North American Freshwater Research Articles

Tissue-specific turnover and diet-tissue discrimination factors of carbon and nitrogen isotopes of a common forage fish held at two temperatures.

The application of stable isotopes to foraging ecology is dependent on understanding life-history and environmental factors unrelated to diet that may influence isotopic composition. Diet-tissue discrimination factors (DTDFs) and turnover rates will increase the accuracy of isotope-based studies. Furthermore, little consideration has been given to the effects of temperature or life-history stage on isotopic ratios despite the prevalence of variation in temperature and growth rates throughout life. We measured δ13 C and δ15 N values with an elemental analyzer coupled to a continuous flow isotope ratio mass spectrometer. These values were used to estimate turnover and DTDFs for Emerald Shiners (Notropis atherinoides), a common North American freshwater forage fish. Fish were assigned to a temperature treatment, either 10°C (Low) or 20°C (High), and provided one of three diets (commercial pellet, Artemia salina, or Hemimysis anomala). At regular intervals fish were sampled and the isotopic compositions of whole body and liver tissues were determined. Tissue turnover rates for fish fed Artemia were faster for liver than for whole body, but were also influenced by temperature. Turnover occurred faster at higher temperatures for body and liver δ15 N values, but not for δ13 C values. The pellet and Hemimysis treatments were in isotopic equilibrium from the start of the experiment and estimated DTDFs based on these treatments were lower than assumed for Δ15 N (+0.6 to 2.7‰) and variable, but within expected ranges for Δ13 C (-1.9 to +1.5‰). The results for Emerald Shiners differed from commonly made assumptions for applying stable isotopes to ecological questions, possibly related to a bias in the use of juveniles in studies of turnover and DTDFs and assumptions regarding thermal-independence of isotopic relationships. The species-specific DTDF and tissue turnover estimates provided here will inform interpretations of stable isotope data for smaller fish species and improve food-web studies.

Valve movement of three species of North American freshwater mussels exposed to elevated carbon dioxide.

Freshwater mussels are at-risk taxa and may be exposed to high levels of carbon dioxide (CO2) because of the potential use of CO2 to control the movement of invasive aquatic fish species. One potential behavioral response to a change in the partial pressure of CO2 (pCO2) may be altered valve movement. In this study, three species of mussels were fitted with modified sensors and exposed to two regimes of pCO2 to define thresholds of impaired valve movement. The first experiment demonstrated that Pyganodon grandis were much more tolerant to rising pCO2 relative to Lampsilis siliquoidea (acute closure at ∼200,000μatm in comparison to ∼80,000μatm). The second experiment consisted of monitoring mussels for 6days and exposing them to elevated pCO2 (∼70,000μatm) over a 2-day period. During exposure to high pCO2, Lampsilis cardium were open for nearly the entire high pCO2 period. Conversely, P. grandis were closed for most of the period following exposure to high pCO2. For L. siliquoidea, the number of closures decreased nearly 40-fold during high pCO2. The valve movement responses observed suggest species differences, and exposure to elevated pCO2 requires a reactive response.

Can introduced species replace lost biodiversity? A test with freshwater molluscs

Human activity has dramatically accelerated both species extinctions and introductions, and the balance of these two processes is generally expected to reduce biodiversity and increase taxonomic homogenization. However, few tests of this hypothesis have been made. We tested whether new macroinvertebrate invaders in North American freshwaters can replace the recent loss of biodiversity, particularly focusing on molluscs. We found that both crustaceans and molluscs are overrepresented among endangered and recently extinct species, as well as among invaders. For molluscs, the number of recently extinct species (79 species) was more than twice that for exotic species (38 species). In addition, molluscan invaders are from different taxonomic families than recently extinct or endangered species. While most extinct and endangered molluscs are from streams and rivers, invaders preferentially colonize lakes and reservoirs. The impact of humans on species introductions and extinctions increases with spatial scale (from local to continental scales), resulting in the increased phylogenetic dissimilarity between introduced species and native communities. Construction of dams and alteration of the flow regimes of lotic systems will continue to lead to the extinction of native species, and promote the spread of invaders, resulting in a loss of biodiversity and taxonomic homogenization.

Connectivity and vagility determine beta diversity and nestedness in North American and European freshwater fish

AbstractAimThis study assesses the extent to which biogeographic patterns in freshwater fish beta diversity and nestedness are due to dispersal limitation, from differences in dispersal opportunity across geographic areas and in dispersal ability across species.LocationEurope and the Atlantic and Pacific realms of North America (NA), east and west of the Continental Divide.MethodsThe effects of glaciation, realm shape, connectivity, current climate and vagility on regional‐level beta diversity and nestedness were investigated. Turnover and nestedness‐resultant dissimilarity components of beta diversity and the nestedness metric based on overlap and decreasing fill (NODF) were calculated from regional species lists and the contributions of turnover and segregation to nestedness structure quantified.ResultsGeographic distance was a stronger predictor of beta diversity than climatic and environmental heterogeneity distances. Species range shapes varied with postglacial colonization direction, being more extensive in an east–west direction in Europe than in Atlantic NA. Turnover increased with declining connectivity, in unglaciated areas, and in non‐migratory species. Species were significantly less nested than expected because of high turnover and within‐realm heterogeneity in regional faunas. Deviations from nestedness were greater in unglaciated areas and in migratory species. Non‐migrants, but not migrants, exhibited coincident range boundaries.Main conclusionsSpatial trends in beta diversity and nestedness in freshwater fish in NA and Europe result primarily from differences in postglacial recolonization opportunity across realms and in dispersal ability across species. Multiple metrics are necessary to identify the processes determining the spatial structure of species assemblages.

The filter feeder Dreissena polymorpha affects nutrient, silicon, and metal(loid) mobilization from freshwater sediments

Organic sediments in aquatic ecosystems are well known sinks for nutrients, silicon, and metal(loid)s. Organic matter-consuming organisms like invertebrate shredders, grazers, and bioturbators significantly affect element fixation or remobilization by changing redox conditions or binding properties of organic sediments. Little is known about the effect of filter feeders, like the zebra mussel Dreissena polymorpha, an invasive organism in North American and European freshwater ecosystems. A laboratory batch experiment exposing D. polymorpha (∼1200 organisms per m2) to organic sediment from a site contaminated with arsenic, copper, lead, and uranium revealed a significant uptake and accumulation of arsenic, copper, iron, and especially uranium both into the soft body tissues and the seashell. This is in line with previous observations of metal(loid) accumulation from biomonitoring studies. Regarding its environmental impact, D. polymorpha significantly contributed to mobilization of silicon, iron, phosphorus, arsenic, and copper and to immobilization of uranium (p < 0.001), probably driven by redox conditions, microbial activity within the gut system, or active control of element homeostasis. No net mobilization or immobilization was observed for zinc and lead, because of their low mobility at the prevailing pH of 7.5–8.5. The present results suggest that D. polymorpha can both ameliorate (nutrient mobilization, immobilization of toxicants mobile under oxic conditions) or aggravate negative effects (mobilization of toxicants mobile under reducing conditions) in ecosystems. Relating the results of the present study to observed population densities in natural freshwater ecosystems suggests a significant influence of D. polymorpha on element cycling and needs to be considered in future studies.

Contact Selectivity for Four Fish Species Sampled with North American Standard Gill Nets

AbstractThe American Fisheries Society (AFS) has recommended standard gears and methods for collecting North American freshwater fish data, but selectivity of these gears, including gill nets, is poorly described for most species. We calculated contact selectivity for species commonly collected with AFS standard gill nets in lakes and reservoirs of North America, including Black Bullhead Ameiurus melas, Channel Catfish Ictalurus punctatus, Walleye Sander vitreus, and Yellow Perch Perca flavescens. The normal selectivity curve provided the best fit to empirical catch data for all species. Inclusion of a tangle factor improved model fit for Black Bullhead, Channel Catfish, and Walleye, indicating that tangling is an important means of capture for these species. Channel Catfish were more susceptible to tangling than the closely related Black Bullhead, likely because larger Channel Catfish are more vulnerable to tangling by spines in the smallest meshes, whereas Black Bullhead in the same meshes were wedged. Failure to include a tangle factor for Channel Catfish and Walleye would have underestimated peak length of captured fish by 80 and 40 mm, respectively. Tangling was not an important factor in Yellow Perch capture. Total selectivity curves for each species captured with the AFS standard gill net were calculated, and relative selectivity values are provided as correction factors for size‐selectivity bias. Our correction factors and modeled selectivity curves for Channel Catfish, Walleye, and Yellow Perch corroborate previous studies that modeled selectivity of the AFS standard gill net for Channel Catfish, Walleye, and Yellow Perch, but our study provided novel information on Black Bullhead. Improved selectivity information for the AFS standard gill net may help to promote further voluntary adoption of AFS standard gears and methods.Received July 15, 2016; accepted October 24, 2016 Published online January 3, 2017

Status of White Sturgeon (Acipenser transmontanusRichardson, 1863) throughout the species range, threats to survival, and prognosis for the future

Summary White Sturgeon, Acipenser transmontanus (WS), are distributed throughout three major river basins on the West Coast of North America: the Sacramento-San Joaquin, Columbia, and Fraser River drainages. Considered the largest North American freshwater fish, some WS use estuarine habitat and make limited marine movements between river basins. Some populations are listed by the United States or Canada as threatened or endangered (upper Columbia River above Grand Coulee Dam; Kootenai River; lower, middle and, upper Fraser River and Nechako River), while others do not warrant federal listing at this time (Sacramento-San Joaquin Rivers; Columbia River below Grand Coulee Dam; Snake River). Threats that impact WS throughout the species’ range include fishing effects and habitat alteration and degradation. Several populations suffer from recruitment limitations or collapse due to high early life mortality associated with these threats. Efforts to preserve WS populations include annual monitoring, harvest restrictions, habitat restoration, and conservation aquaculture. This paper provides a review of current knowledge on WS life history, ecology, physiology, behavior, and genetics and presents the status of WS in each drainage. Ongoing management and conservation efforts and additional research needs are identified to address present and future risks to the species.

Phylogeographic Analysis of Blastomyces dermatitidis and Blastomyces gilchristii Reveals an Association with North American Freshwater Drainage Basins

Blastomyces dermatitidis and Blastomyces gilchristii are dimorphic fungal pathogens that cause serious pulmonary and systemic infections in humans. Although their natural habitat is in the environment, little is known about their specific ecologic niche(s). Here, we analyzed 25 microsatellite loci from 169 strains collected from various regions throughout their known endemic range in North America, representing the largest and most geographically diverse collection of isolates studied to date. Genetic analysis of multilocus microsatellite data divided the strains into four populations of B. dermatitidis and four populations of B. gilchristii. B. dermatitidis isolates were recovered from areas throughout North America, while the B. gilchristii strains were restricted to Canada and some northern US states. Furthermore, the populations of both species were associated with major freshwater drainage basins. The four B. dermatitidis populations were partitioned among (1) the Nelson River drainage basin, (2) the St. Lawrence River and northeast Atlantic Ocean Seaboard drainage basins, (3) the Mississippi River System drainage basin, and (4) the Gulf of Mexico Seaboard and southeast Atlantic Ocean Seaboard drainage basins. A similar partitioning of the B. gilchristii populations was observed among the more northerly drainage basins only. These associations suggest that the ecologic niche where the sexual reproduction, growth, and dispersal of B. dermatitidis and B. gilchristii occur is intimately linked to freshwater systems. For most populations, sexual reproduction was rare enough to produce significant linkage disequilibrium among loci but frequent enough that mating-type idiomorphic ratios were not skewed from 1:1. Furthermore, the evolutionary divergence of B. dermatitidis and B. gilchristii was estimated at 1.9 MYA during the Pleistocene epoch. We suggest that repeated glaciations during the Pleistocene period and resulting biotic refugia may have provided the impetus for speciation as theorized for other species associated with temperate freshwater systems.

Climate and local abundance in freshwater fishes.

Identifying factors regulating variation in numbers of individuals among populations across a species' distribution is a fundamental goal in ecology. A common prediction, often referred to as the abundant-centre hypothesis, suggests that abundance is highest near the centre of a species' range. However, because of the primary focus on the geographical position of a population, this framework provides little insight into the environmental factors regulating local abundance. While range-wide variation in population abundance associated with environmental conditions has been investigated in terrestrial species, the relationship between climate and local abundance in freshwater taxa across species' distributions is not well understood. We used GIS-based temperature and precipitation data to determine the relationships between climatic conditions and range-wide variation in local abundance for 19 species of North American freshwater fishes. Climate predicted a portion of the variation in local abundance among populations for 18 species. In addition, the relationship between climatic conditions and local abundance varied among species, which is expected as lineages partition the environment across geographical space. The influence of local habitat quality on species persistence is well documented; however, our results also indicate the importance of climate in regulating population sizes across a species geographical range, even in aquatic taxa.

Investigation of Freshwater Mussel Glochidia Presence on Asian Carp and Native Fishes of the Illinois River

Densities of introduced Asian carp (Silver Carp and Bighead Carp) in the Illinois River Basin are among the highest in the world. Asian carp have been reported to serve as hosts for Sinanodontawoodiana in their native territories, but no research has been conducted on the potential for Silver or Bighead Carp to host North American freshwater mussels. Our objectives were 1) to examine the presence of glochidia on native and non-native fishes from the Illinois River Basin, 2) to determine an optimal concentration and duration of potassium hydroxide (KOH) exposure for increasing transparency of preserved fish gills to more effectively detect the presence of glochidia and parasites, and 3) identify parasite burdens. Fifteen fish species (12 native and 3 non-native) were collected from the Illinois River Basin during summer of 2014. Preserved fins and gills of native and non-native fishes were examined for glochidia and parasite infections. We determined that a 20 min 5% KOH bath was optimal for increasing gill transparency. We recovered 242 glochidia from 5 native fish species: Bluegill, Largemouth Bass, Smallmouth Bass, Freshwater Drum, and Sauger. Based upon morphometric data, we were able to identify the glochidial larval stage of 5 groups of freshwater mussels: Group A-Lilliput, Group B- Threeridge, Group C- Deertoe or Fawnsfoot, Group D- Threehorn Wartyback, and Group E-Fragile Papershell. We did not locate glochidia on any of the non-native fish species. Future research should include the use of laboratory host trials to elucidate if Asian carp could serve as successful host fishes for native mussels or if they are recruitment sinks, a possibility that could have a major impact on the future stocks of currently imperiled freshwater mussels.

Late Pleistocene fishes of the Tennessee River Basin: an analysis of a late Pleistocene freshwater fish fauna from Bell Cave (site ACb-2) in Colbert County, Alabama, USA.

The Tennessee River Basin is considered one of the most important regions for freshwater biodiversity anywhere on the globe. The Tennessee River Basin currently includes populations of at least half of the described contemporary diversity of extant North American freshwater fishes, crayfish, mussel, and gastropod species. However, comparatively little is known about the biodiversity of this basin from the Pleistocene Epoch, particularly the late Pleistocene (∼10,000 to 30,000 years B.P.) leading to modern Holocene fish diversity patterns. The objective of this study was to describe the fish assemblages of the Tennessee River Basin from the late Pleistocene using a series of faunas from locales throughout the basin documented from published literature, unpublished reports, and an undocumented fauna from Bell Cave (site ACb-2, Colbert County, AL). Herein we discuss 41 unequivocal taxa from 10 late Pleistocene localities within the basin and include a systematic discussion of 11 families, 19 genera, and 24 identifiable species (28 unequivocal taxa) specific to the Bell Cave locality. Among the described fauna are several extirpated (e.g., Northern Pike Esox lucius, Northern Madtom Noturus stigmosus) and a single extinct (Harelip Sucker Moxostoma lacerum) taxa that suggest a combination of late Pleistocene displacement events coupled with more recent changes in habitat that have resulted in modern basin diversity patterns. The Bell Cave locality represents one of the most intact Pleistocene freshwater fish deposits anywhere in North America. Significant preservational, taphonomic, sampling, and identification biases preclude the identification of additional taxa. Overall, this study provides a detailed look into paleo-river ecology, as well as freshwater fish diversity and distribution leading up to the contemporary biodiversity patterns of the Tennessee River Basin and Mississippi River Basin as a whole.

Comparative sodium transport patterns provide clues for understanding salinity and metal responses in aquatic insects

The importance of insects in freshwater ecosystems has led to their extensive use in ecological monitoring programs. As freshwater systems are increasingly challenged by salinization and metal contamination, it is important to understand fundamental aspects of aquatic insect physiology (e.g., osmoregulatory processes) that contribute to insect responses to these stressors. Here we compared the uptake dynamics of Na as NaCl, NaHCO3 and Na2SO4 in the caddisfly Hydropsyche betteni across a range of Na concentrations (0.06–15.22mM) encompassing the vast majority of North American freshwater ecosystems. Sulfate as the major anion resulted in decreased Na uptake rates relative to the chloride and bicarbonate salts. A comparison of Na (as NaHCO3) turnover rates in the caddisfly Hydropsyche sparna and the mayfly Maccaffertium sp. revealed different patterns in the 2 species. Both species appeared to tightly regulate their whole body sodium concentrations (at ∼47±1.8μmol/gwetwt) across a range of Na concentrations (0.06–15.22mM) over 7 days. However, at the highest Na concentration (15.22mM), Na uptake rates in H. sparna (419.1μMNag−1hr−1wetwt) appeared close to saturation while Na uptake rates in Maccaffertium sp. were considerably faster (715gμMNag−1hr−1wetwt) and appeared to not be close to saturation. Na efflux studies in H. sparna revealed that loss rates are commensurate with uptake rates and are responsive to changes in water Na concentrations. A comparison of Na uptake rates (at 0.57mM Na) across 9 species representing 4 major orders (Ephemeroptera, Plecoptera, Trichoptera and Diptera) demonstrated profound physiological differences across species after accounting for the influence of body weight. Faster Na uptake rates were associated with species described as being sensitive to salinization in field studies. The metals silver (Ag) and copper (Cu), known to be antagonistic to Na uptake in other aquatic taxa did not generally exhibit this effect in aquatic insects. Ag only reduced Na uptake at extremely high concentrations, while Cu generally stimulated Na uptake in aquatic insects, rather than suppress it. These results help explain the lack of insect responses to dissolved metal exposures in traditional toxicity testing and highlight the need to better understand fundamental physiological processes in this ecologically important faunal group.

A new polystomatid (Monogenea, Polystomatidae) from the mouth of the North American freshwater turtle Pseudemys nelsoni.

Based on material collected from Pseudemys nelsoni (Reptilia: Chelonia: Emydidae) during a parasite survey of the herpetofauna around Gainesville, Florida, USA, Polystomoides nelsoni sp. n. is described as a new polystome species. This parasite was found in the oral and pharyngeal region of the host. In a sample of nine Pseudemys nelsoni, three specimens were found to release polystome eggs. One turtle was euthanized and dissected and found to be infected in the oral region with 19 specimens belonging to an as-yet-unknown Polystomoides. This is only the fifth Polystomoides recorded from the Nearctic realm. This species is distinguished from known species by a combination of characteristics including marginal hooklet morphology, body length and haptor dimensions.

Biomarker responses to estrogen and androgen exposure in the brook stickleback (Culaea inconstans): A new bioindicator species for endocrine disrupting compounds

Small-bodied freshwater fish are commonly used in regulatory testing for endocrine disrupting compounds (EDCs) but most lack a sensitive and quantifiable androgen-specific biomarker. Brook stickleback (Culaea inconstans) are a North American freshwater fish whose males produce an androgen-regulated glycoprotein in the kidney called spiggin. Although spiggin induction in females has been used as an androgen-specific biomarker of exposure in other stickleback species it has not been characterized in brook stickleback. Therefore, our objective was to develop a bioassay using brook stickleback to measure estrogenic and androgenic responses and establish the sensitivity of traditional and novel biomarkers of exposure. We first developed and optimized a qPCR assay to measure spiggin and vitellogenin transcript levels in kidney and liver tissue, respectively. Basal levels were differentially expressed in mature wild-caught male and female brook stickleback. To determine their sensitivity to EDCs, fish were exposed to nominal concentrations of 1, 10 and 100ng/L of 17α-methyltestosterone (MT) or 17α-ethinylestradiol (EE2) for 21days (sampled at 7 and 21days) under semi-static renewal conditions. MT and EE2 exposure induced spiggin and vitellogenin transcripts in female kidneys and male livers, respectively. Exposure to EE2 also increased hepatosomatic index in both sexes and decreased gonadosomatic index in females. Histopathological alterations were observed in the kidney of EE2-exposed fish and an increase in kidney epithelium cell height occurred in MT-exposed females. Given the sensitivity of these endpoints, the brook stickleback is a promising new freshwater fish model for EDC evaluation and a potential bioindicator for EDCs in North American freshwater environments.

Evolution of the premaxillary fraenum and substratum in snubnose darters and allies (Percidae: Etheostoma)

Darters (Percidae: Etheostomatinae), a species-rich group of North American freshwater fishes, vary in the presence of a premaxillary fraenum, a strip of skin that connects the premaxillary bones to the snout, and it is hypothesized that this trait is a trophic adaptation to particular substrata. Ancestral state reconstructions and analyses of phylogenetic associations between presence of the premaxillary fraenum and preferred stream substratum were conducted in a clade of closely related darters (snubnose darters and allies) that vary in morphology and habitat preferences. The most recent common ancestor of this clade was inferred to possess a fraenum and to inhabit rocky substrata, consistent with previous hypotheses, but a significant correlation between fraenum presence and substratum type across the phylogeny was not found.

Macroecology of North American suckers (Catostomidae): tests of Bergmann's and Rapoport's rules.

Discerning spatial macroecological patterns in freshwater fishes has broad implications for community assembly, ecosystem dynamics, management, and conservation. This study explores the potential interspecific covariation of geographic range (Rapoport's rule) and body size (Bergmann's rule) with latitude in North American sucker fishes (Cypriniformes: Catostomidae). While numerous tests of Rapoport's and Bergmann's rules are documented in the literature, comparatively few of these studies have specifically tested for these patterns, and none have incorporated information reflecting shared ancestry into analyses of North American freshwater fish through a hierarchical model. This study utilized a hierarchical modeling approach with Bayesian inference to evaluate the role that evolution has played in shaping these distributional corollaries. Rapoport's rule was supported at the tribe level but not across family and subfamily groupings. Particularly within the Catostominae subfamily, two tribes reflected strong support for Rapoport's rule while two suggested a pattern was present. Conversely, Bergmann's rule was not supported in Catostomidae. This study provides additional information regarding the pervasiveness of these "rules" by expanding inferences in freshwater fishes and specifically addressing the potential for these macroecological patterns to play a role in the distribution of the understudied group Catostomidae.

Covariates of age-0 walleye Sander vitreus fall recruitment from stocked populations in six midwestern reservoirs

Walleye Sander vitreus are native to central North American freshwater habitats. Introductions within and outside their native range have occurred in all of the lower 48 states. These stockings have occurred due to the increasing popularity of this species, leading to the demand surpassing the supply. Despite the popularity of this species for anglers, and extensive research on the distribution, life history, population dynamics, culture, and stocking strategies, there is disparate information on first year survival of stocked populations. The objective of this study was to test for associations between biotic and abiotic variables for recruitment of age-0 fish in the fall. We tested for the influence of the number of stocking events, moronid stocking density, spring/summer warming rate, and spring/summer maximum discharge on electrofishing catch rates in the fall of age-0 walleye. We used a hierarchical generalized linear model with the Poisson distribution to evaluate these relationships. Parameters of the model were estimated using Bayesian inference. Our model suggested the number of stocking events, moronid stocking density, and maximum discharge/rainfall were the most important factors influencing walleye recruitment. Fall catch rates of age-0 walleye were positively related to stocking events, negatively related to moronid stocking density, and negatively related to maximum discharge/rainfall. However, these relationships were not concordant across all reservoirs. Further, variability we observed across reservoirs support the conclusion that walleye populations must be managed at individual reservoirs. This study will assist managers locally and regionally to determine stocking strategies for walleye to improve recruitment in midwestern reservoirs.

Where are the South American freshwater turtle blood flukes (Trematoda: Spirorchiidae)? The first morphological and molecular analysis of spirorchiid cercariae from freshwater snails in Brazil

Trematodes belonging to the family Spirorchiidae are blood parasites mainly of turtles with a worldwide distribution. These flukes were recently reported in some marine turtles from South America, where the occurrence of spirorchiids in freshwater definitive and intermediate hosts is so far unknown. In the present study, three morphotypes of brevifurcate apharyngeate distome cercariae found in freshwater molluscs from an urban reservoir in Brazil were used for morphological and molecular (nuclear 28S rDNA) evaluation. Two morphotypes of cercariae, probably congeneric species, were found in 12/17,465 specimens of Biomphalaria spp. and differ from each other by body size and sequences (0.1%). They present morphology similar to North American freshwater spirorchiids (Spirorchis spp.), however surprisingly molecular data reveals that these lineages are more closely related to marine spirorchiids. A third species found in 2/777 Pomacea sp. differs morphologically from all previously described spirorchiid cercariae and genetically from spirorchiids with available sequences (16–19%), grouping in the phylogenetic tree with freshwater North American species. This is the first report of freshwater spirorchiids in South America and the first molecular confirmation of the involvement of a caenogastropod in the life cycle of spirorchiids.

Comparing methods to separate components of beta diversity

Summary Two alternative frameworks have been proposed to partition compositional dissimilarity into replacement and nestedness‐resultant component or into replacement and richness‐difference components. These are, respectively, the BAS (Baselga 2010, Global Ecology and Biogeography, 19, 134–143) and POD (Podani &amp; Schmera . Oikos, 120, 1625–1638) frameworks. We conduct a systematic comparison of parallel components in alternative approaches. We test whether the replacement components derived from the BAS and POD frameworks are independent of richness difference. We also evaluate whether previously reported tests of monotonicity between indices and ecological processes are informative to assess the performance of indices. Finally, we illustrate the consequences of differences between the BAS and POD frameworks using the North American freshwater fish fauna as an empirical example. In the BAS framework, the nestedness‐resultant component (βjne or βsne) accounts only for richness differences derived from nested patterns while, in the POD framework, richness‐difference dissimilarity (βrich or βrich.s) accounts for all kind of richness differences. Likewise, the replacement components of both alternative methods account for different concepts. Only the replacement component of the BAS framework (βjtu or βsim) is independent of richness difference, while the parallel component in the POD framework (β−3 or β−3.s) is not (i.e. it is mathematically constrained by richness difference). Therefore, only the BAS framework allows separating (i) the variation in species composition derived from species replacement which is independent of richness difference (i.e. not mathematically constrained by it) and (ii) the variation in species composition derived from nested patterns.

Ecological constraint and the evolution of sexual dichromatism in darters.

It is not known how environmental pressures and sexual selection interact to influence the evolution of extravagant male traits. Sexual and natural selection are often viewed as antagonistic forces shaping the evolution of visual signals, where conspicuousness is favored by sexual selection and crypsis is favored by natural selection. Although typically investigated independently, the interaction between natural and sexual selection remains poorly understood. Here, we investigate whether sexual dichromatism evolves stochastically, independent from, or in concert with habitat use in darters, a species-rich lineage of North American freshwater fish. We find the evolution of sexual dichromatism is coupled to habitat use in darter species. Comparative analyses reveal that mid-water darter lineages exhibit a narrow distribution of dichromatism trait space surrounding a low optimum, suggesting a constraint imposed on the evolution of dichromatism, potentially through predator-mediated selection. Alternatively, the transition to benthic habitats coincides with greater variability in the levels of dichromatism that surround a higher optimum, likely due to relaxation of the predator-mediated selection and heterogeneous microhabitat dependent selection regimes. These results suggest a complex interaction of sexual selection with potentially two mechanisms of natural selection, predation and sensory drive, that influence the evolution of diverse male nuptial coloration in darters.