Effect of salinity change on cardiac activity in Hiatella arctica and Modiolus modiolus, in the White Sea

A great majority of salinity studies have dealt with intertidal species. Little is known about the way subtidal animals respond to salinity fluctuations. Even less details are available on invertebrates from the White Sea, which salinity is ca. 25. The heart rate of two subtidal Bivalvia—Hiatella arctica and Modiolus modiolus—exposed to different salinities was recorded. Changes in cardiac activity were monitored for 9 days of the animals’ acclimation to salinities of 15, 20, 30 and 35, and for 4 days of reacclimation (return to the initial salinity of 25). The initial response to salinity change was a significant heart rate reduction. On the other hand, cardiac activity in M. modiolus intensified at salinities of 30 and 35. Reacclimation induced different HR responses: from a decrease to a rise, depending on the species and the salinity applied in the experiment. The differences in responses to salinity are discussed with respect to the morphological and ecological characteristics of the species.     

A study of the salt lakes and salt springs of Eyre Peninsula, South Australia

An 18-month-study of 40 saline wetlands, ranging from 6 to 336 g l⁻¹, on the west and southern coasts of Eyre Peninsula yielded 88 species of invertebrates, some aquatic plants and a fish. The invertebrates are taxonomically diverse and include 38 crustaceans, 28 insects, 12 molluscs and significantly an aquatic spider, a nemertean, two polychaetes, two sea anemones, a sponge and a bryzoan. Most were tolerant of wide fluctuations in salinity, there being 51 halobionts, 21 halophils and only 16 salt-tolerant freshwater species. Many invertebrates are restricted to the thalassic springs where marine molluscs dominated. Athalassic wetlands were dominated by crustaceans and were of two basic types—coastal and continental. There is evidence of the former evolving biologically into the later, and for some lakes to be still in transition. There is also evidence of increasing salinity in recent decades and already two lakes exhibit severe secondary salinity. Like other salt lakes in Australia the fauna is regionally distinctive. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

The salinity tolerance of freshwater macroinvertebrate eggs and hatchlings in comparison to their older life-stages: a diversity of responses

The tolerance to high salinity of older life stage macroinvertebrates could underestimate the effects of increasing salinity on freshwater macroinvertebrates. The salinity tolerance of the eggs and/or hatchlings of Glyptophysa gibbosa (Planorbidae), Glyptophysa aliciae, Glacidorbis sp. (Glacidorbidae), a Glossiponiidae, Piona cumberlandis (Pionidae), and Chironomus sp. (Chironomidae) were determined. The salinity tolerances of younger life-stages of species studied here and elsewhere were then compared to salinity tolerances of their mature aquatic or dominant life-stage. A diversity of responses have been observed with some species having similar salinity tolerances in all life-stages examined, whilst the eggs or hatchlings of other species had salinity tolerances ranging from 4% to 88% of their older life stages. On present knowledge, this diversity of responses presents some difficulties for simple rules of thumb to approximate sensitivity of young life-stages of freshwater macroinvertebrates inferred from their dominant stage’s tolerance.     

Tolerance of free‐living nematode species to imidacloprid and diuron

The neonicotinoid imidacloprid and the herbicide diuron are long‐lived pesticides commonly detected in European rivers. Both have lethal as well as sublethal effects on aquatic invertebrates dwelling in streambeds. Here, we performed lethality tests of imidacloprid and diuron on seven species of widespread, free‐living nematodes and the model organism Caenorhabditis elegans. Our results indicated that nematodes were relatively tolerant to both pesticides, and only two species (Diploscapter coronatus and Plectus opisthocirculus) showed mortality at high nominal concentrations of imidacloprid (119 mg/L) and diuron (33 mg/L). The changes observed in nematode community structure after imidacloprid and diuron exposure may have been related to trade‐offs between sensitivity to toxicants and changes in competitive abilities of the species. While the former can be tested using single‐species tests, we recommend that the latter be tested in further experiments using multispecies communities. Our results suggest that the presence of these pesticides could favor nematodes over other meiofaunal groups found in freshwater sediments.     

pH and thermal stability studies of carboxymethyl cellulase from intergeneric fusants of Trichoderma reesei/Saccharomyces cerevisiae

The combined effect of pH and temperature on carboxymethyl cellulase from two intergeneric fusants (M 14 and M 62) of Trichoderma reesei QM 9414/Saccharomyces cerevisiae NCIM 3288 was studied using response surface methodology. A central composite design for two variables was employed for the optimization studies. This study was compared with similar studies carried out with Trichoderma reesei QM 9414. The optimal pH and temperature for the enzymes derived from these organisms were: for the fusant M 14—pH 5.7 and 41.7°C, for the fusant M 62—pH 5.3 and 43°C, and for Trichoderma reesei QM 9414—pH 4.31 and 38.3°C. Received 5 May 1997/ Accepted in revised form 17 July 1998     

Effects of macrophyte growth forms on invertebrate communities in saline lakes of the Wyoming High Plains

In saline lakes, areal cover and both species and structural diversity of macrophytes often decline as salinity increases. To assess effects of the loss of certain macrophyte growth forms, we characterized benthic and epiphytic invertebrates in three growth forms (thin-stemmed emergents, erect aquatics, and low macroalgae) in oligosaline lakes (0.8–4.2 mS cm⁻¹) of the Wyoming High Plains, USA. We also measured the biomass and taxonomic composition of epiphytic and benthic invertebrates in two erect aquatics with very similar structure that are found in both oligosaline (Potamogeton pectinatus) and mesosaline (9.3–23.5 mS cm⁻¹) (Ruppia maritima) lakes. Although total biomass of epiphytic invertebrates varied among oligosaline lakes, the relative distribution of biomass among growth forms was similar. For epiphytic invertebrates, biomass per unit area of lake was lowest in emergents and equivalent in erect aquatics and low macroalgae; biomass per unit volume of macrophyte habitat was greatest in low macroalgae. For benthic invertebrates, biomass was less beneath low macroalgae than other growth forms. Taxonomic composition did not differ appreciably between growth forms for either benthic or epiphytic invertebrates, except that epiphytic gastropods were more abundant in erect aquatics. Total biomass of epiphytic and benthic invertebrates for the same growth form (erect aquatic) did not differ between oligosaline (Potamogeton pectinatus) and mesosaline (Ruppia maritima) lakes, but taxonomic composition did change. In the oligosaline to mesosaline range, direct toxic effects of salinity appeared important for some major taxa such as gastropods and amphipods. However, indirect effects of salinity, such as loss of macrophyte cover and typically higher nutrient levels at greater salinities, probably have larger impacts on total invertebrate biomass lake-wide.     

Isolation and characterization of a <Emphasis Type="Italic">Paramecium</Emphasis> cDNA clone encoding a putative serine/threonine protein kinase

Mitogen-activated protein (MAP) kinases, a closely related family of protein kinases, are involved in cell cycle regulation and differentiation in yeast and human cells. They have not been documented in ciliates. We used PCR to amplify DNA sequences of a ciliated protozoan—Paramecium caudatum—using primers corresponding to amino acid sequences that are common to MAP kinases. We isolated and sequenced one putative MAP kinase-like serine/threonine kinase cDNA from P. caudatum. This cDNA, called pcstk1 (Paramecium caudatum Serine/Threonine Kinase 1) shared approximately 35% amino acid identity with MAP kinases from yeast. MAP kinases are activated by phosphorylation of specific threonine and tyrosine residues. These two amino acid residues are conserved in the PCSTK1 sequence at positions Thr 159 and Tyr 161. The PSTAIRE motif, which is characteristic of the CDK2 gene family, cannot be found in ORF of PCSTK1. The highest homology score was to human STK9, which contains MAP type kinase domains. Comparisons of expression level have shown that pcstk1 is expressed equally in cells at different stages (sexual and asexual). We discussed the possibility, as in other organisms, that a family of MAP kinase genes exists in P. caudatum.     

Evaluation of biocathodes in freshwater and brackish sediment microbial fuel cells

Biofilms on biocathodes can catalyze the cathodic oxygen reduction and accordingly guarantee high cathode redox potentials. The present research assessed the use of biocathodes in full-sediment microbial fuel cells. Carbon felt-based biocathodes were evaluated in freshwater systems, and an extension of their application to brackish systems and/or stainless steel webs as base material was considered. Efficient biocathodes could be developed within days through inoculation with active microorganisms. Carbon felt was found most suited for the biocathodes in freshwater with increased performance at salinities around 80–250 mM. Maximum long-term performance reached 12.3 μW cm⁻² cathode. The relative benefit of stainless steel seemed to increase with increasing salinity. A combination of stainless steel cathodes with biofilms could, however, also result in decreased electrical performance. In an efficiently catalyzing cathodic biofilm, an enrichment with an uncultured Proteobacterium—previously correlated with steel waste—was observed.     

Photosynthesis and water relations of four oak species: impact of flooding and salinity

 As global climate changes, sea level rise and increased frequency of hurricanes will expose coastal forests to increased flooding and salinity. Quercus species are frequently dominant in these forest, yet little is known about their salinity tolerance, especially in combination with flooding. In this study, 1-year-old seedlings of Quercus lyrata Walt. (overcup oak), Q. michauxii Nutt. (swamp chestnut oak), Q. nigra L. (water oak), and Q. nuttallii Palmer (Nuttall oak) were chronically (simulating sea level rise) and acutely (simulating hurricane storm surge) exposed to increased flooding and salinity, individually and in combination. The four species demonstrated two response patterns of photosynthesis (A), conductance, and leaf water potential, apparently related to their relative flood tolerance. In Q. lyrata, Q. nuttallii, and Q. nigra (moderately flood-tolerant), A was not immediately reduced after the initiation of the freshwater flooding, but was reduced as the duration of flooding increased. In the second pattern, demonstrated by the weakly flood-tolerant Q. michauxii, A was immediately reduced by freshwater flooding with an increasing impact over time. Watering with 2 parts per thousand (ppt) saline water did not consistently reduce A, but flooding with 2 ppt reduced A of all species, similar to the response with freshwater flooding. Photosynthesis of all species was reduced by 6 ppt watering or flooding, with the latter treatment killing all species within 8 weeks. When acutely exposed to 30 ppt salinity, A was quickly and severely reduced regardless of whether the seedlings were watered or flooded. Acutely flooded seedlings exposed to high salinity died within 2 weeks, but seedlings watered with 30 ppt saline water recovered and A was not reduced the following spring. As saline flooding of coastal areas increases due to sea level rise, photosynthesis of these species will be differentially affected based primarily on their flood tolerance. This suggests that increased flooding associated with sea level rise will impact these tree species to a greater extent than small increases in soil salinity. High salinity accompanying storm surges will be very harmful to all of these species. Received: 20 October 1997 / Accepted: 2 December 1998     

Response to selection and evolvability of invasive populations

While natural selection might in some cases facilitate invasions into novel habitats, few direct measurements of selection response exist for invasive populations. This study examined selection response to changes in salinity using the copepod Eurytemora affinis. This copepod has invaded fresh water from saline habitats multiple times independently throughout the Northern Hemisphere. Selection response to a constant intermediate salinity (5 PSU) was measured in the laboratory for saline source and freshwater invading populations from the St. Lawrence drainage (North America). These populations were reared under three conditions: (1) native salinities (0 or 15 PSU) for at least two generations, (2) 5 PSU for two generations, and (3) 5 PSU for six generations. Full-sib clutches taken from populations reared under these three conditions were split across four salinities (0, 5, 15, and 25 PSU) to determine reaction norms for survival and development time. Contrasts in survival and development time across the three rearing conditions were treated as the selection response. Selection at 5 PSU resulted in a significant decline in freshwater (0 PSU) tolerance for both the saline and freshwater populations. Yet, evolutionary differences in freshwater tolerance persisted between the saline and freshwater populations. The saline and freshwater populations converged in their high-salinity (25 PSU) tolerance, with an increase in the freshwater population and decline in the saline population. Development time did not shift greatly in response to selection at 5 PSU. For all three rearing conditions, the freshwater population exhibited retarded larval development and accelerated juvenile development relative to the saline population. Results from this study indicate that both the saline and freshwater populations exhibit significant responses to selection for a fitness-related trait critical for invasions into a novel habitat. For the Symposium on “Evolvability and Adaptation of Invasive Species,” Society for the Study of Evolution 2004.     

Salinity tolerance of aRhizobium meliloti strain isolated from salt affected soils

The salinity tolerance of aRhizobium meliloti strain isolated from salt-affected soils was examined. Growth of the strain on yeast—mannitol broth containing 0–1.2% NaCl exhibited in all cases the same generation time and simultaneous onset of the stationary phase while the total viable number of cells was the same for three continuous generations. The nodulation, plant yield and elemental composition ofMedicago sativa plants grown on agar slopes, inoculated with cultures from the third generation grown on broth containing 0–1.2% NaCl responded identically to all inocula. The salinity tolerance of the strain in fixing nitrogen was furthermore demonstrated withM. sativa plants grown on either nitrogen-free agar slopes containing 0.2–1.2% NaCl or soil-agar slopes with saline soil in which 0.15 and 0.3% additional NaCl was used.     

Larval salinity tolerance of the South American salt-marsh crab, Neohelice (Chasmagnathus) granulata: physiological constraints to estuarine retention, export and reimmigration

The semiterrestrial crab Neohelice (=Chasmagnathus) granulata (Dana 1851) is a predominant species in brackish salt marshes, mangroves and estuaries. Its larvae are exported towards coastal marine waters. In order to estimate the limits of salinity tolerance constraining larval retention in estuarine habitats, we exposed in laboratory experiments freshly hatched zoeae to six different salinities (5–32‰). At 5‰, the larvae survived for a maximum of 2 weeks, reaching only exceptionally the second zoeal stage, while 38% survived to the megalopa stage at 10‰. Shortest development and negligible mortality occurred at all higher salt concentrations. These observations show that the larvae of N. granulata can tolerate a retention in the mesohaline reaches of estuaries, with a lower limit of ca. 10–15‰. Maximum survival at 25‰ suggests that polyhaline conditions rather than an export to oceanic waters are optimal for successful larval development of this species. In another experiment, we tested the capability of the last zoeal stage (IV) for reimmigration from coastal marine into brackish waters. Stepwise reductions of salinity during this stage allowed for moulting to the megalopa at 4–10‰. Although survival was at these conditions reduced and development delayed, these results suggest that already the zoea-IV stage is able to initiate the reimmigration into estuaries. After further salinity reduction, megalopae survived in this experiment for up to >3 weeks in freshwater, without moulting to juvenile crabs. In a similar experiment starting from the megalopa stage, successful metamorphosis occurred at 4–10‰, and juvenile growth continued in freshwater. Although these juvenile crabs showed significantly enhanced mortality and smaller carapace width compared to a seawater control, our results show that the late larval and early juvenile stages of N. granulata are well adapted for successful recruitment in brackish and even limnetic habitats.     

Lepidium latifolium reproductive potential and seed dispersal along salinity and moisture gradients

Lepidium latifolium is an aggressive plant species that is invading both wetlands and uplands across a wide range of salinities. This study examined how salinity and moisture gradients influence the potential for invasion by L. latifolium. Three sites in the San Francisco Bay Delta with varying salinity and moisture levels were chosen as research sites. These sites corresponded to a dry (18.32%) freshwater (3.88‰) site, a wet (40.53%) brackish (23.16‰) site, and a moderately wet (38.33%) saline (32.33‰) site. Our results showed that inflorescence number and height were unaffected by salinity or soil moisture. Seed production was significantly affected by salinity (P = 0.0297) and moisture levels (P = 0.0004). Seed production at the high salinity site was reduced by 29% from the freshwater site. Seed production at the wettest site had an 87% reduction from the driest site. Seed viability was also reduced by both salinity (P < 0.0001) and soil moisture (P < 0.0001). Viability at the highest salinity site was reduced by 49% from freshwater sites and was reduced by 8% from the wettest to driest sites. Mean seed dispersal distance was 0.23 m greater at the freshwater site, which was not statistically significant (P = 0.1815). The deleterious effects of salinity and moisture resulted in reduced L. latifolium densities in high salinity and moisture locations, but only at the highest salinity site. With increased seed production and viability, drier freshwater sites experience greater propagule pressure, resulting in an increased invasion potential. Therefore, variability along salinity and soil moisture gradients serve as useful metrics for prioritizing control and eradication efforts of L. latifolium.     

Effects of different ionic compositions on survival and growth of <Emphasis Type="Italic">Physa acuta</Emphasis>

Most inland saline waters in southern Australia predominantly contain Na⁺ and Cl⁻ as major ions. The proportions of Ca²⁺, Mg²⁺, SO₄ ²⁻, HCO₃ ⁻ and CO₃ ²⁻ in these waters somewhat vary and might influence salinity tolerance of freshwater organisms. Here the salinity stress of five ionic compositions to the freshwater snail Physa acuta Draparnaud (Gastropoda: Physidae) was compared: commercial sea salt Ocean Nature (ON), synthetic Ocean Nature (ONS) and three saline water types that are common in southern Australia (ONS but without [1]: SO₄ ²⁻, HCO₃ ⁻ and CO₃ ²⁻, [2]: Ca²⁺, HCO₃ ⁻ and CO₃ ²⁻, [3]: Ca²⁺, Mg²⁺), Milli-Q water was used as a negative control. The 96-h LC₅₀ values for all treatments did not differ. However in prolonged sub-lethal exposures, results varied depending on the ionic composition. Growth was negative and shell strength reduced in treatments lacking Ca. Though the content of major cationic elements (Ca, Mg, Na and K) did not differ per unit dry weight of snail across the treatments, the total load of these elements per individual snail varied among treatments. Furthermore, at the sub-lethal salinities tested, 1 and 5 mS cm⁻¹, ionic compositions had more effect on the snail’s growth than salinity. The long-term effects on freshwater animals, especially taxa with calcium-based exoskeletons, from exposure to common saline water types with low calcium concentrations will likely be greater than from exposure to saline waters with an ionic composition similar to seawater.     

Combined effects of salinity and infectious disease on Daphnia dentifera at multiple scales

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Photosynthesis and UV-B Tolerance of the Marine Alga Fucus vesiculosus at Different Sea Water Salinities

The marine algal species in the Baltic Sea are few due to the low sea water salinity. One of the few species that can be found is Fucus vesiculosus. Even this species is affected by the low salinity and becomes smaller in size in the Baltic. In present work the photosynthesis of F. vesiculosus in the northern Baltic (Bothnian Sea) was compared to the photosynthesis of F. vesiculosus in the Atlantic. Oxygen evolution was measured before and after exposure to 2.3 W of UV-B (280–320 nm) radiation for 5 h, as well as after 48 h recovery in low light. The plants were kept in their own sea water salinity as well as in a changed salinity, this to examine possible correlations between salinity and photosynthesis. The results show a significant higher initial maximal photosynthesis (P _max) for Atlantic plants (10.3 nmol O₂ g⁻¹ FW s⁻¹) compared to Baltic plants (4.0 nmol O₂ g⁻¹ FW s⁻¹). The Baltic plants were found more sensitive to UV-B with a 40–50% decrease of P _max as well as a lower degree of recovery (60–70% compared to 75–95% for the Atlantic plants). The higher salinity (35 psu) had a positive effect on the Baltic F. vesiculosus with increased P _max as well as increased tolerance to UV-B. The lower salinity (5 psu) had a negative effect on the Atlantic plants with a decreased P _max as well as a lower tolerance to UV-B. Pigment content was found higher in Atlantic F. vesiculosus. The pigment content decreased then the Atlantic plants were transferred to 5 psu. The concentration of Chl a as well as the total content of violaxanthin, diadinoxanthin and zeaxanthin in Baltic plants increased when transferred to 35 psu. The Atlantic F. vesiculosus can not survive the low salinity in the northern Baltic (died within 8 weeks). It is likely that a long time acclimation or adaptation to low salinity has taken place for F. vesiculosus in northern Baltic. If this is an ecotypic or genotypic development it is too early to say.     

Seasonal plant water uptake patterns in the saline southeast Everglades ecotone

The purpose of this study was to determine the seasonal water use patterns of dominant macrophytes coexisting in the coastal Everglades ecotone. We measured the stable isotope signatures in plant xylem water of Rhizophora mangle, Cladium jamaicense, and Sesuvium portulacastrum during the dry (DS) and wet (WS) seasons in the estuarine ecotone along Taylor River in Everglades National Park, FL, USA. Shallow soilwater and deeper groundwater salinity was also measured to extrapolate the salinity encountered by plants at their rooting zone. Average soil water oxygen isotope ratios (δ ¹⁸O) was enriched (4.8 ± 0.2‰) in the DS relative to the WS (0.0 ± 0.1‰), but groundwater δ ¹⁸O remained constant between seasons (DS: 2.2 ± 0.4‰; WS: 2.1 ± 0.1‰). There was an inversion in interstitial salinity patterns across the soil profile between seasons. In the DS, shallow water was euhaline [i.e., 43 practical salinity units (PSU)] while groundwater was less saline (18 PSU). In the WS, however, shallow water was fresh (i.e., 0 PSU) but groundwater remained brackish (14 PSU). All plants utilized 100% (shallow) freshwater during the WS, but in the DS R. mangle switched to a soil–groundwater mix (δ 55% groundwater) while C. jamaicense and S. portulacastrum continued to use euhaline shallow water. In the DS, based on δ ¹⁸O data, the roots of R. mangle roots were exposed to salinities of 25.4 ± 1.4 PSU, less saline than either C. jamaicense (39.1 ± 2.2 PSU) or S. portulacastrum (38.6 ± 2.5 PSU). Although the salinity tolerance of C. jamaicense is not known, it is unlikely that long-term exposure to high salinity is conducive to the persistence of this freshwater marsh sedge. This study increases our ecological understanding of how water uptake patterns of individual plants can contribute to ecosystem levels changes, not only in the southeast saline Everglades, but also in estuaries in general in response to global sea level rise and human-induced changes in freshwater flows.     

Ammonia exposure of Chasmagnathus granulata (Crustacea, Decapoda) Dana, 1851: accumulation in haemolymph and effects on osmoregulation

In order to study lethal and sublethal effects of ammonia to the estuarine crab Chasmagnathus granulata in the presence of an additional stress factor such as salinity, we determined the LC₅₀ (96 h) of ammonia at 20‰ and in response to osmotic stress (5–40‰) and evaluated ammonia accumulation in the haemolymph of C. granulata and ammonia effects on osmo- and ion-regulation of this species through determinations of the haemolymph Na⁺, Ca²⁺, Cl⁻ and osmotic concentration. The LC₅₀ values (96 h) of total ammonia (NH₃+NH₄⁺) were 10.10, 17.85 and 14.0 mM for crabs maintained at 5, 20 or 40‰ salinity, respectively, suggesting that this crab is fairly resistant to ammonia. The haemolymph ammonia concentration augmented with ambient ammonia during a 6-h exposure to sublethal ammonia concentrations which were not enough to reach equilibrium between external and haemolymph ammonia. At 20‰ salinity, following a 96-h exposure to sublethal concentrations, a significant decrease (P<0.05) of haemolymphatic chloride concentration was registered at 3.3 and 5.5 mM of total ammonia. At 40‰ salinity, a significant increase (P<0.05) of the haemolymph osmotic pressure was apparent at 5.5 mM total ammonia. We postulate that C. granulata gives priority to NH₃ formation as a mechanism to eliminate it by simple diffusion. The differential Na⁺ and Cl⁻ regulation of crabs maintained at 20‰ salinity could modify the strong ion difference, augmenting pH, which in turn should lead the NH₄⁺/NH₃ equilibrium towards NH₃.     

Comparing the low‐salinity tolerance of Ulva species distributed in different environments

The green macroalgal genus Ulva (Ulvales, Ulvophyceae, Chlorophyta) is distributed worldwide from marine to freshwater environments. Comparative analyses of hyposalinity tolerance among marine, brackish, and freshwater Ulva species were performed by fluorescein diacetate viability counts. The subtidal marine species Ulva sp., collected from a depth of 30 m, showed the poorest tolerance to low salinity. This species died in 5 practical salinity units (PSU) artificial seawater or freshwater within 1 day. Its closely related species U. linza L. (an intertidal species) and U. prolifera Müller (a brackish species) showed varying tolerances to low salinity. After 7 days of freshwater exposure, the viability of U. linza L. decreased to approximately 20%, while U. prolifera Müller showed nearly 100% viability. The freshwater species U. limnetica Ichihara et Shimada, not yet found in coastal areas, was highly viable in seawater.     

Tempo and mode of the multiple origins of salinity tolerance in a water beetle lineage

Salinity is one of the most important drivers of the distribution, abundance and diversity of organisms. Previous studies on the evolution of saline tolerance have been mainly centred on marine and terrestrial organisms, while lineages inhabiting inland waters remain largely unexplored. This is despite the fact that these systems include a much broader range of salinities, going from freshwater to more than six times the salinity of the sea (i.e. >200 g/L). Here, we study the pattern and timing of the evolution of the tolerance to salinity in an inland aquatic lineage of water beetles (Enochrus species of the subgenus Lumetus, family Hydrophilidae), with the general aim of understanding the mechanisms by which it was achieved. Using a time‐calibrated phylogeny built from five mitochondrial and two nuclear genes and information about the salinity tolerance and geographical distribution of the species, we found that salinity tolerance appeared multiple times associated with periods of global aridification. We found evidence of some accelerated transitions from freshwater directly to high salinities, as reconstructed with extant lineages. This, together with the strong positive correlation found between salinity tolerance and aridity of the habitats in which species are found, suggests that tolerance to salinity may be based on a co‐opted mechanism developed originally for drought resistance.