Search for: All records

ABSTRACT Exposure to ultraviolet radiation (UVR) can decrease lifespan and reduce fecundity in aquatic invertebrates. Organisms inhabiting shallow waters are often unable to avoid UVR damage. Bdelloid rotifers are known for their resistance to extreme environments and ionizing radiation. However, conflicting results have been reported regarding their response to UVR. We hypothesized that exposure to UVB radiation would negatively affect survival and other life history characteristics (i.e., lifespan, net reproductive rate, intrinsic rate of increase) with increasing intensities and across multiple generations in bdelloids that inhabit shallow rock pools. To test these hypotheses, field‐collected females (F₀) were exposed to two environmentally relevant and one extreme level of UVB for 2 h and individuals that survived were retained for further analysis. Their F₁neonates were then cultured, and their offspring (F₂, F₄) were exposed to the same UVR treatment as the parental generation, and survival was again recorded. Lifetable analyzes were conducted using offspring of exposed mothers in the F₁and F₅generations. As predicted, exposure to UVB radiation negatively affected survival and resulted in cumulative damage after each subsequent exposure to UVB at all intensities. However, maternal exposure to low UVB levels resulted in increased lifespan (97%) and net reproductive rate (215%) in their progeny. At mid UVB intensities, net reproductive rate increased but to a lesser extent. This may reflect an adaptive response to environmental stress, enabling faster reproduction. However, increasing UVR intensity has adverse effects on longevity and fecundity which may compromise population fitness. 

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 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 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 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. 

Afrotropical inland waters remain poorly studied for rotifer diversity. Here, we provide new distribution data from Botswana and connect these local patterns to continental-scale biogeography using an Africa–Europe occurrence dataset. In Botswana, we analyzed rotifer species richness, functional traits, and environmental drivers using 37 samples from 15 water bodies spanning natural and anthropogenic habitats. We recorded 107 rotifer taxa: 92 identified to species or subspecies level, 14 to genus, and one group of unidentified bdelloids. Seventy taxa (~65%) are new records for Botswana, and one species, Donneria sudzukii, is reported for the first time in Africa. Physicochemical gradients explained community structure, with the first two constrained RDA axes accounting for 40.7% and 23.7% of variation. Axis 1 captured a mineralization gradient linked to total dissolved solids and temperature, whereas Axis 2 reflected oxygen concentration and pH. Traits tracked these gradients: warmer, more mineralized waters were associated with specific trophi types, compact body shapes, and intermediate body sizes, whereas less mineralized, better oxygenated sites were related to smaller taxa and alternative feeding morphologies. To place these trait–environment relationships in a broader geographic context, we then analyzed an Africa–Europe dataset (67,170 records) to quantify latitudinal patterns in thermal classes and morphological traits (geometric body shape and trophi type). Diversity showed clear latitudinal structuring: warm-water genera clustered at low latitudes, only Kellicottia and Didymodactylos had mean distributions above 50° N, and bdelloid families were associated with higher latitudes. Morphological traits also varied with latitude, with trilateral truncated pyramid body shapes and malleoramate trophi occurring closest to the equator. Overall, by combining new species-level data from Botswana with continent-scale occurrence patterns, we link local community assembly to macroecological structure in rotifer functional and biogeographical organization. 

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.