Molecular characterization, localization, and physiological roles of ITP and ITP-L in the mosquito, Aedes aegypti

The insect ion transport peptide (ITP) and its alternatively spliced variant, ITP-like peptide (ITP-L), belong to the crustacean hyperglycemic hormone family of peptides and are widely conserved among insect species. While limited, studies have characterized the ITP/ITP-L signaling system within insects, and putative functions including regulation of ion and fluid transport, ovarian maturation, and thirst/excretion have been proposed. Herein, we aimed to molecularly investigate Itp and Itp-l expression profiles in the mosquito, Aedes aegypti, examine peptide immunolocalization and distribution within the adult central nervous system, and elucidate physiological roles for these neuropeptides. Transcript expression profiles of both AedaeItp and AedaeItp-l revealed distinct enrichment patterns in adults, with AedaeItp expressed in the brain and AedaeItp-l expression predominantly within the abdominal ganglia. Immunohistochemical analysis within the central nervous system revealed expression of AedaeITP peptide in a number of cells in the brain and in the terminal ganglion. Comparatively, AedaeITP-L peptide was localized solely within the pre-terminal abdominal ganglia of the central nervous system. Interestingly, prolonged desiccation stress caused upregulation of AedaeItp and AedaeItp-l levels in adult mosquitoes, suggesting possible functional roles in water conservation and feeding-related activities. RNAi-mediated knockdown of AedaeItp caused an increase in urine excretion, while knockdown of both AedaeItp and AedaeItp-l reduced blood feeding and egg-laying in females as well as hindered egg viability, suggesting roles in reproductive physiology and behavior. Altogether, this study identifies AedaeITP and AedaeITP-L as key pleiotropic hormones, regulating various critical physiological processes in the disease vector, A. aegypti.

A novel K+ -dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

AIM

To determine whether Na⁺ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na⁺ /H⁺ Exchangers (NHEs), Na⁺ channels and Na⁺ /Cl^- Cotransporters (NCCs) or if it occurs through a novel mechanism.

METHODS

Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which ²² Na⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K⁺ flux ( J H net ) were measured. The involvement of NHEs, Na⁺ channels, NCCs, K⁺ -channels and K⁺ -dependent Na⁺ /Ca²⁺ exchangers (NCKXs) was evaluated by exposure to Cl^- -free or elevated [K⁺ ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR.

RESULTS

J Na in was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K⁺ as a counter substrate for Na⁺ uptake during acid exposure. Indeed, elevated environmental [K⁺ ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.

CONCLUSIONS

During acid exposure, zebrafish engage a novel Na⁺ uptake mechanism that utilizes the outwardly directed K⁺ gradient as a counter-substrate for Na⁺ and is sensitive to TEA. NKCXs are promising candidates to mediate this K⁺ -dependent Na⁺ uptake, opening new research avenues about Na⁺ uptake in zebrafish and other acid-tolerant aquatic species.

Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus (Kryptolebias marmoratus) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na+-K+-ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.

Effects of Bicarbonate Stress on Serum Ions and Gill Transporters in Alkali and Freshwater Forms of Amur Ide (Leuciscus waleckii)

The Amur ide (Leuciscus waleckii) is a fish in the Cyprinidae family. Compared with other Amur ide living in freshwater ecosystems, the Amur ide population in Lake Dali Nor of China is famous for its high tolerance to the alkaline conditions of 54 mM (pH 9.6). Yet, surprisingly, the ionoregulatory mechanism responsible for this remarkable alkaline adaptation remains unclear. Therefore, this study sought to investigate how bicarbonate affects the acid-base balancing and ionoregulatory responses of this animal. Here, using a comparative approach, the alkali form of Amur ide and its ancestral freshwater form living in other freshwater basins were each exposed to 50 mM (pH 9.59 ± 0.09), a level close to the alkalinity of Lake Dali Nor, and their physiological (AE1) adjustment of ions and acid-base regulation were investigated. This study highlighted differences in blood pH and serum ions (e.g., Na⁺, K⁺, Cl⁻, and Ca²⁺), Na⁺/K⁺ ATPase (NKA) activity and its mRNA level, and mRNA expression of gill transporters (Na⁺/H⁺ exchanger member 2 and/or 3, Na⁺/HCO3- cotransporter (NBC1), Cl⁻/HCO3- exchanger, Na⁺/Cl⁻ cotransporter (NCC), Na⁺/K⁺/2Cl⁻ (NKCC1), SLC26A5, and SLC26A6) for alkalinity adaptation between the two forms of Amur ide differing in alkalinity tolerance. Specifically, close relationships among the serum Na⁺ and mRNA levels of NCC, NKCC1, and NHE, and also NKA and NBC1, in addition to serum Cl⁻ and bicarbonate transporters (e.g., SLC26A5 and SLC26A6), characterized the alkali form of Amur ide. We propose that this ecotype can ensure its transepithelial Cl⁻ and Na⁺ uptake/base secretions are highly functional, by its basolateral NKA with NBC1 and apical ionic transporters, and especially NCC incorporated with other transporters (e.g., SLC26). This suggests an evolved strong ability to maintain an ion osmotic and acid-base balance for more effectively facilitating its adaptability to the high alkaline environment. This study provides new insights into the physiological responses of the alkaline form of the Amur ide fish for adapting to extreme alkaline conditions. This information could be used as a reference to cultivating alkaline-tolerant fish species in abandoned alkaline waters.

Molecular basis for epithelial morphogenesis and ion transport in the Malpighian tubule

During development, the insect Malpighian tubule undergoes several programmed morphogenetic events that give rise to the tubule's ability to transport ions and water at unparalleled speed. Studies in Diptera, in particular, have greatly increased our understanding of the molecular pathways underlying embryonic tubule development. In this review, we discuss recent work that has revealed new insights into the molecular players required for the development and maintenance of structurally and functionally intact adult Malpighian tubules. We highlight the contribution of the smooth septate junction (sSJ) proteins to the morphogenesis and transport function of the epithelial cells of the Drosophila melanogaster Malpighian tubule and also discuss new findings on the role of the GATAe transcription factor. We also consider the roles of sSJ proteins in the fly midgut, as compared to the Malpighian tubule, and the importance of cellular context for the functions of these proteins.

Boron Oxide Nanoparticles Exhibit Minor, Species-Specific Acute Toxicity to North-Temperate and Amazonian Freshwater Fishes

Boron oxide nanoparticles (nB₂O₃) are manufactured for structural, propellant, and clinical applications and also form spontaneously through the degradation of bulk boron compounds. Bulk boron is not toxic to vertebrates but the distinctive properties of its nanostructured equivalent may alter its biocompatibility. Few studies have addressed this possibility, thus our goal was to gain an initial understanding of the potential acute toxicity of nB₂O₃ to freshwater fish and we used a variety of model systems to achieve this. Bioactivity was investigated in rainbow trout (Oncorhynchus mykiss) hepatocytes and at the whole animal level in three other North and South American fish species using indicators of aerobic metabolism, behavior, oxidative stress, neurotoxicity, and ionoregulation. nB₂O₃ reduced O. mykiss hepatocyte oxygen consumption (ṀO₂) by 35% at high doses but whole animal ṀO₂ was not affected in any species. Spontaneous activity was assessed using ṀO₂ frequency distribution plots from live fish. nB₂O₃ increased the frequency of high ṀO₂ events in the Amazonian fish Paracheirodon axelrodi, suggesting exposure enhanced spontaneous aerobic activity. ṀO₂ frequency distributions were not affected in the other species examined. Liver lactate accumulation and significant changes in cardiac acetylcholinesterase and gill Na⁺/K⁺-ATPase activity were noted in the north-temperate Fundulus diaphanus exposed to nB₂O₃, but not in the Amazonian Apistogramma agassizii or P. axelrodi. nB₂O₃ did not induce oxidative stress in any of the species studied. Overall, nB₂O₃ exhibited modest, species-specific bioactivity but only at doses exceeding predicted environmental relevance. Chronic, low dose exposure studies are required for confirmation, but our data suggest that, like bulk boron, nB₂O₃ is relatively non-toxic to aquatic vertebrates and thus represents a promising formulation for further development.

The physiology of fish in acidic waters rich in dissolved organic carbon, with specific reference to the Amazon basin: Ionoregulation, acid-base regulation, ammonia excretion, and metal toxicity

Although blackwaters, named for their rich content of dissolved organic carbon (DOC), are often very poor in ions and very acidic, they support great fish biodiversity. Indeed, about 8% of all freshwater fish species live in the blackwaters of the Rio Negro watershed in the Amazon basin. We review how native fish survive these harsh conditions that would kill most freshwater fish, with a particular focus on the role of DOC, a water quality parameter that has been relatively understudied. DOC, which is functionally defined by its ability to pass through a 0.45-µm filter, comprises a diverse range of compounds formed by the breakdown of organic matter and is quantified by its carbon component that is approximately 50% by mass. Adaptations of fish to acidic blackwaters include minimal acid-base disturbances associated with a unique, largely unknown, high-affinity Na⁺ uptake system that is resistant to inhibition by low pH in members of the Characiformes, and very tight regulation of Na⁺ efflux at low pH in the Cichliformes. Allochthonous (terrigenous) DOC, which predominates in blackwaters, consists of larger, more highly colored, reactive molecules than autochthonous DOC. The dissociation of protons from allochthonous components such as humic and fulvic acids is largely responsible for the acidity of these blackwaters, yet at the same time, these components may help protect organisms against the damaging effects of low water pH. DOC lowers the transepithelial potential (TEP), mitigates the inhibition of Na⁺ uptake and ammonia excretion, and protects against the elevation of diffusive Na⁺ loss in fish exposed to acidic waters. It also reduces the gill binding and toxicity of metals. At least in part, these actions reflect direct biological effects of DOC on the gills that are beneficial to ionoregulation. After chronic exposure to DOC, some of these protective effects persist even in the absence of DOC. Two characteristics of allochthonous DOC, the specific absorbance coefficient at 340 nm (determined optically) and the PBI (determined by titration), are indicative of both the biological effectiveness of DOC and the ability to protect against metal toxicity. Future research needs are highlighted, including a greater mechanistic understanding of the actions of DOCs on gill ionoregulatory function, morphology, TEP, and metal toxicity. These should be investigated in a wider range of native fish Orders that inhabit one of the world's greatest biodiversity hotspots for freshwater fishes.

Temperature Effects During a Sublethal Chronic Metal Mixture Exposure on Common Carp (Cyprinus carpio)

The aquatic environment is the final sink of various pollutants including metals, which can pose a threat for aquatic organisms. Waterborne metal mixture toxicity might be influenced by environmental parameters such as the temperature. In the present study, common carp were exposed for 27 days to a ternary metal mixture of Cu, Zn, and Cd at two different temperatures, 10 and 20°C. The exposure concentrations represent 10% of the 96 h-LC₅₀ (concentration lethal for the 50% of the population in 96 h) for each metal (nominal metal concentrations of Cu: 0.08 μM; Cd: 0.02 μM and Zn: 3 μM). Metal bioaccumulation and toxicity as well as changes in the gene expression of enzymes responsible for ionoregulation and induction of defensive responses were investigated. Furthermore the hepatosomatic index and condition factor were measured as crude indication of overall health and energy reserves. The obtained results showed a rapid Cu and Cd increase in the gills at both temperatures. Cadmium accumulation was higher at 20°C compared to 10°C, whereas Cu and Zn accumulation was not, suggesting that at 20°C, fish had more efficient depuration processes for Cu and Zn. Electrolyte (Ca, Mg, Na, and K) levels were analyzed in different tissues (gills, liver, brain, muscle) and in the remaining carcasses. However, no major electrolyte losses were observed. The toxic effect of the trace metal ion mixture on major ion uptake mechanisms may have been compensated by ion uptake from the food. Finally, the metal exposure triggered the upregulation of the metallothionein gene in the gills as defensive response for the organism. These results, show the ability of common carp to cope with these metal levels, at least under the condition used in this experiment.

Does dissolved organic carbon from Amazon black water (Brazil) help a native species, the tambaqui Colossoma macropomum to maintain ionic homeostasis in acidic water?

To assess how the quality and properties of the natural dissolved organic carbon (DOC) could drive different effects on gill physiology, we analysed the ionoregulatory responses of a native Amazonian fish species, the tambaqui Colossoma macropomum, to the presence of dissolved organic carbon (DOC; 10 mg l^-1 ) at both pH 7.0 and pH 4.0 in ion-poor water. The DOC was isolated from black water from São Gabriel da Cachoeira (SGC) in the upper Rio Negro of the Amazon (Brazil) that earlier been shown to protect a non-native species, zebrafish Danio rerio against low pH under similar conditions. Transepithelial potential (TEP), net flux rates of Na⁺ , Cl^- and ammonia and their concentrations in plasma and Na⁺ , K⁺ ATPase; v-type H⁺ ATPase and carbonic anhydrase activities in gills were measured. The presence of DOC had negligible effects at pH 7.0 apart from lowering the TEP, but it prevented the depolarization of TEP that occurred at pH 4.0 in the absence of DOC. However, contrary to our initial hypothesis, SGC DOC was not protective against the effects of low pH. Colossoma macropomum exposed to SGC DOC at pH 4.0 experienced greater net Na⁺ and Cl^- losses, decreases of Na⁺ and Cl^- concentrations in plasma and elevated plasma ammonia levels and excretion rates, relative to those exposed in the absence of DOC. Species-specific differences and changes in DOC properties during storage are discussed as possible factors influencing the effectiveness of SGC DOC in ameliorating the effects of the acid exposure.

The Air-Breathing Paradise Fish (Macropodus opercularis) Differs From Aquatic Breathers in Strategies to Maintain Energy Homeostasis Under Hypoxic and Thermal Stresses

Two major strategies are used by most fish to maintain energy homeostasis under hypoxia. One is to utilize alternative metabolic pathways to increase energy production, and the other is to limit energy expenditure by suppressing energy-consuming processes, especially ionoregulation. Some anabantoid fishes live in tropical rivers, where hypoxic environments occur frequently. We previously found that under ambient hypoxia, anabantoid fishes do not downregulate Na⁺/K⁺-ATPase (NKA) activity to conserve energy in gills but instead increase the frequency of air-breathing respiration (ABR). In addition to the hypoxic condition, another factor that may cause cellular hypoxia in fish is abnormally high environmental temperatures. The frequency of such extreme thermal events has increased due to global climate change. In the present study, we examined whether the anabantoid fish, Macropodus opercularis employs the two strategies mentioned above to resist both ambient hypoxic and elevated thermal (cellular hypoxic) conditions. Results indicate that neither glucose metabolism nor gill NKA activity were altered by hypoxia (DO = 1.5 ± 1 mg/L), but glucose metabolism was increased by thermal stress (34 ± 1°C). NH₄ ⁺ excretion and ABR frequency were both increased under hypoxia, thermal or hypoxic-and-thermal treatments. In fish that were restricted from breathing air, increased mortality and glucose metabolism were observed under hypoxic or thermal treatments. These results suggest that for M. opercularis, increasing ABR is an important strategy for coping with unmet oxygen demand under hypoxic or thermal stress. This behavioral compensation allows anabantoid fish to physiologically withstand hypoxic and thermal stresses, and constitutes a mechanism of stress resistance that is unavailable to water-breathing fishes.

Mechanisms of calcium sequestration by isolated Malpighian tubules of the house cricket Acheta domesticus

Hemolymph calcium homeostasis in insects is achieved by the Malpighian tubules, primarily by sequestering excess Ca²⁺ within internal calcium stores (Ca-rich granules) most often located within type I (principal) tubule cells. Using both the scanning ion-selective electrode technique and the Ramsay secretion assay this study provides the first measurements of basolateral and transepithelial Ca²⁺ fluxes across the Malpighian tubules of an Orthopteran insect, the house cricket Acheta domesticus. Ca²⁺ transport was specific to midtubule segments, where 97% of the Ca²⁺ entering the tubule is sequestered within intracellular calcium stores and the remaining 3% is secreted into the lumen. Antagonists of voltage-gated (L-type) calcium channels decreased Ca²⁺ influx ≥fivefold in adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated tubules, suggesting basolateral Ca²⁺ influx is facilitated by voltage-gated Ca²⁺ channels. Increasing fluid secretion through manipulation of intracellular levels of cAMP or Ca²⁺ had opposite effects on tubule Ca²⁺ transport. The adenylyl cyclase-cAMP-PKA pathway promotes Ca²⁺ sequestration whereas both 5-hydroxytryptamine and thapsigargin inhibited sequestration. Our results suggest that the midtubules of Acheta domesticus are dynamic calcium stores, which maintain hemolymph calcium concentration by manipulating rates of Ca²⁺ sequestration through stimulatory (cAMP) and inhibitory (Ca²⁺ ) regulatory pathways.

Acid-sensing ion channels are involved in epithelial Na+ uptake in the rainbow trout Oncorhynchus mykiss

A role for acid-sensing ion channels (ASICs) to serve as epithelial channels for Na(+) uptake by the gill of freshwater rainbow trout was investigated. We found that the ASIC inhibitors 4',6-diamidino-2-phenylindole and diminazene decreased Na(+) uptake in adult rainbow trout in a dose-dependent manner, with IC50 values of 0.12 and 0.96 μM, respectively. Furthermore, we cloned the trout ASIC1 and ASIC4 homologs and demonstrated that they are expressed differentially in the tissues of the rainbow trout, including gills and isolated mitochondrion-rich cells. Immunohistochemical analysis using custom-made anti-zASIC4.2 antibody and the Na(+)-K(+)-ATPase (α5-subunit) antibody demonstrated that the trout ASIC localizes to Na(+)/K(+)-ATPase-rich cells in the gill. Moreover, three-dimensional rendering of confocal micrographs demonstrated that ASIC is found in the apical region of mitochondrion-rich cells. We present a revised model whereby ASIC4 is proposed as one mechanism for Na(+) uptake from dilute freshwater in the gill of rainbow trout.

Acid-base and ion balance in fishes with bimodal respiration

The evolution of air breathing during the Devonian provided early fishes with bimodal respiration with a stable O2 supply from air. This was, however, probably associated with challenges and trade-offs in terms of acid-base balance and ionoregulation due to reduced gill:water interaction and changes in gill morphology associated with air breathing. While many aspects of acid-base and ionoregulation in air-breathing fishes are similar to water breathers, the specific cellular and molecular mechanisms involved remain largely unstudied. In general, reduced ionic permeability appears to be an important adaptation in the few bimodal fishes investigated but it is not known if this is a general characteristic. The kidney appears to play an important role in minimizing ion loss to the freshwater environment in the few species investigated, and while ion uptake across the gut is probably important, it has been largely unexplored. In general, air breathing in facultative air-breathing fishes is associated with an acid-base disturbance, resulting in an increased partial pressure of arterial CO2 and a reduction in extracellular pH (pHE ); however, several fishes appear to be capable of tightly regulating tissue intracellular pH (pHI ), despite a large sustained reduction in pHE , a trait termed preferential pHI regulation. Further studies are needed to determine whether preferential pHI regulation is a general trait among bimodal fishes and if this confers reduced sensitivity to acid-base disturbances, including those induced by hypercarbia, exhaustive exercise and hypoxia or anoxia. Additionally, elucidating the cellular and molecular mechanisms may yield insight into whether preferential pHI regulation is a trait ultimately associated with the early evolution of air breathing in vertebrates.

Interactions between hypoxia tolerance and food deprivation in Amazonian oscars, Astronotus ocellatus

Oscars are often subjected to a combination of low levels of oxygen and fasting during nest-guarding on Amazonian floodplains. We questioned whether this anorexia would aggravate the osmo-respiratory compromise. We compared fed and fasted oscars (10-14 days) in both normoxia and hypoxia (10-20 Torr, 4 h). Routine oxygen consumption rates (O2) were increased by 75% in fasted fish, reflecting behavioural differences, whereas fasting improved hypoxia resistance and critical oxygen tensions (Pcrit) lowered from 54 Torr in fed fish to 34 Torr when fasting. In fed fish, hypoxia reduced liver lipid stores by approximately 50% and total liver energy content by 30%. Fasted fish had a 50% lower hepatosomatic index, resulting in lower total liver protein, glycogen and lipid energy stores under normoxia. Compared with hypoxic fed fish, hypoxic fasted fish only showed reduced liver protein levels and even gained glycogen (+50%) on a per gram basis. This confirms the hypothesis that hypoxia-tolerant fish protect their glycogen stores as much as possible as a safeguard for more prolonged hypoxic events. In general, fasted fish showed lower hydroxyacylCoA dehydrogenase activities compared with fed fish, although this effect was only significant in hypoxic fasted fish. Energy stores and activities of enzymes related to energy metabolism in muscle or gills were not affected. Branchial Na(+) uptake rates were more than two times lower in fed fish, whereas Na(+) efflux was similar. Fed and fasted fish quickly reduced Na(+) uptake and efflux during hypoxia, with fasting fish responding more rapidly. Ammonia excretion and K(+) efflux were reduced under hypoxia, indicating decreased transcellular permeability. Fasted fish had more mitochondria-rich cells (MRC), with larger crypts, indicating the increased importance of the branchial uptake route when feeding is limited. Gill MRC density and surface area were greatly reduced under hypoxia, possibly to reduce ion uptake and efflux rates. Density of mucous cells of normoxic fasted fish was approximately fourfold of that in fed fish. Overall, a 10-14 day fasting period had no negative effects on hypoxia tolerance in oscars, as fasted fish were able to respond more quickly to lower oxygen levels, and reduced branchial permeability effectively.

Uptake, handling and excretion of Na+ and Cl- from the diet in vivo in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus, an agastric teleost

A radiotracer approach using diets labelled with (22)Na(+), (36)Cl(-) and [(14)C]polyethylene-4000 (PEG-4000) was employed to investigate the role of intestinal uptake from the food in ion homeostasis in the killifish Fundulus heteroclitus. This euryhaline teleost lacks both a stomach and the capacity for Cl(-) uptake at the gills in freshwater. PEG-4000 appearance in the water was minimal up to 10-11 h post-feeding, indicating the virtual absence of Na(+) and Cl(-) loss in the faeces up until this time. Rapid uptake of dietary Na(+) and Cl(-) occurred and more than 88% of (22)Na(+) and (36)Cl(-) were absorbed in the intestine by 3 h post-feeding; excretion rates of Na(+) and Cl(-) originating from the food were greatest during this period. Uptake and excretion of Cl(-) from the diet was fivefold to sixfold greater than that of Na(+) in freshwater, and approximately threefold greater in seawater. Excretions of dietary Na(+) and Cl(-) by seawater-acclimated killifish were far greater than by freshwater-acclimated killifish in this time frame, reflecting the much greater branchial efflux rates and turnover rates of the internal exchangeable pools. At both 3 and 9 h post-feeding, the largest fraction of dietary Na(+) was found in the carcass of freshwater-acclimated fish, followed by the external water, and finally the digestive tract. However, in seawater-acclimated fish, more was excreted to the water, and less was retained in the carcass. For Cl(-), which was taken up and excreted more rapidly than Na(+), the majority of the dietary load had moved to the external water by 9 h in both freshwater and seawater animals. After 7 days training on a low-salt natural diet (live Lumbriculus variegatus worms; 31.5 μmol Na(+) g(-1) wet mass) versus a high-salt synthetic pellet diet (911 μmol Na(+) g(-1) dry food mass), freshwater killifish exhibited a lower absolute excretion rate of Na(+) from the low-salt diet, but relative uptake from the intestine and retention in the carcass were virtually identical from the two diets. Seawater killifish excreted relatively more Na(+) from the low-salt diet. Overall, our results emphasize the importance of dietary Na(+) and Cl(-) in the electrolyte economy of the killifish, particularly in freshwater, and especially for Cl(-).