered by a lack of both taxonomic expertise and concomitantly adequate guides to the identification of some taxa. These deficiencies are particularly true for the sessile species. To help alleviate these impediments, we assembled information from the literature on easily recognizable characters of all nine valid species in one notable genus: Floscularia (Monogononta; Gnesiotrocha; Flosculariidae). Using that information we developed a simple, dichotomous key to enable workers to identify species in this genus. Our key emphasizes easily observable characters of adult female morphology, including features of their tubes, anterior ends, trophi, and colony formation abilities, thereby allowing for relatively quick identification.  

Accurate identification of species is key to understanding their ecological roles and evolutionary history. It is also essential in cataloging biodiversity for comparisons among habitat types, responses to climate change, effective management practices, and more. The paucity of taxonomic expertise is increasing and with it the ability to competently identify species, this is particularly true for small taxa including rotifers. In an effort to improve this situation, we collated information on morphological characters from the literature on all valid species of sessile Gnesiotrocha (phylum Rotifera) currently assigned to two orders and four families. We review Order Collothecaceae, which comprises families Atrochidae (3 spp.) and Collothecidae (50 spp.) and Order Flosculariaceae, which includes families Conochilidae (7 spp.) and Flosculariidae (71 species). Based on that information, we provide dichotomous keys to the Families, monospecific species in Flosculariidae, and species of Atrochidae, Conochilidae, and Limnias. These keys will aid researchers to identify species in these families and lead to a better understanding of freshwater biodiversity and eco-evolutionary processes.   

Diapausing embryos of invertebrates represent investments in future populations. Thus, these embryos must be capable of withstanding a variety of environmental assaults. Consequently, their eggshells should be adapted to resist injuries from predators, sediments, or excessive shrinkage if desiccated. To date, there have been no direct nanomechanical measurements of the eggshells of most diapausing invertebrates. Here, we used three approaches to understand how eggshells of two rotifers, a freshwater species (Brachionus calyciflorus) and a brackish water species (B. plicatilis), tolerate harsh conditions: (1) atomic force microscopy to measure elasticity and hardness; (2) transmission electron microscopy to study ultrastructure; (3) scanning electron microscopy to examine surface features. We compare these values to measurements of brine shrimp (Artemia salina) cysts and mosquito (Aedes aegypti) overwintering eggs. Our results revealed that rotifer eggshells are structurally similar and have comparable nanomechanical values. While rotifer eggshells had lower Young’s moduli (ca. 13–16 MPa) and hardness values (1.84–1.85x10-2 GPa) than eggshells of Artemia and Aedes, eggshells of all species were relatively elastic and not particularly resistant to deformation. Pliancy of shells that form egg banks (i.e., Artemia, Brachionus) may be an adaptation to resist cracking under the physical forces of buried sediments. Though there are no obvious relationship between eggshell thickness, ultrastructure, ornamentation, or nanomechanical values in rotifer eggshells, we hypothesize that eggshell chemistry may play an important role in determining elasticity and hardness. Future studies should consider an integrative approach to understand importance of eggshell structure, chemistry, and mechanics in protecting diapausing embryos.