Rapid evolution of body fluid regulation following independent invasions into freshwater habitats

Genome architecture underlying salinity adaptation in the invasive copepod Eurytemora affinis species complex: A review

With climate change, habitat salinity is shifting rapidly throughout the globe. In addition, many destructive freshwater invaders are recent immigrants from saline habitats. Recently, populations of the copepod Eurytemora affinis species complex have invaded freshwater habitats multiple times independently from saline estuaries on three continents. This review discusses features of this species complex that could enhance their evolutionary potential during rapid environmental change. Remarkably, across independent freshwater invasions, natural selection has repeatedly favored the same alleles far more than expected. This high degree of parallelism is surprising, given the expectation of nonparallel evolution for polygenic adaptation. Factors such as population structure and the genome architecture underlying critical traits under selection might help drive rapid adaptation and parallel evolution. Given the preponderance of saline-to-freshwater invasions and climate-induced salinity change, the principles found here could provide invaluable insights into mechanisms operating in other systems and the potential for adaptation in a changing planet.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Strategies of Invertebrate Osmoregulation: an Evolutionary Blueprint for Transmuting Into Fresh Water from the Sea

Early marine invertebrates like the Branchiopoda began their sojourn into dilute media some 500 million years ago in the Middle Cambrian. Others like the Mollusca, Annelida and many crustacean taxa have followed, accompanying major marine transgressions and regressions, shifting landmasses, orogenies, and glaciations. In adapting to these events and new habitats, such invertebrates acquired novel physiological abilities that attenuate the ion loss and water gain that constitute severe challenges to life in dilute media. Among these taxon-specific adaptations, selected from the subcellular to organismal levels of organization, and constituting a feasible evolutionary blueprint for invading fresh water, are reduced body permeability and surface (S) to volume (V) ratios, lowered osmotic concentrations, increased osmotic gradients, increased surface areas of interface epithelia, relocation of membrane proteins in ion-transporting cells, and augmented transport enzyme abundance, activity and affinity. We examine these adaptations in taxa that have penetrated into fresh water, revealing diversified modifications, a consequence of distinct body plans, morpho-physiological resources, and occupation routes. Contingent on life history and reproductive strategy, numerous patterns of osmotic regulation have emerged, including intracellular isosmotic regulation in weak hyper-regulators and well-developed anisosmotic extracellular regulation in strong hyper-regulators, likely reflecting inertial adaptations to early life in an estuarine environment. In this review, we address osmoregulation in those freshwater invertebrate lineages that have successfully invaded this biotope. Our analyses show that across sixty-six freshwater invertebrate species from six phyla/classes that have transmuted into fresh water from the sea, hemolymph osmolalities decrease logarithmically with increasing S: V ratios. The arthropods have the highest osmolalities, from 300 to 650 mOsmoles/kg H2O in the Decapoda with 220 to 320 mOsmoles/kg H2O in the Insecta; osmolalities in the Annelida range from 150 to 200 mOsmoles/kg H2O, the Mollusca showing the lowest osmolalities at 40 to 120 mOsmoles/kg H2O. Overall, osmolalities reach a cut-off at ∼200 mOsmoles/kg H2O, independently of increasing S: V ratio. The ability of species with small S: V ratios to maintain large osmotic gradients is mirrored in their putatively higher Na+/K+-ATPase activities that drive ion uptake processes. Selection pressures on these morpho-physiological characteristics have led to differential osmoregulatory abilities, rendering possible the conquest of fresh water while retaining some tolerance of the ancestral medium.

Recognizing salinity threats in the climate crisis

Climate change is causing habitat salinity to transform at unprecedented rates across the globe. While much of the research on climate change has focused on rapid shifts in temperature, far less attention has focused on the effects of changes in environmental salinity. Consequently, predictive studies on the physiological, evolutionary, and migratory responses of organisms and populations to the threats of salinity change are relatively lacking. This omission represents a major oversight, given that salinity is among the most important factors that define biogeographic boundaries in aquatic habitats. In this perspective, we briefly touch on responses of organisms and populations to rapid changes in salinity occurring on contemporary time scales. We then discuss factors that might confer resilience to certain taxa, enabling them to survive rapid salinity shifts. Next, we consider approaches for predicting how geographic distributions will shift in response to salinity change. Finally, we identify additional data that are needed to make better predictions in the future. Future studies on climate change should account for the multiple environmental factors that are rapidly changing, especially habitat salinity.

Cyphoderia ampulla (Cyphoderiidae: Rhizaria), a tale of freshwater sailors. The causes and consequences of ecological transitions through the salinity barrier in a family of benthic protists

The salinity barrier that separates marine and freshwater biomes is probably the most important division in biodiversity on Earth. Those organisms that successfully performed this transition had access to new ecosystems while undergoing changes in selective pressure, which often led to major shifts in diversification rates. While these transitions have been extensively investigated in animals, the tempo, mode, and outcome of crossing the salinity barrier have been scarcely studied in other eukaryotes. Here, we reconstructed the evolutionary history of the species complex Cyphoderia ampulla (Euglyphida: Cercozoa: Rhizaria) based on DNA sequences from the nuclear SSU rRNA gene and the mitochondrial cytochrome oxidase subunit I gene, obtained from publicly available environmental DNA data (GeneBank, EukBank) and isolated organisms. A tree calibrated with euglyphid fossils showed that four independent transitions towards freshwater systems occurred from the Mid Miocene onwards, coincident with important fluctuations in sea level. Ancestral trait reconstructions indicated that the whole family Cyphoderiidae had a marine origin and suggest that ancestors of the freshwater forms were euryhaline and lived in environments with fluctuating salinity. Diversification rates did not show any obvious increase concomitant with ecological transitions, but morphometric analyses indicated that species increased in size and homogenized their morphology after colonizing the new environments. This suggests adaptation to changes in selective pressure exerted by life in freshwater sediments.

Ion Transporter Gene Families as Physiological Targets of Natural Selection During Salinity Transitions in a Copepod

Salinity is a key factor that structures biodiversity on the planet. With anthropogenic change, such as climate change and species invasions, many populations are facing rapid and dramatic changes in salinity throughout the globe. Studies on the copepod Eurytemora affinis species complex have implicated ion transporter gene families as major loci contributing to salinity adaptation during freshwater invasions. Laboratory experiments and population genomic surveys of wild populations have revealed evolutionary shifts in genome-wide gene expression and parallel genomic signatures of natural selection during independent salinity transitions. Our results suggest that balancing selection in the native range and epistatic interactions among specific ion transporter paralogs could contribute to parallel freshwater adaptation. Overall, these studies provide unprecedented insights into evolutionary mechanisms underlying physiological adaptation during rapid salinity change.

Recreated Ancestral Opsin Associated with Marine to Freshwater Croaker Invasion Reveals Kinetic and Spectral Adaptation

Rhodopsin, the light-sensitive visual pigment expressed in rod photoreceptors, is specialized for vision in dim-light environments. Aquatic environments are particularly challenging for vision due to the spectrally dependent attenuation of light, which can differ greatly in marine and freshwater systems. Among fish lineages that have successfully colonized freshwater habitats from ancestrally marine environments, croakers are known as highly visual benthic predators. In this study, we isolate rhodopsins from a diversity of freshwater and marine croakers and find that strong positive selection in rhodopsin is associated with a marine to freshwater transition in South American croakers. In order to determine if this is accompanied by significant shifts in visual abilities, we resurrected ancestral rhodopsin sequences and tested the experimental properties of ancestral pigments bracketing this transition using in vitro spectroscopic assays. We found the ancestral freshwater croaker rhodopsin is redshifted relative to its marine ancestor, with mutations that recapitulate ancestral amino acid changes along this transitional branch resulting in faster kinetics that are likely to be associated with more rapid dark adaptation. This could be advantageous in freshwater due to the redshifted spectrum and relatively narrow interface and frequent transitions between bright and dim-light environments. This study is the first to experimentally demonstrate that positively selected substitutions in ancestral visual pigments alter protein function to freshwater visual environments following a transition from an ancestrally marine state and provides insight into the molecular mechanisms underlying some of the physiological changes associated with this major habitat transition.

Physiological basis of interactive responses to temperature and salinity in coastal marine invertebrate: Implications for responses to warming

Developing physiological mechanistic models to predict species' responses to climate-driven environmental variables remains a key endeavor in ecology. Such approaches are challenging, because they require linking physiological processes with fitness and contraction or expansion in species' distributions. We explore those links for coastal marine species, occurring in regions of freshwater influence (ROFIs) and exposed to changes in temperature and salinity. First, we evaluated the effect of temperature on hemolymph osmolality and on the expression of genes relevant for osmoregulation in larvae of the shore crab Carcinus maenas. We then discuss and develop a hypothetical model linking osmoregulation, fitness, and species expansion/contraction toward or away from ROFIs. In C. maenas, high temperature led to a threefold increase in the capacity to osmoregulate in the first and last larval stages (i.e., those more likely to experience low salinities). This result matched the known pattern of survival for larval stages where the negative effect of low salinity on survival is mitigated at high temperatures (abbreviated as TMLS). Because gene expression levels did not change at low salinity nor at high temperatures, we hypothesize that the increase in osmoregulatory capacity (OC) at high temperature should involve post-translational processes. Further analysis of data suggested that TMLS occurs in C. maenas larvae due to the combination of increased osmoregulation (a physiological mechanism) and a reduced developmental period (a phenological mechanisms) when exposed to high temperatures. Based on information from the literature, we propose a model for C. maenas and other coastal species showing the contribution of osmoregulation and phenological mechanisms toward changes in range distribution under coastal warming. In species where the OC increases with temperature (e.g., C. maenas larvae), osmoregulation should contribute toward expansion if temperature increases; by contrast in those species where osmoregulation is weaker at high temperature, the contribution should be toward range contraction.

Rapid evolution of genome-wide gene expression and plasticity during saline to freshwater invasions by the copepod Eurytemora affinis species complex

Saline migrants into freshwater habitats constitute among the most destructive invaders in aquatic ecosystems throughout the globe. However, the evolutionary and physiological mechanisms underlying such habitat transitions remain poorly understood. To explore the mechanisms of freshwater adaptation and distinguish between adaptive (evolutionary) and acclimatory (plastic) responses to salinity change, we examined genome-wide patterns of gene expression between ancestral saline and derived freshwater populations of the Eurytemora affinis species complex, reared under two different common-garden conditions (0 versus 15 PSU). We found that evolutionary shifts in gene expression (between saline and freshwater inbred lines) showed far greater changes and were more widespread than acclimatory responses to salinity (0 versus 15 PSU). Most notably, 30-40 genes showing evolutionary shifts in gene expression across the salinity boundary were associated with ion transport function, with inorganic cation transmembrane transport forming the largest Gene Ontology category. Of particular interest was the sodium transporter, the Na⁺ /H⁺ antiporter (NHA) gene family, which was discovered in animals relatively recently. Thirty key ion regulatory genes, such as NHA paralogue #7, demonstrated concordant evolutionary and plastic shifts in gene expression, suggesting the evolution of ion transporter function and plasticity during rapid invasions into novel salinities. Moreover, freshwater invasions were associated with the evolution of reduced plasticity in the freshwater population, again for the same key ion transporters, consistent with the predicted evolution of canalization following adaptation to stressful conditions. Our results have important implications for understanding evolutionary and physiological mechanisms of range expansions by some of the most widespread invaders in aquatic habitats.

Adaptation potential of the copepod Eurytemora affinis to a future warmer Baltic Sea

To predict effects of global change on zooplankton populations, it is important to understand how present species adapt to temperature and how they respond to stressors interacting with temperature. Here, we ask if the calanoid copepod Eurytemora affinis from the Baltic Sea can adapt to future climate warming. Populations were sampled at sites with different temperatures. Full sibling families were reared in the laboratory and used in two common garden experiments (a) populations crossed over three temperature treatments 12, 17, and 22.5°C and (b) populations crossed over temperature in interaction with salinity and algae of different food quality. Genetic correlations of the full siblings' development time were not different from zero between 12°C and the two higher temperatures 17 and 22.5°C, but positively correlated between 17 and 22.5°C. Hence, a population at 12°C is unlikely to adapt to warmer temperature, while a population at ≥17°C can adapt to an even higher temperature, that is, 22.5°C. In agreement with the genetic correlations, the population from the warmest site of origin had comparably shorter development time at high temperature than the populations from colder sites, that is, a cogradient variation. The population with the shortest development time at 22.5°C had in comparison lower survival on low quality food, illustrating a cost of short development time. Our results suggest that populations from warmer environments can at present indirectly adapt to a future warmer Baltic Sea, whereas populations from colder areas show reduced adaptation potential to high temperatures, simply because they experience an environment that is too cold.

An evolutionary perspective on marine invasions

Species distributions are rapidly changing as human globalization increasingly moves organisms to novel environments. In marine systems, species introductions are the result of a number of anthropogenic mechanisms, notably shipping, aquaculture/mariculture, the pet and bait trades, and the creation of canals. Marine invasions are a global threat to human and non-human populations alike and are often listed as one of the top conservation concerns worldwide, having ecological, evolutionary, and social ramifications. Evolutionary investigations of marine invasions can provide crucial insight into an introduced species' potential impacts in its new range, including: physiological adaptation and behavioral changes to exploit new environments; changes in resident populations, community interactions, and ecosystems; and severe reductions in genetic diversity that may limit evolutionary potential in the introduced range. This special issue focuses on current research advances in the evolutionary biology of marine invasions and can be broadly classified into a few major avenues of research: the evolutionary history of invasive populations, post-invasion reproductive changes, and the role of evolution in parasite introductions. Together, they demonstrate the value of investigating marine invasions from an evolutionary perspective, with benefits to both fundamental and applied evolutionary biology at local and broad scales.

Epigenetic patterns associated with an ascidian invasion: a comparison of closely related clades in their native and introduced ranges

Environmentally induced epigenetic modifications have been proposed as one mechanism underlying rapid adaptive evolution of invasive species. Didemnum vexillum is an invasive colonial ascidian that has established in many coastal waters worldwide. Phylogenetic analyses have revealed that D. vexillum populations consist of two distinct clades; clade B appears to be restricted to the native range (Japan), whereas clade A is found in many regions throughout the world, including New Zealand. The spread of D. vexillum clade A suggests that it might be intrinsically more invasive than clade B, despite low levels of genetic diversity compared to populations from the native region. This study investigated whether D. vexillum clade A exhibits epigenetic signatures (specifically differences in DNA methylation) associated with invasiveness. Global DNA methylation patterns were significantly different between introduced clade A colonies, and both clades A and B in the native range. Introduced colonies also showed a significant reduction in DNA methylation levels, which could be a mechanism for increasing phenotypic plasticity. High levels of DNA methylation diversity were maintained in the introduced population, despite reduced levels of genetic diversity, which may allow invasive populations to respond quickly to changes in new environments. Epigenetic changes induced during the invasion process could provide a means for rapid adaptation despite low levels of genetic variation in introduced populations.

Response to salinity and temperature changes in the alien Asian copepod Pseudodiaptomus marinus introduced in the Black Sea

The salinity tolerance and the effect of temperature were studied on the behavior and motor activity of the nonindigenous Indo-Pacific calanoid copepod Pseudodiaptomus marinus, first found in Sevastopol Bay (Black Sea) in autumn 2016. According to the index of median lethal salinity (LS₅₀ ), the salinity tolerance range of adult P. marinus collected at 18.0 psu in November 2016 and subsequently reared in the laboratory amounted to 5.0-44.0 psu, independently of the acclimation regime. Females of P. marinus collected in December 2016 at 12.0°C became torpid at 8.0°C, a value typical of winter-spring Black Sea coastal areas. An increase in temperature from 8.0°C to 27.0°C led to an increase in the beat frequency of mouth appendages, swimming speed, and time spent cruising. However, at the same high temperature, the mean cruising speed in the feeding-current feeder P. marinus was 2-fold lower than that of the native, similarly sized cruise feeder Pseudocalanus elongatus. On the contrary, mouthpart beat frequency while cruising was 2-fold higher reaching 80 Hz, due to the creation of feeding currents in P. marinus. The results of our experiments confirm the euryhaline character of P. marinus, and point to an apparent ability to survive cold temperatures in a torpid state. This suggests the possibility of entering an overwintering stage to survive the adverse cold winter-spring environmental conditions of the Black Sea, similarly to the recent thermophilic Indo-Pacific invader Oithona davisae which established a successful population in the same area.

Immunohistochemical analysis of the distribution of molecules involved in ionic and pH regulation in the lancelet Branchiostoma floridae (Hubbs, 1922)

The aim of present work is to analyse the distribution of carbonic anhydrase II (CAII), cystic fibrosis transmembrane regulator (CFTR), vacuolar-type H⁺-ATPase (V-H⁺-ATPase), Na⁺/K⁺ ATPase, Na⁺/H⁺ exchanger (NHE) and SLC26A6 (solute carrier family 26, member 6), also known as pendrin protein, in the lancelet Branchiostoma floridae in order to go in depth in the evolution of osmoregulation and pH regulation in Chordates. In view of their phylogenetic position, lancelets may indeed provide a critical point of reference for studies on osmoregulation evolution in Chordates. The results of present work demonstrated that, except to Na⁺/K⁺ ATPase that is strongly expressed in nephridia only, all the other studied molecules are abundantly present in skin, coelomic epithelium, renal papillae and nephridia and hepatic coecum. Thus, it is possible to hypothesize that also in lancelet, as in fish, these organs are involved in pH control and ionic regulation. In the digestive tract of B. floridae, the intestine epithelium was weakly immune-reactive to all tested antibodies, while the hepatic coecum showed an intense immunoreactivity to all molecules. Since in amphioxus the hepatic coecum functions simultaneously as stomach, liver and pancreas, these immunohistochemical results proved the secretion of H+ and HCO₃^- ions, typical of digestive process. Colocalization studies indicated a co-expression of the studied proteins in all considered organs, excluding NHE and pendrin for renal papillae, since some renal papillae are NHE immunopositive only.

A pedigree-based experiment reveals variation in salinity and thermal tolerance in the salmon louse, Lepeophtheirus salmonis

The salmon louse is a highly abundant ectoparasitic copepod of salmonids in the North Pacific and Atlantic. Widespread and rapid development of resistance to chemical agents used to delouse salmonids on marine farms is now threatening the continued development of the aquaculture industry and have served as a potent catalyst for the development of alternative pest management strategies. These include freshwater and warm-water treatments to which the louse is sensitive. However, given the well-documented evolutionary capacity of this species, the risk of developing tolerance towards these environmental treatments cannot be dismissed. Two common-garden experiments were performed using full-sibling families of lice identified by DNA parentage testing to investigate whether one of the fundamental premises for evolution, in this context genetic variation in the capacity of coping with fresh or warm water, exists within this species. Significant differences in survival were observed among families in both experiments, although for the salinity experiment, it was not possible to unequivocally disentangle background mortality from treatment-induced mortality. Thus, our data demonstrate genetic variation in tolerance of warm water and are suggestive of genetic variation in salinity tolerance. We conclude that extensive use of these environmental-based treatments to delouse salmonids on commercial farms may drive lice towards increased tolerance.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Marine invasions enter the genomic era: three lessons from the past, and the way forward

The expanding scale and increasing rate of marine biological invasions have been documented since the early 20th century. Besides their global ecological and economic impacts, non-indigenous species (NIS) also have attracted much attention as opportunities to explore important eco-evolutionary processes such as rapid adaptation, long-distance dispersal and range expansion, and secondary contacts between divergent evolutionary lineages. In this context, genetic tools have been extensively used in the past 20 years. Three important issues appear to have emerged from such studies. First, the study of NIS has revealed unexpected cryptic diversity in what had previously been assumed homogeneous entities. Second, there has been surprisingly little evidence of strong founder events accompanying marine introductions, a pattern possibly driven by large propagule loads. Third, the evolutionary processes leading to successful invasion have been difficult to ascertain due to faint genetic signals. Here we explore the potential of novel tools associated with high-throughput sequencing (HTS) to address these still pressing issues. Dramatic increase in the number of loci accessible via HTS has the potential to radically increase the power of analyses aimed at species delineation, exploring the population genomic consequences of range expansions, and examining evolutionary processes such as admixture, introgression, and adaptation. Nevertheless, the value of this new wealth of genomic data will ultimately depend on the ability to couple it with expanded "traditional" efforts, including exhaustive sampling of marine populations over large geographic scales, integrated taxonomic analyses, and population level exploration of quantitative trait differentiation through common-garden and other laboratory experiments.

Do saline taxa evolve faster? Comparing relative rates of molecular evolution between freshwater and marine eukaryotes

The major branches of life diversified in the marine realm, and numerous taxa have since transitioned between marine and freshwaters. Previous studies have demonstrated higher rates of molecular evolution in crustaceans inhabiting continental saline habitats as compared with freshwaters, but it is unclear whether this trend is pervasive or whether it applies to the marine environment. We employ the phylogenetic comparative method to investigate relative molecular evolutionary rates between 148 pairs of marine or continental saline versus freshwater lineages representing disparate eukaryote groups, including bony fish, elasmobranchs, cetaceans, crustaceans, mollusks, annelids, algae, and other eukaryotes, using available protein-coding and noncoding genes. Overall, we observed no consistent pattern in nucleotide substitution rates linked to habitat across all genes and taxa. However, we observed some trends of higher evolutionary rates within protein-coding genes in freshwater taxa-the comparisons mainly involving bony fish-compared with their marine relatives. The results suggest no systematic differences in substitution rate between marine and freshwater organisms.

The Legs Have It: In Situ Expression of Ion Transporters V-Type H(+)-ATPase and Na(+)/K(+)-ATPase in the Osmoregulatory Leg Organs of the Invading Copepod Eurytemora affinis

The copepod Eurytemora affinis has an unusually broad salinity range, as some populations have recently invaded freshwater habitats independently from their ancestral saline habitats. Prior studies have shown evolutionary shifts in ion transporter activity during freshwater invasions and localization of ion transporters in newly discovered "Crusalis organs" in the swimming legs. The goals of this study were to localize and quantify expression of ion transport enzymes V-type H(+)-ATPase (VHA) and Na(+)/K(+)-ATPase (NKA) in the swimming legs of E. affinis and determine the degree of involvement of each leg in ionic regulation. We confirmed the presence of two distinct types of ionocytes in the Crusalis organs. Both cell types expressed VHA and NKA, and in the freshwater population the location of VHA and NKA in ionocytes was, respectively, apical and basal. Quantification of in situ expression of NKA and VHA established the predominance of swimming leg pairs 3 and 4 in ion transport in both saline and freshwater populations. Increases in VHA expression in swimming legs 3 and 4 of the freshwater population (in fresh water) relative to the saline population (at 15 PSU) arose from an increase in the abundance of VHA per cell rather than an increase in the number of ionocytes. This result suggests a simple mechanism for increasing ion uptake in fresh water. In contrast, the decline in NKA expression in the freshwater population arose from a decrease in ionocyte area in legs 4, likely resulting from decreases in number or size of ionocytes containing NKA. Such results provide insights into mechanisms of ionic regulation for this species, with added insights into evolutionary mechanisms underlying physiological adaptation during habitat invasions.

Immunolocalization and expression of Na(+)/K(+) -ATPase in embryos, early larval stages and adults of the freshwater shrimp Palaemonetes argentinus (Decapoda, Caridea, Palaemonidae)

The euryhaline shrimp Palaemonetes argentinus exemplifies an evolutionary transition from brackish to freshwater habitats that requires adequate osmoregulatory capacities. Hyperosmoregulation is functional at hatching and it likely begins during the embryonic phase allowing this species to develop entirely in fresh water. Here, we investigated the Na(+)/K(+)-ATPase α-subunit gene (nka-α) expression using quantitative real-time PCR and localized Na(+)/K(+)-ATPase (NKA) in ion-transporting epithelia through immunofluorescence microscopy. We reared shrimps from spawning to juvenile stages at two salinities (1, 15 ‰) and maintained adults for 3 weeks at three salinity treatments (1, 15, 25 ‰). nka-α gene expression was measured in: (1) embryos at an early (SI), intermediate (SII) and late (SIII) stage of embryonic development; (2) newly hatched larvae (Zoea I, ZI); and (3) isolated gill tissue of adults. The nka-α expression was low in SI and SII embryos and reached maximum levels prior to hatching (SIII), which were similar to expression levels detected in the ZI. The nka-α expression in SIII and ZI was highest at 15 ‰, whereas salinity did not affect expression in earlier embryos. In SIII, in ZI and in a later zoeal stage ZIV, NKA was localized in epithelial cells of pleurae, in the inner-side epithelium of branchiostegite and in the antennal glands. Gills appeared in the ZIV but NKA immunolabeling of the cells of the gill shaft occurred in a subsequent developmental larval stage, the decapodid. Extrabranchial organs constitute the main site of osmoregulation in early ontogenetic stages of this freshwater shrimp.

Evolutionary mechanisms of habitat invasions, using the copepod Eurytemora affinis as a model system

The study of the copepod Eurytemora affinis has provided unprecedented insights into mechanisms of invasive success. In this invited review, I summarize a subset of work from my laboratory to highlight key insights gained from studying E. affinis as a model system. Invasive species with brackish origins are overrepresented in freshwater habitats. The copepod E. affinis is an example of such a brackish invader, and has invaded freshwater habitats multiple times independently in recent years. These invasions were accompanied by the evolution of physiological tolerance and plasticity, increased body fluid regulation, and evolutionary shifts in ion transporter (V‐type H⁺ATPase, Na⁺, K⁺‐ATPase) activity and expression. These evolutionary changes occurred in parallel across independent invasions in nature and in laboratory selection experiments. Selection appears to act on standing genetic variation during invasions, and maintenance of this variation is likely facilitated through ‘beneficial reversal of dominance’ in salinity tolerance across habitats. Expression of critical ion transporters is localized in newly discovered Crusalis leg organs. Increased freshwater tolerance is accompanied by costs to development time and greater requirements for food. High‐food concentration increases low‐salinity tolerance, allowing saline populations to invade freshwater habitats. Mechanisms observed here likely have relevance for other taxa undergoing fundamental niche expansions.

Niche Limits of Symbiotic Gut Microbiota Constrain the Salinity Tolerance of Brine Shrimp

Symbiosis generally causes an expansion of the niche of each partner along the axis for which a service is mutually provided. However, for other axes, the niche can be restricted to the intersection of each partner's niche and can thus be constrained rather than expanded by mutualism. We explore this phenomenon using Artemia as a model system. This crustacean is able to survive at very high salinities but not at low salinities, although its hemolymph's salinity is close to freshwater. We hypothesized that this low-salinity paradox results from poor performance of its associated microbiota at low salinity. We showed that, in sterile conditions, Artemia had low survival at all salinities when algae were the only source of carbon. In contrast, survival was high at all salinities when fed with yeast. We also demonstrated that bacteria isolated from Artemia's gut reached higher densities at high salinities than at low salinities, including when grown on algae. Taken together, our results show that Artemia can survive at low salinities, but their gut microbiota, which are required for algae digestion, have reduced fitness. Widespread facultative symbiosis may thus be an important determinant of niche limits along axes not specific to the mutualistic interaction.

From local adaptation to ecological speciation in copepod populations from neighboring lakes

Continental copepods have been derived from several independent invasive events from the sea, but the subsequent evolutionary processes that account for the current diversity in lacustrine environments are virtually unknown. Salinity is highly variable among lakes and constitutes a source of divergent selection driving potential reproductive isolation. We studied four populations of the calanoid copepod Leptodiaptomus cf. sicilis inhabiting four neighboring lakes with a common history (since the Late Pleistocene) located in the Oriental Basin, Mexico; one lake is shallow and varies in salinity periodically (1.4-10 g L(-1)), while three are deep and permanent, with constant salinity (0.5, 1.1 and 6.5 g L(-1), respectively). We hypothesized that (1) these populations belong to a different species than L. sicilis sensu stricto and (2) are experiencing ecologically based divergence due to salinity differences. We assessed morphological and molecular (mtDNA) COI variation, as well as fitness differences and tests of reproductive isolation. Although relationships of the Mexican populations with L. sicilis s.s. could not be elucidated, we identified a clear pattern of divergent selection driven by salinity conditions. The four populations can still be considered a single biological species (sexual recognition and hybridization are still possible in laboratory conditions), but they have diverged into at least three different phenotypes: two locally adapted, specialized in the lakes of constant salinity (saline vs. freshwater), and an intermediate generalist phenotype inhabiting the temporary lake with fluctuating salinity. The specialized phenotypes are poorly suited as migrants, so prezygotic isolation due to immigrant inviability is highly probable. This implication was supported by molecular evidence that showed restricted gene flow, persistence of founder events, and a pattern of allopatric fragmentation. This study showed how ecologically based divergent selection may explain diversification patterns in lacustrine copepods.

Comparison of whole mitochondrial genome sequences from two clades of the invasive ascidian, Didemnum vexillum

The mitochondria are the main source of cellular energy production and have an important role in development, fertility, and thermal limitations. Adaptive mitochondrial DNA mutations have the potential to be of great importance in determining aspects of the life history of an organism. Phylogenetic analyses of the globally invasive marine ascidian Didemnum vexillum using the mitochondrial cytochrome c oxidase 1 (COX1) coding region, revealed two distinct clades. Representatives of one clade (denoted by 'B') are geographically restricted to D. vexillum's native region (north-west Pacific Ocean, including Japan), whereas members of the other clade (denoted by 'A') have been introduced and become invasive in temperate coastal areas around the world. Persistence of clade B's restricted distribution may reflect it being inherently less invasive than clade A. To investigate this we sought to determine if the two clades differ significantly in other mitochondrial genes of functional significance, specifically, alterations in amino acids encoded in mitochondrial enzyme subunits. Differences in functional mitochondrial genes could indicate an increased ability for clade A colonies to tolerate a wider range of environmental temperature. Full mitochondrial genomic sequences from D. vexillum clades A and B were obtained and they predict significant sequence differences in genes encoding for enzymes involved in oxidative phosphorylation. Diversity levels were relatively high and showed divergence across almost all genes, with p-distance values between the two clades indicating recent divergence. Both clades showed an excess of rare variants, which is consistent with balancing selection or a recent population expansion. Results presented here will inform future research focusing on examining the functional properties of the corresponding mitochondrial respiration enzymes, of A and B clade enzymes. By comparing closely related taxa that have differing distributions it is possible to identify genes and phenotypes suited to particular environments. The examination of mitochondrial genotypes, and associated enzyme functioning, across populations may aid in our understanding of thermal tolerance and environmental adaptation.

Testing for beneficial reversal of dominance during salinity shifts in the invasive copepod Eurytemora affinis, and implications for the maintenance of genetic variation

Maintenance of genetic variation at loci under selection has profound implications for adaptation under environmental change. In temporally and spatially varying habitats, non-neutral polymorphism could be maintained by heterozygote advantage across environments (marginal overdominance), which could be greatly increased by beneficial reversal of dominance across conditions. We tested for reversal of dominance and marginal overdominance in salinity tolerance in the saltwater-to-freshwater invading copepod Eurytemora affinis. We compared survival of F1 offspring generated by crossing saline and freshwater inbred lines (between-salinity F1 crosses) relative to within-salinity F1 crosses, across three salinities. We found evidence for both beneficial reversal of dominance and marginal overdominance in salinity tolerance. In support of reversal of dominance, survival of between-salinity F1 crosses was not different from that of freshwater F1 crosses under freshwater conditions and saltwater F1 crosses under saltwater conditions. In support of marginal overdominance, between-salinity F1 crosses exhibited significantly higher survival across salinities relative to both freshwater and saltwater F1 crosses. Our study provides a rare empirical example of complete beneficial reversal of dominance associated with environmental change. This mechanism might be crucial for maintaining genetic variation in salinity tolerance in E. affinis populations, allowing rapid adaptation to salinity changes during habitat invasions.

Feasting in fresh water: impacts of food concentration on freshwater tolerance and the evolution of food × salinity response during the expansion from saline into fresh water habitats

Saline to freshwater invasions have become increasingly common in recent years. A key hypothesis is that rates of freshwater invasions have been amplified in recent years by increased food concentration, yet this hypothesis has remained unexplored. We examined whether elevated food concentration could enhance freshwater tolerance, and whether this effect evolves following saline to freshwater invasions. We examined physiological response to salinity and food concentration in a 2 × 2 factorial design, using ancestral brackish and freshwater invading populations of the copepod Eurytemora affinis. We found that high food concentration significantly increases low-salinity tolerance. This effect was reduced in the freshwater population, indicating evolution following the freshwater invasion. Thus, ample food could enable freshwater invasions, allowing subsequent evolution of low-salinity tolerance even under food-poor conditions. We also compared effects of food concentration on freshwater survival between two brackish populations from the native range. Impacts of food concentration on freshwater survival differed between the brackish populations, suggesting variation in functional properties affecting their propensity to invade freshwater habitats. The key implication is that high food concentration could profoundly extend range expansions of brackishwater species into freshwater habitats, potentially allowing for condition-specific competition between saline invaders and resident freshwater species.