Platichthys Stellatus Research Articles

A field test for correlates of poor reproductive success and genetic damage in contaminated populations of starry flounder, Platichthys stellatus

A field test for correlates of poor reproductive success and genetic damage in contaminated populations of starry flounder, Platichthys stellatus

Creatine turnover in the starry flounder,Platichthys stellatus

Starry flounder (Platichthys stellatus) were cannulated and a bolus of 9 μCi(14)C-creatine in saline was injected into the caudal vein. The fish were sacrificed at intervals ranging from 1h to 36d after label injection. Creatine pool size (PCr+Cr) and creatinine (Crn) content in blood, muscle, gills and liver were analyzed and specific activities (SA) determined.Mean concentrations of PCr+Cr/Crn in PCA-extracts of muscle, gills, liver and blood of experimental fish (at rest) were 38.1/2.40, 4.1/0.25, 5.6/0.45 and 0.3/n.d. μmol.g(-1) respectively.Within 10 min, plasma SA had decreased by approximately 90%. In white muscle, the rate of(14)C-Cr appearance as well as label disappearance was slow compared to gills and liver. In fish examined 36d postinjection, mean SA in muscle had decreased to 23% of maximum SA which occurred 24h after injection.(14)C-Cr was incorporated into the liver tissue at a very high rate, SA being two orders of magnitude higher in liver than in white muscle. Over the first 6d, retention of label was observed in liver; after 36d only 3% of the original label was detected.Creatine pool size (PCr+Cr) in white muscle decreased with food deprivation. In flounder sacrificed after 36d, PCr+Cr was only 52% that of fed control fish, suggesting that creatine or precursors for its biosynthesis are supplied with the diet.

Effects of organic contaminants on reproduction of the starry flounder Platichthys stellatus in San Francisco Bay

Concentrations of neutral organic contaminants and activities of microsomal P-450 mixed-function oxidase (MFO) were measured in the livers of the starry flounder Platichthys stellatus (Pallas), collected from more- and less-contaminated sites in San Francisco Bay, during the 1984–1985 reproductive season. Starry flounder collected at the Berkeley (Bk) site, located in the more urbanized central portion of San Francisco Bay, had greater liver concentrations of polychlorinated biphenyls (PCBs) and polynuclear aromatic hydrocarbons (PAH) (fluorescence equivalents of pyrene) than those collected at a site in northern San Pablo Bay (SP), where urban development is less intense and more distant. Hepatic aryl hydrocarbon hydroxylase (AHH) activity, a particular MFO, in males and in gonadally immature females fluctuated significantly from September 1984 through April 1985 with the Bk population exhibiting significantly greater activities during this period. Site differences were especially notable during the time of spawning (January–March), as AHH activities of starry flounder from SP declined and those from Bk remained elevated. These site differences appear to be due to P-450 isozymes (e.g. P-450E) whose activities are inducible by some PCBs and PAHs, as males and gonadally immature females from the two sites were not different in their hepatic AHH activities when assayed in the presence of an inhibitor of P-450E, 7,8-benzoflavone (7,8-BF). Greatly reduced hepatic AHH activities in females coincided with the onset of vitellogenesis; however, a comparison of female starry flounder bearing yolky eggs from the two sites during several successive reproductive seasons indicated significantly greater AHH activity in those caught at Bk than those from SP. There was a linear relationship between hepatic AHH activity and its inhibition by 7,8-BF in all P. stellatus assayed and more than 98% of individuals caught in San Francisco Bay had hepatic AHH activites that were suppressed by 7,8-BF. Therefore, widespread induction of hepatic microsomal P-450 by PAH-type compounds in San Francisco Bay is indicated and, further, P-450 induction is apparent in female starry flounder in a portion of San Francisco Bay during gametogenesis and the time of spawning.

Aryl hydrocarbon hydroxylase activity in a population of brown bullhead ( Ictalurus nebulosus) environmentally exposed to polycyclic hydrocarbons

Two marine flatfish species, English sole (Pleuronectes vetulus) and starry flounder (Platichthys stellatus), were exposed intermuscularly to tributyltin chloride (TBT) at 0.93 mg tin/kg body weight. The tissue concentrations of TBT and its biotransformation products dibutyltin (DBT) and monobutyltin (MBT) were monitored over a three week period. Tributyltin concentrations generally reached a maximum in all tissues by 24 h post exposure. At 24 h, the bile contained the greatest concentrations of tin, followed by the liver and kidney, and then by spleen and muscle. Total butyltins were generally present at higher concentrations in starry flounder than in English sole for all tissues, except gall bladder, at all time points. In both species the concentration of hepatic DBT increased with time. The time-point at which TBT and DBT concentrations became approximately equal was between 1 and 3 days for English sole and between 7 and 14 days for starry flounder. The ratios of hepatic TBT to DBT concentrations at all time-points from 1 to 21 days were significantly lower in English sole compared to starry flounder, suggesting more rapid biotransformation of TBT in English sole. First-order elimination rate constants for TBT in liver were 0.061 and 0.068 day−1 for English sole and starry flounder, respectively. The kidney TBT half-lives were shorter in flatfish than in rainbow trout.

Morphologic similarities and parallels in geographic distribution of suspected toxicopathic liver lesions in rock sole ( Lepidopsetta bilineata), starry flounder ( Platichthys stellatus), Pacific staghorn sculpin ( Leptocottus armatus), and Dover sole ( Microstomus pacificus) as compared to English sole ( Parophrys vetulus) from urban and non-urban embayments in Puget Sound, Washington

Morphologic similarities and parallels in geographic distribution of suspected toxicopathic liver lesions in rock sole ( Lepidopsetta bilineata), starry flounder ( Platichthys stellatus), Pacific staghorn sculpin ( Leptocottus armatus), and Dover sole ( Microstomus pacificus) as compared to English sole ( Parophrys vetulus) from urban and non-urban embayments in Puget Sound, Washington

Organic contaminants and hepatic mixed-function oxidase activities during the reproductive season in starry flounder from two sites in San Francisco Bay

Organic contaminants and hepatic mixed-function oxidase activities during the reproductive season in starry flounder from two sites in San Francisco Bay

Chlorophenols and Chlorinated Hydrocarbons in Starry Flounder (Platichthys stellatus) and Contaminants in Estuarine Sediments near a Large Municipal Outfall

Abstract Starry flounders, collected near a large municipal sewage outfall in the Fraser River Estuary, between March 1984 and April 1985, were quantified for chlorophenols, DDTs, and PCBs. Sediments from the polluted zone were analyzed for EPA Priority Pollutants and herbicides. Muscle tissue contained maximum chlorophenol levels about one half the Canadian limit and one tenth the limit for PCBs in edible fish tissue. Contaminants in matched tissue samples from the polluted zone and an adjacent control area showed statistically significant higher levels of all target compounds in muscle tissue from the outfall area except for DDD which was greater in the control. Bone and liver tissue were rich in PCBs and DDTs. The sediments contained phthalate esters, PCBs, PAHs and phenol. However the levels of pesticides, chlorophenols and herbicides were close to or below the detection limits. The sediment data were compared with values from nine other sites in the estuary.

Chemical carcinogenesis in feral fish: uptake, activation, and detoxication of organic xenobiotics.

The high prevalence of liver neoplasms in English sole (Parophrys vetulus) and substantially lower prevalence of neoplasms in a closely related species, starry flounder (Platichthys stellatus) captured from industrialized waterways, provide a unique opportunity to compare biochemical processes involved in chemical carcinogenesis in feral fish species. Because levels of aromatic hydrocarbons (AHs) in urban sediments are correlated with prevalences of liver neoplasms in English sole, we have initiated detailed studies to evaluate the effects of endogenous and exogenous factors on uptake, activation and detoxication of carcinogenic AHs, such as benzo[a]pyrene (BaP), using spectroscopic, chromatographic, and radiometric techniques. The results obtained thus far show that sole readily takes up AHs associated with sediment from urban areas and that the presence of other xenobiotics, such as PCBs, in sediment increases tissue concentrations of BaP metabolites. Extensive metabolism of BaP occurred whether sole was exposed to this AH via sediment, per os, or intraperitoneally. Substantial modification of hepatic DNA occurred and persisted for a period of 2-4 weeks after a single exposure to BaP. The level of covalent binding of BaP intermediates to hepatic DNA was 10-fold higher in juvenile than adult sole and 90-fold higher in juvenile sole than in Sprague-Dawley rat, a species which is resistant to BaP-induced hepatocarcinogenesis. The level of chemical modification of hepatic DNA in juvenile flounder was 2-4 fold lower than that for juvenile sole and concentration of BaP 7,8-diol glucuronide in bile of sole was significantly higher than that in flounder bile, although the rate of formation of BaP 7,8-diol by hepatic microsomes was comparable for both species. Moreover, liver microsomes from both species, in the presence of exogenous DNA, metabolized BaP into essentially a single adduct, identified as (+)anti-7,8-diol-9,10-epoxy-7,8,9,10-tetrahydroBaP-dG. These results, along with our findings that hepatic GST activity in flounder was two times higher than in sole, demonstrate that microsomal metabolism of BaP does not accurately reflect the differences in the ability of these fish to form BaP-DNA adducts in vivo and also suggest that detoxication of reactive intermediates is an important factor in determining the levels of DNA modification by AHs and resulting toxic effects in feral fish.

Volume-sensitive taurine transport in fish erythrocytes.

Taurine plays an important role in cell volume regulation in both vertebrates and invertebrates. Erythrocytes from two euryhaline fish species, the eel (Anguilla japonica) and the starry flounder (Platichthys stellatus) were found to contain high intracellular concentrations of this amino acid (approximately equal to 30 mmol per liter of cell water). Kinetic studies established that the cells possessed a saturable high-affinity Na+-dependent beta-amino-acid transport system which also required Cl- for activity (apparent Km (taurine) 75 and 80 microM; Vmax 0.85 and 0.29 mumol/g Hb per hr for eel (20 degrees C) and flounder cells (10 degrees C), respectively. This beta-system operated with an apparent Na+/Cl-/taurine coupling ratio of 2:1:1. A reduction in extracellular osmolarity, leading to an increase in cell volume, reversibly decreased the activity of the transporter. In contrast, low medium osmolarity stimulated the activity of a Na+-independent nonsaturable transport route selective for taurine, gamma-amino-n-butyric acid and small neutral amino acids, producing a net efflux of taurine from the cells. Neither component of taurine transport was detected in human erythrocytes. It is suggested that these functionally distinct transport routes participate in the osmotic regulation of intracellular taurine levels and hence contribute to the homeostatic regulation of cell volume. Volume-induced increases in Na+-independent taurine transport activity were suppressed by noradrenaline and 8-bromoadenosine-3', 5'-cyclic monophosphate, but unaffected by the anticalmodulin drug, pimozide.

Descriptions of Neolebouria tinkerbellae n.sp. (Trematoda: Digenea: Opecoelidae) from experimental fish hosts, and of metacercariae of N. tinkerbellae and an unidentified digenean from Pandalus jordani (Decapoda: Penaeidae) from the Pacific coast of Canada

Neolebouria tinkerbellae n.sp. (Trematoda: Opecoelidae) is described from specimens obtained from experimental infections in Platichthys stellatus (Pallas), Gasterosteus aculeatus Linnaeus, and Leptocottus armatus Girard. The metacercarial stage occurs in the heart muscle of Pandalus jordani Rathbun (smooth pink shrimp) from the Pacific coast of Canada and forms a translucent white cyst. A second, unidentified species of metacercaria found in P. jordani forms a cyst surrounded by a brown pigmented layer in the somatic musculature. Both metacercariae are described.

Effects of Strenuous Activity on Intracellular and Extracellular Acid-Base Status and H+Exchange with the Environment in the Inactive, Benthic Starry Flounder Platichthys stellatus

Exhaustive activity in starry flounder resulted in an acidosis in the whole-body extracellular fluid (ECF) and intracellular fluid (ICF) compartments. In the ECF, the acidosis consisted of a short-lived respiratory component (increase in CO₂ tension [Pco₂]) followed by a longer-lived metabolic component (decrease in []). The acid-base disturbance was corrected by 8-12 h. There was little lactate accumulation in the blood, with levels rarely greater than 1-2 mmol/liter, and at all times the blood metabolic acid load (ΔH⁺m) was in excess of the blood lactate load (ΔLa⁻). Blood [glucose] increased by 50%. Whole-body extracellular fluid volume (ECFV) fell by 17% owing to a shift of fluid into the intercellular fluid volume (ICFV), causing a general hemoconcentration. Exercise also caused an acidosis in the whole-body intracellular compartment, with intracellular pH dropping from a rest value of 7.58 to a low of 7.24. The whole-body intracellular acidosis was corrected ~4 h sooner than the extracellular disturbance and became alkalotic at 8 h, returning to normal at 12 h. Associated with this acid-base disturbance was an increased efflux of acidic equivalents (H⁺) to the environmental water, coincident with a large increase in the titratable-acidity flux. Ammonia excretion increased only slightly. Analysis of the distribution of metabolic acid between the ECF, ICF, and environmental water revealed that until 4 h postexercise, the bulk of the acid load remained in the intracellular compartment. Approximately 20% passed through the extracellular fluid and was transiently stored in the environmental water at 4-12 h. This flux of H⁺ to the water was associated with an intracellular alkalosis and thus appeared to hasten correction of intracellular acid-base status, perhaps as a means of aiding metabolic recovery.

Adrenergic Analysis of Extracellular and Intracellular Lactate and H+Dynamics after Strenuous Exercise in the Starry Flounder Platichthys stellatus

Adrenergic blockade with a selective a antagonist (yohimbine) and β antagonist (propranolol) was employed to test whether catecholamine mobilization was involved in lactate and acidic equivalents (H⁺) dynamics following strenuous exercise in chronically cannulated starry flounder. In control fish (not blocked) sampled repetitively for 8 h after exercise, arterial [lactate] remained characteristically low (<2 mmol/liter), and acid-base, hematological, and glucose changes were similar to those seen in other studies. Neither α nor β blockade immediately after exercise nor β blockade before exercise had significant influence on most responses, though hyperglycemia was inhibited by α blockade and prolonged by β blockade. Blood [lactate] was also marginally elevated, and red blood cell (RBC) swelling was reduced by β antagonism. A second series involving single-terminal sampling of cannulated flounder demonstrated that RBC intracellular pH (pHi), [NTP], and arterial PO₂ remained unchanged in the face of severe ...

Muscle and Liver Intracellular Acid-Base and Metabolite Status after Strenuous Activity in the Inactive, Benthic Starry Flounder Platichthys stellatus

In addition to an extracellular acidosis in which blood metabolic acid load greatly exceeded lactate load, exhaustive activity in starry flounder resulted in an intracellular acidosis of largely metabolic origin in the white muscle, with intracellular pH dropping from 7.56 to 7.27, as measured by DMO distribution. An accumulation of lactate and depletion of glycogen in addition to a shift of fluid from the extracellular to intracellular space were associated with the postexercise acidosis. Pyruvate levels increased in blood and later in muscle; the relative rise in pyruvate was greater than that in lactate so the lactate:pyruvate ratio declined. The muscle intracellular acidosis was corrected sooner than the extracellular acidosis (4–8 h vs. 8–12 h). The restoration of muscle pHi was associated with an increase in pyruvate, a restoration of glycogen stores, and clearance of the lactate load. It is suggested that both lactate and acidic equivalents (H⁺) were cleared from the muscle via in situ oxidation and/or glyconeogenesis and that the rapid correction of the intracellular acidosis through efflux of part of the H⁺ load facilitated metabolic recovery. The liver showed a progressive alkalinization after exercise. This alkalinization was of metabolic origin and not associated with lactate accumulation. Except for a short-lived depression 0.5 h after exercise, red cell intracellular pH remained virtually constant.

Host-Parasite Relationships between Gyrodactylus stellatus (Monogenea: Gyrodactylidae) and Parophrys vetulus (Pleuronectidae: English Sole) from Coastal Waters of Oregon

Gyrodactylus stellatus Crane and Mizelle, 1967, was found on juvenile English sole (Parophrys vetulus), speckled sanddabs (Citharichthys stigmaeus) and starry flounder (Platichthys stellatus) in Yaquina Bay, Oregon. Prevalence and intensity of infections were higher on juvenile English sole than on speckled sanddabs or starry flounders. Over a 12-mo period from July 1981 to June 1982, prevalence and intensity of infection on juvenile English sole averaged 69.2% per sampling and 5.5 worms per fish, respectively. Prevalence and intensity were maximum in June (98.0% and 10.1 worms/fish) and minimum in October (37.0% and 1.7 worms/ fish). In the laboratory, intensity on juvenile English sole reached higher levels than observed on hosts in the estuary. Intensity increased for 9 wk then decreased over the next 3-wk period. Rate of G. stellatus population growth increased with temperature from 6 to 18 C, and was higher on unfed than on fed English sole. Salinity and previous experience with high infection levels did not significantly influence the rate of G. stellatus increase. English sole (Parophrys vetulus Girard, 1854) is one of the most common pleuronectid species found along the Pacific Coast of the United States and an important component of the commercial fishery (Hart, 1973). Large numbers of juvenile English sole utilize Yaquina Bay as a nursery ground during their first year of life (Olson and Pratt, 1973) and according to Olson (1978) are commonly parasitized by Gyrodactylus stellatus Crane and Mizelle, 1967. Gyrodactyliasis caused by several species of monogenetic trematodes in this genus has been reported in both freshwater and marine teleosts (Cone and Odense, 1984). This study examines the host-parasite relationship between English sole and G. stellatus. MATERIALS AND METHODS Fishes from Yaquina Bay, Oregon were collected with a 5-m otter trawl and transported to the Oregon State University Marine Science Center. Juvenile English sole (P. vetulus) to be used in experiments were held in circular tanks (87 cm in diameter with 125 L of seawater) supplied with running ultraviolet light filtered seawater (3-4 L/min) for 1-2 wk to allow feeding to begin. Experimental fish were of similar size (7.5 + 3.2 cm long) and free of abnormalities. All fish were maintained on a diet of commercial moist salmon feed. Uninfected experimental fish were obtained by treating with a 1:4,000 formalin solution for 1 hr (Putz and Hoffman, 1963). Laboratory infections were produced by placing uninfected fish in a tank with 2-3 infected fish for 1-3 days. Received 16 April 1985; revised 27 September 1985; accepted 30 September 1985. Host specificity of G. stellatus was studied ecologically by examining a variety of potential host species from Yaquina Bay and experimentally by exposing potential hosts to the parasite in the laboratory. Prevalence and intensity of infection of G. stellatus on English sole from Yaquina Bay by season and by size of fish were determined from samples taken monthly from July 1981 to June 1982. Shortly after capture, 50 fish were selected randomly and the number of Gyrodactylus present on the fins of each fish was counted under a dissecting microscope. If the catch was less than 50 fish, all were used as samples. The method by which G. stellatus transfers from one host to another was examined in an experiment that consisted of 4 treatments. In the first and second treatments infected English sole were kept separate from uninfected hosts in flowing and static water. In the third treatment, infected and uninfected fish were allowed direct contact in static water. In the last treatment infected English sole were removed from the container before introduction of uninfected sole. Each container contained 2 infected and 3 uninfected English sole. The rate at which G. stellatus increased in number on juvenile English sole (8.8 ? 1.08 cm long) under laboratory conditions and ambient temperature was determined by following the course of infection on 50 naturally infected individuals. The number of worms on pectoral, ventral and caudal fins on each fish was counted weekly for 1 yr. To determine the effect of temperature on intensity of infection, naturally infected English sole were held in circular tanks of running seawater maintained at 6, 12, or 18 C. Each treatment was replicated with 25 fish (10.3 ? 1.12 cm long) per replicate. The experiment extended for 6 wk and the number of G. stellatus per fish was counted weekly. In an experiment to determine the effect of salinity on intensity of G. stellatus infections, infected fish (10.1 + 0.62 cm long) were maintained at salinities of 14, 21 and 28%o in static water.

Sea star and walrus predation on bivalves in Norton Sound, Bering Sea, Alaska

Abstract Pacific walruses (Odobenus rosmarus divergens) and sea stars (Asterias amurensis) are the primary predators on bivalve mollusks in Norton Sound and probably in most of the Bering and Chukchi Seas. Secondary predators in these areas include starry flounder (Platichthys stellatus) and king (Paralithodes spp.) and Tanner (Chionoecetes spp.) crabs. Walruses and sea stars consume the same species, but not the same sizes of bivalves. The primary walrus prey are large individuals of several bivalve species, which have no refuge from walrus predation. Sea stars feed on smaller species and on smaller individuals of the large bivalve species. Laboratory experiments and field observations indicate that large bivalves have refuges from sea stars. Large Serripes groenlandicus escape sea star predation by leaping, large Yoldia hyperborea by rapid burrowing into the sediment. Large Mya truncata and Macoma calcarea live too deep in the sediment (>15 cm) for capture by the shallow digging (<10 cm) Asterias amuren...

Reproductive success, xenobiotic contaminants and hepatic mixed-function oxidase (MFO) activity in Platichthys stellatus populations from San Francisco Bay

To identify some specific effects of organic contaminants on fisheries in an urbanized estuary we compared the reproductive success of starry flounder from San Francisco Bay with concentrations of tissue contaminants and hepatic mixed-function oxidase (MFO) activity. We found significantly lower (P < 0·05) sediment concentrations of total identified polynuclear aromatic hydrocarbons (PAHs) in the less urbanized San Pablo Bay (SP) area (Fig. 1) than in the more urbanized central bay (CB) stations (Table 1). For flounder in early gametogenesis (August and September) the SP fish (n = 20) had significantly lower (P < 0·01) liver concentrations of Aroclor 1260 (0·34 ± 0·14 μg/g) than those at the CB stations: Berkeley (BK, n = 20, 1·6 ± 1·6 μg/g); Oakland (OK, n = 16, 2·3 ± 2·8 μg/g); and Alameda (AL, n = 4, 2·2 ± 1 μg/g). A similar pattern existed for DDT concentrations: SP = 0·2 ± 0·16 μg/g; BK = 0·1 ± 0·34 μg/g; OK = 0·4 ± 0·53 μg/g; and AL = 0·4 ± 0·33 μg/g. Total PAHs in livers were as follows: SP = 0·14 μg/g; BK = 2·6 μg/g; OK = 1·4 μg/g; and AL = 14 μg/g. Although gonad index, liver index, and presence of fin rot are inversely related to aryl hydrocarbon hydroxylase (AHH) activity, healthy fish in a similar reproductive state have lower AHH activities in the SP area. For example, in August and September, 1984, mean AHH activities were as follows: SP = 203 ± 89, and CB = 355 ± 200 pmol 3-OH-B[ a]P mg microsomal protein min. We found a log-linear relationship for AHH activity and its percent inhibition by 7,8-benzoflavone ( 10 −4 m ) and only a few fish from SP showed enhanced AHH activity after addition of 7,8-benzoflavone. This suggests that most of the starry flounder in San Francisco Bay are induced.

On the inheritance of directional asymmetry (sidedness) in the starry flounder, Platichthys stellatus: Additional analyses of Policansky's data

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Microstructural growth patterns in the otoliths of larval and juvenile starry flounder, Platichthys stellatus

Microstructural growth patterns were examined and interpreted in the otoliths of both wild and laboratory-reared starry flounders, Platichthys stellatus. Growth increments were not formed with a daily periodicity in laboratory-reared larvae. However, increment counts increased with the degree of sample preparation, suggesting that increments near the resolving limit of light microscopy were not being observed. Increments in wild flounder sagittae were more clearly defined under both light and electron microscopy; in addition, larval and juvenile growth patterns could be easily differentiated. A transition zone between the growth regions corresponded to the size and age at metamorphosis. An increase of increment width with larval age resulted from a curvilinear relationship between otolith diameter and fish size. Larval growth rates of wild fish remained relatively constant at 0.25 mm/day until metamorphosis; juvenile growth rates were substantially higher. Metamorphosis was characterized by a sudden but temporary decline in growth rate.

Comparison of Age Determination Methods for the Starry Flounder

Sagittal otoliths from 808 starry flounders Platichthys stellatus from Bellingham Bay, Washington were examined for annular structures by two techniques: Traditional external readings of whole otoliths and internal readings of “cracked and burnt” otoliths. The annular nature of rings observed by both techniques was confirmed by seasonal progression of the width of opaque otolith margins in fish 1–3 years old. Annular rings were also confirmed in internally read otoliths with 4–10 rings. More annuli could be identified in sexually mature fish when read internally (maximum, 24) than externally (maximum, 11). Anteroposterior growth of starry flounder sagittae was attenuated with age, although the sagittae continued to thicken; this accounts in large measure for the relative success of the two techniques. Despite the likelihood of geographical variation, it appears that starry flounders have a longer lifetime than was previously suspected. Received November 17, 1983 Accepted April 8, 1984

Passive and active ion transport by the urinary bladder of a euryhaline flounder.

The effects of voltage clamping on the flux ratios and unidirectional and net fluxes of Na and Cl were used to gain insight into the mechanisms of active and passive ion transport across urinary bladders isolated from seawater-(SW) and freshwater-acclimated (FW) flounder, Platichthys stellatus. Although the transepithelial conductance (Gt = 2.77 mS X cm-2) of FW bladders was much greater than that of SW bladders (Gt = 0.40 mS X cm-2), the current-voltage relationships of both SW and FW bladders were markedly nonlinear. Under short-circuit conditions there was a large difference in the serosal-to-mucosal Na flux (JNasm) between SW (0.10 mueq X cm-2 X h-1) and FW (1.71 mueq X cm-2 X h-1) bladders, but their mannitol permeabilities were identical. The results indicate that 1) the paracellular pathway of both SW and FW bladders is Cl selective and Cl movements through the shunt account for a maximum of 90% of Gt in SW bladders and 19% in FW bladders; 2) the larger Gt of FW bladders is due to greater conductance of the apical cell membrane; 3) the majority of the passive ion movement across these epithelia proceeds through nonconductive, presumably transcellular, pathways; and 4) active transport of Na and Cl occurs by neutral coupling to each other and to other unidentified ions.