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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 Aquatic species found in habitats with limited shade and little dissolved organic carbon (DOC) have increased vulnerability to ultraviolet radiation (UVR) damage. Pigmentation is a common mechanism used by animals for protection from UVR. A pigmented bdelloid rotifer,Philodina, occurs in high densities in shallow rock pools in El Paso Co., TX, and is subject to repeated desiccation and high UVR. To understand the roles of DOC, pigmentation, and dormancy in reducing the effects of UVR exposure in these rotifers: (1) DOC levels in rock pools were measured before and after the summer monsoon season and (2) hydrated or dormant bdelloids (desiccated for 0, 1, 7, or 32 d) that differed in degree of pigmentation (highly, moderately, lightly, and none) were exposed to three intensities of UVB radiation (low, mid, or high) and monitored for survival after 48 h. Pigmented bdelloids were found in rock pools with lower DOC concentrations. Logistic regression analysis indicated that pigmentation level, desiccation time, and UVB intensity all affected survival. Bdelloids in the dormant form for 1 d were more resistant to UVB exposure at all pigmentation levels. However, as desiccation time increased, the odds of surviving decreased. Hydrated highly pigmented bdelloids were three times more likely to survive desiccation, UVB radiation, and their combined effects. Prolonged periods of drought due to the changing climate will alter DOC concentrations, causing photoprotection to become an increasingly important survival strategy for aquatic invertebrates, especially those inhabiting shallow waters. 

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 Coloniality may grant colony members an energetic advantage in the form of lower individual respiration rates as colony size increases. If this occurs it should be apparent as negative allometric scaling of respiration with colony size, and colonial organisms should have scaling factors < 1. However, colonial members from phylum Rotifera have yet to be examined. To test if colonial rotifers possess allometric scaling relationships between respiration rate and colony size, we measured respiration rates for four solitary and three colonial rotifer species; from these respiration rates we estimated scaling factors. We found mixed evidence for allometric scaling of respiration rate in colonial rotifers. Both rotifers with allometric scaling of respiration rate,Conochilus hippocrepisandLacinularia flosculosa, have extensive mucilaginous coverings. These coverings may represent an investment of colony members into a shared structure, lowering individual metabolic costs and thus respiratory needs. Additionally, we determined which traits are associated with allometric scaling of respiration. We compiled known scaling factors for animal phyla from a wide phylogenetic spectrum with colonial representatives and conducted a hierarchical mixed regression that included attributes of colonies. Traits associated with allometric scaling in colonial animals included colony shape, the presence of shared extrazooidal structures, and planktonic lifestyle. There are many other colonial rotifers and animal taxa for which allometric scaling factors have yet to be estimated, knowing these may enhance our understanding of the benefits of coloniality in animals. 

Abstract The rotifer fauna of Africa has been studied for >100 years, but there has been no recent synthesis. We compiled data from 265 publications that reported information on African rotifers. Our dataset yielded information on the distribution of 765 taxa from ~1850 separate sites; these included both natural and artificial habitats such as lakes, ponds, puddles, oases, artificial systems, rivers and wetlands. A heat map of predicted rotifer biodiversity indicated that the greatest diversity should be present in the sub-Saharan region including a large hotspot in Mali and several smaller ones scattered in that region. Lakes Kariba, Tanganyika and Malawi showed high-predicted diversity, but surprisingly, Lake Victoria had lower diversity than expected. Two regions showed unusually high-predicted diversity: northwestern Algeria extending into Morocco and Egypt. Equatorial Africa is rich in habitats well suited for rotifers, yet their predicted biodiversity seems low. Latitude and elevation were negatively correlated with richness, while permanent water source and littoral zone were positively correlated according to generalized linear modeling results. Partial RDA analyses showed significant correlations among several environmental features and species occurrences. It is clear that more survey work remains to be done to achieve a better understanding of African rotifers. 

Abstract AimThe long history of isolation of the Antarctic continent, coupled with the harsh ecological conditions of freezing temperatures, could affect the patterns of genetic diversity in the organisms living there. We aim (a) to test whether such pattern can be seen in a mitochondrial marker of bdelloid rotifers, a group of microscopic aquatic and limno‐terrestrial animals and (b) to speculate on the potential mechanisms driving the pattern. LocationFocus on Antarctica. TaxonRotifera Bdelloidea. MethodsWe analysed different metrics of genetic diversity, also spatially explicit ones, including number of haplotypes, accumulation curves, genetic distances, time to the most recent common ancestor, number of independently evolving units from DNA taxonomy, strength of the correlation between geographical and genetic distances, population genetics neutrality and differentiation indices, potential historical processes, obtained from an extensive sample of cytochrome oxidase subunit I (COI) sequences obtained from bdelloid rotifers. We included 2242 individuals from 23 species in a comparison between Antarctic and non‐Antarctic taxa, correcting for sample size directly in the analyses and then by confirming the results also using only a restricted dataset of nine well‐sampled species. ResultsAntarctic species had consistently lower genetic diversity and potential younger relative age than non‐Antarctic species, even if they were similar in sample size, geographical extent, neutrality and differentiation indices, and correlation between genetic and geographical distances. Main conclusionsThe extensive survey of genetic diversity in one mitochondrial marker in Antarctic bdelloids supports previous suggestions from other organisms that the origin and maintenance of terrestrial Antarctic fauna are different from those of other continents. Such differences could be speculated to be due, in the case of bdelloid rotifers, to the more recent origin of the species living there in comparison to non‐Antarctic species. 

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. 

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.   

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. 

Afrotropical inland waters are highly diverse ecosystems; however, they remain poorly studied, especially for rotifers. Here, we contributed to the knowledge of the rotifer species richness in the largely understudied African countries of Angola and Ghana. We assessed the roles of habitat type and a suite of abiotic environmental factors in determining rotifer species richness of Ghana. A total of 37 sites (Ghana 32, Angola 5) in 19 water bodies from a variety of aquatic habitat types were sampled. In Ghana, we identified 118 taxa (105 species or subspecies level, 13 identified to genus). We identified 15 taxa (13 species) in the Angola samples. For Ghana, 100 of 118 (~85%) taxa were new records for the country, of which 13 species (~11%) were also new records for Africa. Nearly all the species (~93%) were new records for Angola. Species richness was positively correlated with conductivity and reservoir habitat type and negatively with pH. Redundancy analysis (RDA), conducted at the species level for the Ghana dataset, indicated suites of species associated with latitude, longitude, temperature, TDS, or pH. We also evaluated the effect of climate on species distribution in 27 African countries by conducting a review of all reports from Africa to determine factors associated with species richness. A Spearman’s correlation confirmed a significant positive correlation between the number of rotifer species and the number of climatic regions (R = 0.53, p < 0.001) for certain countries, based on species distributions in relation to Köppen–Geiger climate regions. This fact validates the environmental heterogeneity hypothesis for African rotifers. Lastly, we predicted that rotifer species richness in Ghana, as a country with a tropical climate, could approach ~190 taxa, while in climatically heterogeneous Angola we predict ~200 taxa. This study contributes to our knowledge of rotifer biogeography and species richness patterns in Africa.