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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.  

All species of Flosculariidae (Rotifera; Monogononta; Gnesiotrocha; Flosculariaceae) possess a flexible integument; they are termed illoricate. But species of four genera (Beauchampia, Floscularia, Limnias, and Ptygura) possess a distinctive hardened region on the dorsal side of their anterior end, which unfortunately has received little attention beyond the species level. In our work we study the ultrastructure of these hardened regions. In Beauchampia, Limnias, and some species of Ptygura the hardened zone takes the form of a dorsal plate (hereafter DP) with or without additional protruding processes. The DP has the shape of a ~50 μm diameter disc enfolding the anterior dorsal region of the animal, with a perimetral inward fold, smooth surface, and pores. The DP of Limnias ceratophylli shows three main ultrastructural differences from the rest of the body: (1) type of glycocalyx, (2) absence of small mounds on its surface, and (3) presence of broad cisternae at the base of the pores. In Floscularia and some Ptygura the hardened structures are reduced to a pair of dorsal projections protecting the dorsal antenna: i.e., lacking a full DP. These structures deserve attention because their adaptive importance has not been studied. Here we augment those observations with other characters: i.e., presence/absence of a birefringent body in late-stage embryos and morphology of their trophi and corona. Our goal was to provide information to help develop better taxonomy, as well as advance our understanding of the evolution of sessile taxa in general and of the potentially polyphyletic Ptygura genus in particular. 

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.   

In recent years, trait-based research on plankton has gained interest because of its potential to uncover general roles in ecology. While trait categories for phytoplankton and crustaceans have been posited, rotifer trait assessment has lagged behind. Here, we reviewed the literature to assess traits key to their life histories and provided a data matrix for the 138 valid genera of phylum Rotifera. We considered seven traits: habitat type, trophi type, presence of lorica and foot, predation defense attributes, corona type, and feeding traits. While most traits were morphological attributes and supposedly easy to assess, we were faced with several challenges regarding trait assignment. Feeding traits were especially difficult to assess for many genera because relevant information was missing. Our assembled trait matrix provides a foundation that will initiate additional research on rotifer functional diversity, diminish the misclassification of rotifer genera into trait categories, and facilitate studies across trophic levels.