Groundwater seepage along a Barrier Island

Resistivity and water level measurements were made on a barrier island on the south shore of Long Island, New York to examine the distribution of fresh groundwater and the potential for recirculation of saline groundwater. The depth to the base of the freshwater lens was overpredicted by calculations of the static-equilibrium depth to a sharp interface apparently because of the sensitivity of the calculation to the low water-table elevations which are in turn sensitive to variations in sea level because of the existence of a transition zone between fresh and saline groundwater. Mixing and recirculation of saline groundwater at the base of the lens produced a transition zone up to 9.65 m thick. Measurements also support model forecasts of a mean bay level several centimeters above sea level, augmented by atmospheric forcing and wave setup. A time lag of about 8 hours between the response of the ocean level to longshore winds and the corresponding response of the bay level can result in a difference in elevation between the bay and the ocean that is up to four times that produced by other agents such as Stokes transport and density differences. In the presence of differential hydraulic head, bay and ocean water may be exchanged via groundwater flow between the base of the freshwater lens under the barrier beach and a deeper clay layer.     

Decreased precipitation exacerbates the effects of sea level on coastal dune ecosystems in open ocean islands

The alteration of fresh and marine water cycling is likely to occur in coastal ecosystems as climate change causes the global redistribution of precipitation while simultaneously driving sea‐level rise at a rate of 2–3 mm yr^?1. Here, we examined how precipitation alters the ecological effects of ocean water intrusion to coastal dunes on two oceanic carbonate islands in the Bahamas. The approach was to compare sites that receive high and low annual rainfall and are also characterized by seasonal distribution (wet and dry season) of precipitation. The spatial and temporal variations in precipitation serve as a proxy for conditions of altered precipitation which may occur via climate change. We used the natural abundances of stable isotopes to identify water sources (e.g., precipitation, groundwater and ocean water) in the soil–plant continuum and modeled the depth of plant water uptake. Results indicated that decreased rainfall caused the shallow freshwater table on the dune ecosystem to sink and contract towards the inland, the lower freshwater head allowed ocean water to penetrate into the deeper soils, while shallow soils became exceedingly dry. Plants at the drier site that lived nearest to the ocean responded by taking up water from the deeper and consistently moist soil layers where ocean water intruded. Towards the inland, decreased rainfall caused the water table to sink to a depth that precluded both recharge to the upper soil layers and access by plants. Consequently, plants captured water in more shallow soils recharged by infrequent rainfall events. The results demonstrate dune ecosystems on oceanic islands are more susceptible to ocean water intrusion when annual precipitation decreases. Periods of diminished precipitation caused drought conditions, increased exposure to saline marine water and altered water‐harvesting strategies. Quantifying species tolerances to ocean water intrusion and drought are necessary to determine a threshold of community sustainability.     

On the possibility for generic modeling of submarine groundwater discharge

We simulate large-scale dynamics of submarine groundwater discharge (SGD) in three different coastal aquifers on the Mediterranean Sea. We subject these aquifers to a wide range of different groundwater management conditions, leading to widely different net groundwater drainage from land to sea. The resulting SGD at steady-state is quantifiable and predictable by simple linearity in the net land-determined groundwater drainage, defined as total fresh water drainage minus groundwater extraction in the coastal aquifer system. This linearity appears to be general and independent of site-specific, variable and complex details of hydrogeology, aquifer hydraulics, streamlines and salinity transition zones in different coastal systems. Also independently of site-specifics, low SGD implies high seawater content due to seawater intruding into the aquifer and mixing with fresh groundwater within a wide salinity transition zone in the aquifer. Increasing SGD implies decreasing seawater content, decreased mixing between seawater and fresh groundwater and narrowing of the salinity transition zone of brackish groundwater in the aquifer.     

Response of Juniperus phoenicea on sandy dunes in the Camargue (France) to water and saline constraint in summer

The ‘Bois des Rièges’ woodland occurs on the relic littoral dunes in the National Reserve of the Camargue. Although surrounded by brackish ponds and saline lands with very salt groundwater close to the surface, the dunes are covered with non-halophytic vegetation of herbaceous shrubs and trees, including the dominant shrub Juniperus phoenicea. This is due to the presence of a freshwater lens beneath the dunes supplied by rains, and floating in hydrodynamic equilibrium upon the saline aquifer. The importance and duration of the freshwater stock depends on the precipitation-evapotranspiration balance as well as on the size of the dune. From the end of the spring to the autumn rain period this freshwater stock is considerably reduced, while the capillary potential in the zone of aeration of soil and the osmotic potential of the soil solution, influenced by capillary rise of the brackish groundwater, decrease. In summer the vegetation is thus subjected to severe drought caused by lack of water or salt excess, to which it must adjust its biological activity. The water relationships in the soil-plant system have been studied along a transect between the top and the borders of a dune surrounded by saline lands. Using simultaneous water potential measurements of the sunny and shady sides of Juniperus trees daily and seasonal transpiration regulations were studied. The preliminary results indicate that trees on the border of the dune as well as on tops are absorbing water from the same freshwater lens in the middle of the dune.     

Pelagic sea snakes dehydrate at sea

Secondarily marine vertebrates are thought to live independently of fresh water. Here, we demonstrate a paradigm shift for the widely distributed pelagic sea snake, Hydrophis (Pelamis) platurus, which dehydrates at sea and spends a significant part of its life in a dehydrated state corresponding to seasonal drought. Snakes that are captured following prolonged periods without rainfall have lower body water content, lower body condition and increased tendencies to drink fresh water than do snakes that are captured following seasonal periods of high rainfall. These animals do not drink seawater and must rehydrate by drinking from a freshwater lens that forms on the ocean surface during heavy precipitation. The new data based on field studies indicate unequivocally that this marine vertebrate dehydrates at sea where individuals may live in a dehydrated state for possibly six to seven months at a time. This information provides new insights for understanding water requirements of sea snakes, reasons for recent declines and extinctions of sea snakes and more accurate prediction for how changing patterns of precipitation might affect these and other secondarily marine vertebrates living in tropical oceans.     

Competition between Hardwood Hammocks and Mangroves

A bstract The boundaries between mangroves and freshwater hammocks in coastal ecotones of South Florida are sharp. Further, previous studies indicate that there is a discontinuity in plant predawn water potentials, with woody plants either showing predawn water potentials reflecting exposure to saline water or exposure to freshwater. This abrupt concurrent change in community type and plant water status suggests that there might be feedback dynamics between vegetation and salinity. A model examining the salinity of the aerated zone of soil overlying a saline body of water, known as the vadose layer, as a function of precipitation, evaporation and plant water uptake is presented here. The model predicts that mixtures of saline and freshwater vegetative species represent unstable states. Depending on the initial vegetation composition, subsequent vegetative change will lead either to patches of mangrove coverage having a high salinity vadose zone or to freshwater hammock coverage having a low salinity vadose zone. Complete or nearly complete coverage by either freshwater or saltwater vegetation represents two stable steady-state points. This model can explain many of the previous observations of vegetation patterns in coastal South Florida as well as observations on the dynamics of vegetation shifts caused by sea level rise and climate change.     

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.     

The effects of depth and salinity on juvenile Chinook salmon Oncorhynchus tshawytscha (Walbaum) habitat choice in an artificial estuary

The energetic cost for juvenile Chinook salmon Oncorhynchus tshawytscha to forage in habitats of different salinity and depth was quantified using a behavioural titration based on ideal free distribution theory. When given a choice between freshwater habitats of different depths (>0·83 or <0·83 m), a greater proportion of fish used the deeper habitat. When the deeper habitat was saltwater, the proportion of fish using it increased. When food was added to both the shallow freshwater and deep saline habitats, however, fish distribution returned to that observed when both habitats were fresh water. This indicates that the preference for deep saline habitats during the stratified phase was driven by some benefit associated with residency in deeper water, rather than salinity. The low perceived cost of low salinity might be in part due to the fish's ability to minimize this cost by only making brief forays into the alternate freshwater habitat. When the food ration delivered to the more costly, shallow habitat was 50% greater than that delivered to the less costly, deep habitat, fish distributed themselves equally between the two habitats, presumably because of equal net benefits. This study demonstrates that juvenile Chinook salmon prefer deep saline habitat to shallow freshwater habitats but will make brief forays into the freshwater habitat if food availability is sufficiently high.     

Evaluating climatic and non-climatic influences on ion chemistry in natural and man-made lakes of Nebraska,USA

Conductivity and major ion chemistry data were analyzed for a suite of Nebraska (USA) natural lakes, reservoirs, sand pits, and barrow pits to evaluate the magnitude of climatic versus non-climatic influence on ionic concentration and composition. In both natural lakes and sand and barrow pits, conductivity is positively related to longitude and reflects decreasing effective moisture from east to west. Reservoirs showed no relationship between lake conductivity and location, probably because the reservoirs are very strongly influenced by groundwater and surface water inflow and have shorter residence times relative to the other lake types. At smaller spatial scales, conductivity among natural lakes is variable. Lakes that are at low elevation within a groundwater flow system were fresh, because of substantial input of fresh groundwater. In contrast, lakes at high elevation exhibited a wide range of conductivity, probably because of differences in the degree of connection to groundwater and surface to volume ratio impacts on evaporation rates. Differences also were evident among natural lakes in terms of their response to seasonal changes in precipitation. Sub-saline and saline lakes showed more seasonal variation in conductivity than freshwater lakes, and lakes in the more arid part of the state showed larger responses to precipitation change than those in areas to the east that receive higher precipitation.     

The effects of 11-ketotestosterone on occupation of downstream location and seawater in the New Zealand shortfinned eel, Anguilla australis

The androgen 11-ketotestosterone (11KT) is associated with the physiological and morphological changes that occur during the transformation of sedentary ('yellow') freshwater eels (Anguilla spp.) into their migratory form ('silver') prior to their spawning migration in the ocean. In this study, we investigate the possible role of 11KT in modulating behaviors consistent with downstream migration; i.e., downstream and salinity preference in the New Zealand shortfinned eel (A. australis). Unlike silvering, 11KT did not induce preference for downstream locations, scored as presence at the downstream ends of 35 m raceways. Likewise, there was no evidence for increased salinity preference in 11KT-treated yellow eels, scored as preference for sea water over fresh water in a choice experiment. However, the 11KT treatment induced higher frequency of movements between fresh water and sea water, which may indicate restlessness.     

Abbreviation of larval development and extension of brood care as key features of the evolution of freshwater Decapoda

The transition from marine to freshwater habitats is one of the major steps in the evolution of life. In the decapod crustaceans, four groups have colonized fresh water at different geological times since the Triassic, the freshwater shrimps, freshwater crayfish, freshwater crabs and freshwater anomurans. Some families have even colonized terrestrial habitats via the freshwater route or directly via the sea shore. Since none of these taxa has ever reinvaded its environment of origin the Decapoda appear particularly suitable to investigate life‐history adaptations to fresh water. Evolutionary comparison of marine, freshwater and terrestrial decapods suggests that the reduction of egg number, abbreviation of larval development, extension of brood care and lecithotrophy of the first posthatching life stages are key adaptations to fresh water. Marine decapods usually have high numbers of small eggs and develop through a prolonged planktonic larval cycle, whereas the production of small numbers of large eggs, direct development and extended brood care until the juvenile stage is the rule in freshwater crayfish, primary freshwater crabs and aeglid anomurans. The amphidromous freshwater shrimp and freshwater crab species and all terrestrial decapods that invaded land via the sea shore have retained ocean‐type planktonic development. Abbreviation of larval development and extension of brood care are interpreted as adaptations to the particularly strong variations of hydrodynamic parameters, physico‐chemical factors and phytoplankton availability in freshwater habitats. These life‐history changes increase fitness of the offspring and are obviously favoured by natural selection, explaining their multiple origins in fresh water. There is no evidence for their early evolution in the marine ancestors of the extant freshwater groups and a preadaptive role for the conquest of fresh water. The costs of the shift from relative r‐ to K‐strategy in freshwater decapods are traded‐off against fecundity, future reproduction and growth of females and perhaps against size of species but not against longevity of species. Direct development and extension of brood care is associated with the reduction of dispersal and gene flow among populations, which may explain the high degree of speciation and endemism in directly developing freshwater decapods. Direct development and extended brood care also favour the evolution of social systems, which in freshwater decapods range from simple subsocial organization to eusociality. Hermaphroditism and parthenogenesis, which have evolved in some terrestrial crayfish burrowers and invasive open water crayfish, respectively, may enable populations to adapt to restrictive or new environments by spatio‐temporal alteration of their socio‐ecological characteristics. Under conditions of rapid habitat loss, environmental pollution and global warming, the reduced dispersal ability of direct developers may turn into a severe disadvantage, posing a higher threat of extinction to freshwater crayfish, primary freshwater crabs, aeglids and landlocked freshwater shrimps as compared to amphidromous freshwater shrimps and secondary freshwater crabs.     

Rectal gland morphology of freshwater and seawater acclimated bull sharks Carcharhinus leucas

To compare rectal gland morphology of bull sharks Carcharhinus leucas , animals captured in the freshwater reaches of the Brisbane River, Australia, were acclimated to sea water over 17 days with 1 week in the final salinity. A control group was left in fresh water for 17 days. Animals in fresh water and sea water were strongly hyper- and hypo-ionic with respect to plasma Na⁺ and Cl^–, respectively. This difference necessitates NaCl secretion by the rectal gland in sea water and conservation of NaCl in fresh water. Structural differences in the rectal gland of freshwater and seawater acclimated bull sharks were limited. There was no difference in rectal gland cross-sectional area, lumen area, rectal gland vein area, number of secretory tubules or secretory cells per secretory tubule in freshwater and seawater acclimated animals. At a cellular level, there was no difference between the degree of basolateral and lateral folding, number of mitochondria or number of desmosomes per tight junction. Tight junction width was significantly greater in seawater acclimated animals. The number of red blood cells in the interstitial tissue was also significantly higher in seawater acclimated animals, possibly as a result of increased blood perfusion of the secretory epithelia. The lack of major structural changes in the rectal glands of bull sharks acclimated to fresh water and sea water most likely represents the salinity gradient in the Brisbane River where animals are found throughout the river and can experience large fluctuations in salinity over short distances. Differences in rectal gland morphology of bull sharks in fresh water and sea water are discussed in terms of their relevance to osmoregulation in elasmobranchs.     

Habitat and phylogeny influence salinity discrimination in crocodilians: implications for osmoregulatory physiology and historical biogeography

Crocodylids are better adapted than alligatorids, through a suite of morphological specializations, for life in hyperosmotic environments. The presence of such specializations even in freshwater crocodylids has been interpreted as evidence for a marine phase in crocodylid evolution, consistent with the trans-osceanic migration hypothesis of crocodilian biogeography. The ability to discriminate fresh water from hyperosmotic sea water, and to avoid drinking the latter, is known to be an important osmoregulatory mechanism for estuarine crocodylids. This study was undertaken to determine whether the ability to discriminate between hyper- and hypo-osmotic salinities is determined by habitat, as it is in other normally freshwater reptiles, or whether, like morphological adaptations associated with estuarine life, it has a phylogenetic basis. Two species of freshwater alligatorid were found to drink fresh water and hyperosmotic sea water indiscriminately, while an estuarine population of a normally freshwater alligatorid species drank only fresh water. This indicated that salinity discrimination is determined at least in part by habitat. However, all three crocodylid species tested drank fresh water but not hyperosmotic sea water, suggesting that, in crocodilians, the ability to distinguish between fresh water and sea water is influenced by phylogeny as well as by habitat. The implications of this result are discussed in the context of two alternate hypotheses for the historical biogeography of the Crocodilia.     

Salinity as a constraint affecting food and habitat choice of mussel-feeding diving ducks

Observations of freshwater drinking in Eiders feeding primarily on mussels led us to hypothesize that the highly saline sea water enclosed in mussels could cause salt-related dehydration problems in the ducks, since they consume entire mussels. The proportion of sea water increases with increasing mussel size. Smaller duck species are more sensitive to the higher salt content of larger mussels than are larger ducks; however, salt stress may be avoided by feeding in habitats with lower salinity, by feeding on less salty food items, by utilizing smaller mussels, by drinking fresh water, or by employing low energy foraging techniques. A possible evolutionary strategy for solving the salt problem might be to increase body mass, enabling ducks to utilize larger mussels without passing an upper salt consumption limit. At the same time, foraging on larger mussels is more economical. Although large size facilitates the utilization of brackish and marine environments, it may be selected against in ducks breeding in fresh water, where fish competition may reduce optimal food item size. In conclusion, salinity is an important habitat barrier in both breeding and overwintering diving ducks, but there are various ways of crossing this barrier. To understand better how ducks utilize their habitats, however, it is necessary to measure habitat salinity levels and the size of both ducks and their preferred and less-preferred food types.     

Uptake of saline groundwater by plants: An analytical model for semi-arid and arid areas

An analytical model, based on unsaturated zone water and solute balances, was developed to describe the uptake of saline groundwater by plants in dry regions. It was assumed that: i. initially, the profile had low water and salt contents to some depth; ii. both water and solutes move upwards from the water table by piston flow due only to plant water extraction; iii. the uptake of water concentrates solutes in the soil solution until some threshold salinity is reached, above which plants can no longer extract water due to osmotic effects; iv. uptake of the groundwater does not affect the water table level; and v. uptake of groundwater is only limited by transmission of groundwater through the soil. Model predictions were compared with measurements of groundwater uptake made over 15 months at five sites in aEucalyptus forest in a semi-arid area, using independently measured model parameters. Depth and salinity of groundwater, and soil type varied greatly between sites. Predicted groundwater uptake rates were close to measured values, generally being within ∼ 0.1 mm day^-1. Sensitivity analysis showed that groundwater depth and salinity were the main controls on uptake of groundwater, while soil properties appeared to have a lesser effect. The model showed that uptake of groundwater would result in complete salinisation of the soil profile within 4 to 30 yr at the sites studied, unless salts were leached from the soil by rainfall or flood waters. However, a relatively small amount of annual leaching may be sufficient to allow groundwater uptake to continue. Thus groundwaters, even when saline, may be important sources of water to plants in arid and semi-arid areas.     

Diverse migration strategy between freshwater and seawater habitats in the freshwater eel genus Anguilla

The freshwater eels of the genus Anguilla, which are catadromous, migrate between freshwater growth habitats and offshore spawning areas. A number of recent studies, however, found examples of the temperate species Anguilla anguilla, Anguilla rostrata, Anguilla japonica, Anguilla australis and Anguilla dieffenbachii that have never migrated into fresh water, spending their entire life history in the ocean. Furthermore, those studies found an intermediate type between marine and freshwater residents, which appear to frequently move between different environments during their growth phase. The discovery of marine and brackish-water residents Anguilla spp. suggests that they do not all have to be catadromous, and it calls into question the generalized classification of diadromous fishes. There has been little available information, however, concerning migration in tropical Anguilla spp. Anguilla marmorata, shows three fluctuation patterns: (1) continuous residence in fresh water, (2) continuous residence in brackish water and (3) residence in fresh water after recruitment, while returning to brackish water. Such migratory patterns were found in other tropical species, Anguilla bicolor bicolor and Anguilla bicolor pacifica. In A. b. bicolor collected in a coastal lagoon of Indonesia, two further patterns of habitat use were found: (1) constantly living in either brackish water or sea water with no freshwater life and (2) habitat shift from fresh water to brackish water or sea water. The wide range of environmental habitat use indicates that migratory behaviour of tropical Anguilla spp. is facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas. Further, the migratory behaviours of tropical Anguilla spp. appear to differ in each habitat in response to inter and intra-specific competition. The results suggest that tropical Anguilla spp. have a flexible pattern of migration, with an ability to adapt to various habitats and salinities. The ability of anguillids to reside in environments of various salinities would be a common feature between tropical and temperate species without a latitudinal cline. Thus, the migration of Anguilla spp. into fresh water is clearly not an obligatory behaviour. This evidence of geographical variability among Anguilla spp. suggests that habitat use is determined by environmental conditions in each site.     

Fluctuating deposition of ocean water drives plant function on coastal sand dunes

Sea‐level rise will alter the hydrology of terrestrial coastal ecosystems. As such, it becomes increasingly important to decipher the present role of ocean water in coastal ecosystems in order to assess the coming effects of sea‐level rise scenarios. Sand dunes occur at the interface of land and sea. Traditionally, they are conceived as freshwater environments with rain and ground water as the only water sources available to vegetation. This study investigates the possibility of ocean water influx to dune soils and its effect on the physiology of sand dune vegetation. Stable isotopes are used to trace the path of ocean water from the soil to the vegetation. Soil salinity, water content and δ¹⁸O values are measured concurrently with stem water and leaf tissue of eight species during the wet and dry season and from areas proximal and distal to the ocean. Our results indicate the dune ecosystem is a mixed freshwater and marine water system characterized by oceanic influence on dune hydrology that is spatially heterogeneous and fluctuates temporally. Ocean water influx to soil occurs via salt spray in areas 5–12 m from the ocean during dry season. Accordingly, vegetation nearest to the sea demonstrate a plastic response to ocean water deposition including elevated integrated water use efficiency (δ¹³C_leaf) and uptake of ocean water that comprised up to 52% of xylem water. We suggest physiological plasticity in response to periodic ocean water influx may be a functional characteristic common to species on the leading edge of diverse coastal habitats and an important feature that should be included in modeling coastal ecosystems. Rising sea level would likely cause a repercussive landward shift of dune species in response to encroaching maritime influences. However, human development would restrict this process, potentially causing the demise of dune systems and the protection from land erosion they provide.     

In situ and laboratory studies on the behaviour and survival of Pacific salmon (genus Oncorhynchus)

Juvenile Pacific salmon display a marked surface water orientation during downstream migration, estuarine and nearshore coastal rearing phases. Many estuaries in British Columbia are vertically stratified with a shallow, well-defined halocline which can restrict the dispersion of wastes discharged into less saline surface waters and impose constraints upon aquatic organisms. In situ experiments in an estuary receiving a surface discharge of treated pulp mill wastes, revealed conditions which were lethal to underyearling salmon at, and below the halocline (4.0–6.5 m depth). Behavioural bioassays determined that juvenile chinook salmon were biased towards the water surface and avoided waters at depth. Dissolved oxygen was the variable which affected this distribution most significantly. Surface waters receiving effluent from another pulp mill were lethal to juvenile salmon within 350 m, and a significant vertical avoidance response occurred within 350–950 m of the outfalls. The behavioural response was significantly correlated with in situ temperature, pH and colour (effluent).As a complement to field experiments we developed a 4500 l water column simulator (WCS) to examine salmon behaviour in the laboratory. We investigated the surface water orientation behaviour of juvenile salmon in relation to variations in salinity and dissolved oxygen. Under simulated vertically stratified estuarine conditions, the fish moved freely between overlying fresh water and salt water. Induction of hypoxic conditions in fresh water elicited a downward distribution shift towards the halocline and oxygenated, but more saline, waters. Avoidance reactions (50% level) occurred consistently up to 7–8 mg · l^–1 dissolved oxygen. Salmon continued to examine the hypoxic freshwater zone despite sub-optimal conditions.     

Evolutionary bottlenecks in brackish water habitats drive the colonization of fresh water by stingrays

Species richness in freshwater bony fishes depends on two main processes: the transition into and the diversification within freshwater habitats. In contrast to bony fishes, only few cartilaginous fishes, mostly stingrays (Myliobatoidei), were able to colonize fresh water. Respective transition processes have been mainly assessed from a physiological and morphological perspective, indicating that the freshwater lifestyle is strongly limited by the ability to perform osmoregulatory adaptations. However, the transition history and the effect of physiological constraints on the diversification in stingrays remain poorly understood. Herein, we estimated the geographic pathways of freshwater colonization and inferred the mode of habitat transitions. Further, we assessed habitat‐related speciation rates in a time‐calibrated phylogenetic framework to understand factors driving the transition of stingrays into and the diversification within fresh water. Using South American and Southeast Asian freshwater taxa as model organisms, we found one independent freshwater colonization event by stingrays in South America and at least three in Southeast Asia. We revealed that vicariant processes most likely caused freshwater transition during the time of major marine incursions. The habitat transition rates indicate that brackish water species switch preferably back into marine than forth into freshwater habitats. Moreover, our results showed significantly lower diversification rates in brackish water lineages, whereas freshwater and marine lineages exhibit similar rates. Thus, brackish water habitats may have functioned as evolutionary bottlenecks for the colonization of fresh water by stingrays, probably because of the higher variability of environmental conditions in brackish water.     

The anal portion as a salt-excreting organ in a seawater mosquito larva,A?des togoi Theobald

Summary Larvae of the marine mosquito,Aëdes togoi Theobald, tolerate environmental salinities ranging from fresh water to 300% sea water. When they were transferred from fresh water to sea water, sodium concentration in the haemolymph increased for the first 2 days and decreased to the seawater-adapted level within 4 days. When transferred from sea water to fresh water, the sodium concentration decreased markedly for the first 2 days and attained the freshwater adapted level after 4 days. When the larvae in sea water were ligated near the anus, they died within 3 days, showing an increased sodium level in the haemolymph. The larvae ligated at the neck lost considerable body weight and died within 4 days.When the anal portion, a terminal portion of the hindgut, was catheterized, the larvae maintained in sea water showed an increase in haemolymph sodium. The anal portion epithelium of the larvae adapted to 100 and 150% sea water demonstrated a strong positive reaction to the histochemical assay for chloride ions, whereas the reaction was negative or weakly positive in freshwater adapted larvae. In the larvae with the anal papillae ligated, a slight increase in haemolymph sodium occurred while in sea water. The anal papillae were weakly positive to chloride ions. Unlike salt-water mosquito larvae of the other species, in which the rectum is considered to be involved in hyperosmotic urine production and the anal papillae appear to be the extrarenal organ, the anal portion inA. togoi larvae seems to play an important role in excretion of excess ions when placed in hyperosmotic media.