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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. 

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 Many planktonic rotifers carry their oviposited eggs until hatching. In some species, the eggs are attached to the mother via secretions from her style gland, which forms a thread that extends from her cloaca. In species ofPompholyx, the mother possesses the rare ability to change the tension on the secreted thread, which alters the proximity of the egg with respect to her body. In this study, we used behavioral observations, confocal microscopy, and transmission electron microscopy to study the functional morphology of the stalk gland, which secretes a similar thread to the style gland. Our observations reveal that six longitudinal muscles insert on a stalk‐gland complex, which is a combination of a two‐headed gland and an epithelial duct that connects to the posterior cloaca. The gland secretes a single, long, electron‐dense thread that traverses the duct and attaches to the egg surface through the cloaca. Three retractor muscles insert on the stalk gland and function to pull the entire complex anteriorly, thereby increasing tension on the thread and moving the egg close to the mother's body. A set of three (two pairs and a single dorsal) protractor muscles antagonize these actions, and their contraction pulls the gland complex close to the cloaca, thereby releasing tension on the thread and allowing the egg to distance itself from the mother. The stalk gland complex does not appear to be homologous to the style glands of other rotifers, but we hypothesize that it functions as a form of maternal protection as is the case with style glands. 

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. 

Abstract Freshwater gastrotrichs have a biphasic lifecycle that reputedly involves the production of three types of eggs: apomictic and fast hatching (tachyblastic ova), apomictic and delayed hatching (opsiblastic ova), and plaque‐bearing eggs (potentially derived from mixis). While some details of oogenesis and eggshell structure are known for tachyblastic ova, there are few details on other egg types. Here, we provide the first ultrastructural description of the oviposited opsiblastic eggs of the freshwater gastrotrich,Lepidodermella squamata. Scanning electron microscopy revealed the eggshell surface to be ornamented with long flattened pillar‐like structures centered on polygonal plates that are pitted along their periphery. Transmission electron microscopy showed the pits to lead to a vast labyrinth of tubular spaces and larger cavities throughout the thick apical layer of the shell. The basal layer of the shell is amorphous and connected to a network of fine fibers that traverse an extra‐oocyte space and forms a protective sheet around the uncleaved oocyte. The uncleaved oocyte has a dense layer of peripheral ooplasm surrounding a core of organelles including mitochondria, membrane‐bound secretion granules, endoplasmic reticulum, and a single nucleus in a granular, ribosome‐rich cytoplasm. Secretion granules are the most abundant organelles and presumably contain lipid‐rich yolk that will be used as energy for delayed cleavage, thus functioning in temporal dispersal. These data are compared to the fine structure of invertebrate resting eggs across the phylogenetic spectrum to determine the novelty of opsiblastic egg structure inL. squamata. 

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 Eggshell characteristics can be important components of fitness, providing protection to the developing embryo against environmental stressors. Many invertebrates, however, produce multiple egg types as part of their reproductive strategies. In monogonont rotifers, for example, reproduction occurs via cyclical parthenogenesis-a process involving extended periods of asexual reproduction interrupted by brief phases of sexuality. Asexual reproduction yields diploid subitaneous (amictic) eggs or haploid eggs that develop into males, while sexual reproduction produces diploid diapausing (resting) eggs. To date, only the eggshells of diapausing eggs have received substantial ultrastructural investigation. Here, we examine the ultrastructure of subitaneous eggshells in monogonont rotifers across diverse taxa and lifestyles, using light and electron microscopy. Our results reveal considerable variation in eggshell thickness, layering, and staining properties among taxa. Generally, sessile rotifers exhibited thinner eggshells with fewer layers and limited morphological diversity, whereas species that brood or oviposit eggs on substrates had thicker, more layered shells with more complex staining profiles. These findings indicate that subitaneous eggshell ultrastructure is more diverse than previously recognized and may hold value for future ecological and evolutionary studies. 

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.  

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.