Evidence for regional aeolian transport of freshwater micrometazoans in arid regions

To what extent are ephippia of Mexican Anomopoda (Crustacea, Cladocera) identifiable?

Diapausing embryos encased within cladoceran ephippia result from sexual reproduction and increase genetic diversity. They are also important means by which species bypass harsh environmental conditions and disperse in space and time. Once released, ephippia usually sink to the benthos and remain there until hatching. Using the Sars' method (incubating sediments to identify cladoceran hatchlings), ephippial egg bank biodiversity can be evaluated. Yet, even when samples are incubated under a variety of conditions, it is not possible to warrant that all have hatched. Few keys are available that facilitate the identification of cladocerans by using only ephippial morphology. Our goal was to analyze some cladoceran ephippia from Mexico, to develop a means to identify them using easily recognizable characteristics. Ephippia of 23 cladoceran species from waters in Aguascalientes (México) in 11 genera (Alona, Biapertura, Ceriodaphnia, Chydorus, Daphnia, Dunhevedia, Ilyocryptus, Macrothrix, Moina, Pleuroxus, and Simocephalus) were analyzed. In our analysis six morphological features were selected that permitted the identification of ephippia to species(-group) level. The results demonstrate that with a proper catalog of features, some ephippia can be identified.

Genetic differentiation and phylogeography of rotifer Polyarthra dolichoptera and P. vulgaris populations between Southeastern China and eastern North America: High intercontinental differences

Genetic differentiations and phylogeographical patterns of small organisms may be shaped by spatial isolation, environmental gradients, and gene flow. However, knowledge about genetic differentiation of rotifers at the intercontinental scale is still limited. Polyarthra dolichoptera and P. vulgaris are cosmopolitan rotifers that are tolerant to environmental changes, offering an excellent model to address the research gap. Here, we investigated the populations in Southeastern China and eastern North America and evaluated the phylogeographical patterns from their geographical range sizes, geographic–genetic distance relationships and their responses to spatial‐environmental factors. Using the mitochondrial cytochrome c oxidase subunit I gene as the DNA marker, we analyzed a total of 170 individuals. Our results showed that some putative cryptic species, also known as entities were widely distributed, but most of them were limited to single areas. The divergence of P. dolichoptera and P. vulgaris indicated that gene flow between continents was limited while that within each continent was stronger. Oceanographic barriers do affect the phylogeographic pattern of rotifers in continental waters and serve to maintain genetic diversity in nature. The genetic distance of P. dolichoptera and P. vulgaris populations showed significant positive correlation with geographic distance. This might be due to the combined effects of habitat heterogeneity, long‐distance colonization, and oceanographic barriers. Furthermore, at the intercontinental scale, spatial distance had a stronger influence than environmental variables on the genetic differentiations of both populations. Wind‐ and animal‐mediated transport and even historical events of continental plate tectonics are potential factors for phylogeography of cosmopolitan rotifers.

Patterns of Rotifer Diversity in the Chihuahuan Desert

Desert aquatic systems are widely separated, lack hydrologic connections, and are subject to drought. However, they provide unique settings to investigate distributional patterns of micrometazoans, including rotifers. Thus, to understand rotifer biodiversity we sampled 236 sites across an array of habitats including rock pools, springs, tanks, flowing waters, playas, lakes, and reservoirs in the Chihuahuan Desert of the USA (n = 202) and Mexico (n = 34) over a period of >20 years. This allowed us to calculate diversity indices and examine geographic patterns in rotifer community composition. Of ~1850 recognized rotifer species, we recorded 246 taxa (~13%), with greatest diversity in springs (n = 175), lakes (n = 112), and rock pools (n = 72). Sampling effort was positively related to observed richness in springs, lakes, rivers, and tanks. Nestedness analyses indicated that rotifers in these sites, and most subsets thereof, were highly nested (support from 4 null models). Distance was positively correlated with species composition dissimilarity on small spatial scales. We predicted species richness for unsampled locations using empirical Bayesian kriging. These findings provide a better understanding of regional rotifer diversity in aridlands and provide information on potential biodiversity hotspots for aquatic scientists and resource managers.

Facing Adversity: Dormant Embryos in Rotifers

An in-depth look at the basic aspects of dormancy in cyclic parthenogenetic organisms is now possible thanks to research efforts conducted over the past two decades with rotifer dormant embryos. In this review, we assemble and compose the current knowledge on four central themes: (1) distribution of dormancy in animals, with an overview on the phylogenetic distribution of embryo dormancy in metazoans, and (2) physiological and cellular processes involved in dormancy, with a strong emphasis on the dormant embryos of cyclically parthenogenetic monogonont rotifers; and discussions of (3) the selective pressures and (4) the evolutionary and population implications of dormancy in these animals. Dormancy in metazoans is a widespread phenomenon with taxon-specific features, and rotifers are among the animals in which dormancy is an intrinsic feature of their life cycle. Our review shows that embryo dormancy in rotifers shares common functional pathways with other taxa at the molecular and cellular level, despite the independent evolution of dormancy across phyla. These pathways include the arrest of similar metabolic routes and the usage of common metabolites for the stabilization of cellular structures and to confer stress resistance. We conclude that specific features of recurrent harsh environmental conditions are a powerful selective pressure for the fine-tuning of dormancy patterns in rotifers. We hypothesize that similar mechanisms at the organism level will lead to similar adaptive consequences at the population level across taxa, among which the formation of egg banks, the coexistence of species, and the possibility of differentiation among populations and local adaptation stand out. Our review shows how studies of rotifers have contributed to improved knowledge of all of these aspects.

Biogeography and co-occurrence of 16 planktonic species of Keratella Bory de St. Vincent, 1822 (Rotifera, Ploima, Brachionidae) in lakes and reservoirs of the United States

We report current distributions of 16 species of Keratella Bory de St. Vincent, 1822 within lakes and reservoirs of the U.S. Specimens were identified from 988 lakes and reservoirs during spring and summer 2012 as part of the U.S. Environmental Protection Agency's National Lakes Assessment (NLA). We performed a co-occurrence analysis to determine correlations between species-pair occurrences and a niche-centroid analysis to predict optimal water quality conditions for each species. While a high degree of overlap in geographic range was observed among the species, distribution maps showed that K. quadrata was largely confined to more northern latitudes and K. americana was confined primarily to the eastern U.S. Other common species were either ubiquitously or sparsely distributed across ecoregions, suggesting that their distribution may be more related to inter-species competition or local water quality parameters. This study expands the limited pool of knowledge on rotifer biogeography within the U.S.

Anemochory of diapausing stages of microinvertebrates in North American drylands

1. Dry, ephemeral, desert wetlands are major sources of windblown sediment, as well as repositories for diapausing stages (propagules) of aquatic invertebrates. Zooplankton propagules are of the same size range as sand and dust grains. They can be deflated and transported in windstorm events. This study provides the evidence that dust storms aid in dispersal of microinvertebrate propagules via anemochory (aeolian transport). 2. We monitored 91 windstorms at six sites in the southwestern U.S. over a 17-year period. The primary study site was located in El Paso, Texas in the northern Chihuahuan Desert. Additional samples were collected from the Southern High Plains region. Dust carried by these events was collected and rehydrated to hatch viable propagules transported with it. 3. Using samples collected over a six-year period, 21 m above the ground which included 59 storm events, we tested the hypothesis that transport of propagules is correlated with storm intensity by monitoring meteorological conditions such as storm duration, wind direction, wind speed, and PM₁₀ (fine dust concentration). An air quality monitoring site located adjacent to the dust samplers provided quantitative hourly measurements. 4. Rehydration results from all events showed that ciliates were found in 92% of the samples, rotifers in 81%, branchiopods in 29%, ostracods in 4%, nematodes in 13%, gastrotrichs in 16%, and tardigrades in 3%. Overall, four bdelloid and 11 monogonont rotifer species were identified from rehydrated windblown dust samples. 5. PCA results indicated gastrotrichs, branchiopods, nematodes, tardigrades, and monogonont rotifer occurrence positively correlated with PM₁₀ and dust event duration. Bdelloid rotifers were correlated with amount of sediment deposited. NMDS showed a significant relationship between PM₁₀ and occurrence of some taxa. Zero-inflated, general linear models with mixed-effects indicated significant relationships with bdelloid and nematode transport and PM₁₀. 6. Thus, windstorms with high particulate matter concentration and long duration are more likely to transport microinvertebrate diapausing stages in drylands.

Using Proales similis (Rotifera) for toxicity assessment in marine waters

There is a need to develop more animal species for assessing toxicity in marine environments. Cyst-based toxicity tests using invertebrates are especially fast, technically simple, cost-effective, and sensitive to a variety of toxicants. Over the past 30 years, a variety of toxicity endpoints have been measured using the marine rotifer Brachionus plicatilis hatched from cysts, including mortality, reproduction, ingestion, swimming, enzyme activity, and gene expression. A consensus has developed that the most ecologically relevant toxicity measurements should be made using more than one species. Furthermore, it has been noted that the rotifer species toxicant sensitivity distribution is much broader than which endpoint is measured. This implies that toxicity should be measured with the simplest, fastest, least expensive test available on as many species as feasible. If a battery of test species is to be used to estimate toxicity, diapause egg-based toxicity tests that do not require culturing of test animals will be key. In this paper, we describe how diapause eggs of a new marine rotifer, Proales similis, can be produced, stored and hatched under controlled conditions to produce animals for toxicity tests. Methods are described for quantifying the toxicity of copper, mercury and cadmium based on mortality, ingestion, reproduction, and diapause egg hatching endpoints. We found that reproduction and ingestion endpoints were generally more sensitive to the metals than mortality or diapause egg hatching. When the copper sensitivity of P. similis was compared to Brachionus manjavacas and B. plicatilis using an ingestion test, similar EC50s were observed. In contrast, the B. rotundiformis ingestion EC50 for copper was about 4× more sensitive. Although diapause egg hatching was not the most sensitive endpoint, it is the most ecologically relevant for assessing sediment toxicity. Our discovery of diapausing eggs in the P. similis life cycle has created a conundrum. We have not observed males or sex in P. similis populations, which is a direct contradiction to the orthodox view of the monogonont rotifer life cycle. Work is needed to clarify how diapause egg production is accomplished by P. similis and whether sexual reproduction is involved.

Long-distance passive dispersal in microscopic aquatic animals

Given their dormancy capability (long-term resistant stages) and their ability to colonise and reproduce, microscopic aquatic animals have been suggested having cosmopolitan distribution. Their dormant stages may be continuously moved by mobile elements through the entire planet to any suitable habitat, preventing the formation of biogeographical patterns. In this review, I will go through the evidence we have on the most common microscopic aquatic animals, namely nematodes, rotifers, and tardigrades, for each of the assumptions allowing long-distance dispersal (dormancy, viability, and reproduction) and all the evidence we have for transportation, directly from surveys of dispersing stages, and indirectly from the outcome of successful dispersal in biogeographical and phylogeographical studies. The current knowledge reveals biogeographical patterns also for microscopic organisms, with species-specific differences in ecological features that make some taxa indeed cosmopolitan with the potential for long-distance dispersal, but others with restricted geographic distributions.