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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. 

Abstract The retrocerebral organ (RCO) is a complex glandular system that is widely distributed across species of phylum Rotifera (sensu stricto). This system is hypothesized to secrete mucus that aids in benthic locomotion, adhesion, and/or reproduction. Unfortunately, the ultrastructure of the RCO is mostly unknown, having only been partially examined in one species. We used transmission electron microscopy and confocal laser scanning microscopy to describe the RCO in the planktonic freshwater rotiferTrichocerca similis. Results reveal the RCO to be a singular syncytial organ composed of a posterior glandular region, an expansive reservoir, and an anterior duct. The glandular portion has an active synthetic cytoplasm with paired nuclei, abundant rER, ribosomes, Golgi, and mitochondria. Electron‐dense secretion granules accumulate at the anterior end of the gland and undergo homotypic fusion to create larger, more electron‐lucent granules with numerous mesh‐like contents that gradually fuse into tubular secretions that accumulate in the reservoir. Ultrastructure of these secretions suggests they may be hydrated glycoproteins. Cross‐striated longitudinal muscles form a partial sleeve around the reservoir and may function to squeeze the secretions through the single cytoplasmic duct that penetrates the cerebral ganglion. A review of the RCOs from other rotifers suggests that further ultrastructural analyses are required before attempting to discern their functions and homologies. 

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. 

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. 

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. 

Most species of Keratella possess dome-shaped, dorsal plates comprising a network of polyhedral units (facets), delineated by slightly raised ridges. The arrangement of facets define a species’ facet pattern (FP), with the resulting structure resembling a geodesic dome. Researchers have sorted species into categories based on their FPs, but those have not been analyzed. Additionally, while a strong lorica has been suggested to protect Keratella from predatory attack or other actions causing blunt force trauma (BFT), we know little of how that occurs. Thus, in our study we tested two hypotheses. (1) There is support for categorizing Keratella species into unique groupings based on their FPs. (2) FPs provide resistance to physical stresses. To test that hypothesis we used the structural analysis software SkyCiv©. Our results indicate support for four FP categories. Additionally, the SkyCiv analysis provided preliminary ‘proof-of-concept’ that Keratella FPs have a functional significance: i.e., adding or subtracting facets in our model was followed by a change in predicted structural reliability. We posit that FPs are adaptations protecting Keratella from fractures to the lorica that may result from BFT incurred during predatory attack by copepods or while caught within the branchial chambers of daphnids