Changes in osmotic water permeability of the eel gills during seawater and freshwater adaptation

Summary Changes in osmotic water permeability of the isolated gills of the Japanese eel,Anguilla japonica were studied during transfer to seawater or to fresh water. The water permeability increased gradually during the course of seawater transfer and attained a maximal level after 2 weeks. The water permeability of the freshwater eel gills was reduced when calcium ions were added to the incubation medium at a concentration of 1 mM, where-as no effect of the ion was observed on the gills of the seawater-adapted eel even at a higher concentration (10 mM). In contrast to seawater transfer, the water permeability decreased to a low level almost immediately (3 h) after transfer from seawater to fresh water. The acute reduction of the water permeability was also seen in the gills of the hypophysectomized eel after transfer to fresh water.The gradual increase in the gill water permeability during seawater transfer is correlated with an increase in the number of chloride cells. In scanning electron microscopy, chloride cells of seawater-adapted eel gills exhibit a pit-like structure, which was larger than in the freshwater eel. On transfer from seawater to fresh water, the pit diameter became smaller within 6 h. Hypophysectomy did not affect the change in gill surface structures during transfer to fresh water. The junctions between the chloride cells of seawater eel gills are reported to be of the leaky type. The parallel change in osmotic water permeability and in pit size of the chloride cells during seawater or freshwater transfer or after hypophysectomy suggests that these cells could provide a major route of water as well as ion movement.This paper is a portion of a thesis presented to Hokkaido University by t. Ogasawara in partial fulfilment of the requirements for Doctor of Fisheries     

Burrowing behavior and ecology of the crab-eating Indian snake eel Pisoodonophis boro

Synopsis The snake eel Pisoodonophis boro burrows, causing leaks in the embankments and damaging the paddy fields and salt pans near estuaries. Field observations and laboratory experiments were made to study this behavior. P. boro was burrowing to eat the fiddler crab Uca annulipes in the mud flats. The eel showed a patchy distribution within the Uca zone. Salinity and the physical nature of the deposits controlled the distribution of the eel. Eel population density was low when the estuary was completely filled with neritic waters during the summer and fresh water during the monsoon period. The region of greatest abundance contained a good mixture of sand, silt and clay. Eels were not found where medium and fine sand formed the bulk of the substratum. The laboratory experiments showed that P. boro preferred loam soil although it could invariably burrow into hard substratum like sand for protection. The eel adapted itself to the experimental substrates ranging from sand to fine clay. However, their natural distribution was determined by Uca distribution. As U. annulipes is not found either in salt pans or in paddy fields P. boro rarely occurs in these habitats.     

稻虾田黄鳝的食物组成及其对克氏原螯虾捕食强度研究

文章研究了稻田黄鳝(Monopterus albus)天然饵料生物资源、稻田黄鳝对克氏原螯虾(Procambarus clarkii)捕食选择及不同投喂策略下黄鳝对克氏原螯虾的捕食强度,旨在为构建与优化稻-虾-鳝综合种养模式提供依据。结果表明,稻田中黄鳝天然饵料生物种类丰富,达16种(属);克氏原螯虾是黄鳝最喜猎物,其次为米虾,再次为蚯蚓和水生寡毛类;稻田黄鳝自然生长期间,其胃和肠前端食物团中,克氏原螯虾重量百分比均显著高于其他猎物,其中8月份占比最大,达93.90%, 4月份占比最小,为76.85%;当克氏原螯虾为唯一食物时,每尾大规格成鳝(≥200 g)日均捕食量为(1.63±0.065) g;当克氏原螯虾、米虾和蚯蚓作为食物时,黄鳝主要捕食克氏原螯虾且不捕食蚯蚓,对三者的选择指数分别为0.066、–0.266和–1;若以克氏原螯虾、鱼糜-饲料为食物时,黄鳝主要摄食鱼糜-饲料,极少捕食克氏原螯虾,对两者的选择指数分别为–0.846和0.591。     

A novel guanylin family (guanylin,uroguanylin, and renoguanylin) in eels: possible osmoregulatory hormones in intestine and kidney

As the intestine is an essential organ for fish osmoregulation, the intestinal hormone guanylins may perform major functions, especially in euryhaline fish such as eels and salmonids. From the intestine of an eel, we identified cDNAs encoding three distinct guanylin-like peptides. Based on the sequence of mature peptide and sites of production, we named them guanylin, uroguanylin, and renoguanylin. Renoguanylin is a novel peptide that possesses the characteristics of both guanylin and uroguanylin and was abundantly expressed in the kidney. By immunohistochemistry, guanylin was localized exclusively in goblet cells, but not enterochromaffin cells, of the intestine. After transfer of eels from fresh water to seawater, mRNA expression of guanylin and uroguanylin did not change for 3 h, but it increased after 24 h. The increase was profound (2-6-fold) after adaptation to seawater. The expression of uroguanylin was also up-regulated in the kidney of seawater-adapted eels, but that of renoguanylin was not so prominent as other guanylins in both intestine and kidney. Collectively, the novel eel guanylin family appears to have important functions for seawater adaptation, particularly long-term adaptation. Eel guanylin may be secreted from goblet cells into the lumen with mucus in response to increased luminal osmolality and act on the epithelium to regulate water and salt absorption.     

Induced oogenesis of the European eel (Anguilla anguilla L.) in freshwater condition

European eel is a catadromous fish species, which means that after living in freshwater premature individuals adapt to sea water, and migrate to the Sargasso Sea for spawning. Although male eel can be sexually matured even in freshwater, to date, it was believed that female eel can be matured only in seawater. Here we show that the process of sexual maturation may be induced in freshwater by treating female eels with carp pituitary (GSI = 9.87 ± 1.55%). It is thus proposed that seawater condition is not an obligatory environment for stimulating gametogenesis and for artificial maturation of the European eel in neither gender.     

The testis histology of artificially maturated European eel (Anguilla anguilla L.) at the end of sexual maturation, and spermatozoa ultrastructure in freshwater rearing. Short communication

The artificial induction of sexual maturation of European eel males was carried out by using weekly hCG administrations. Histological pictures showed that the testis tissues developed and regressed naturally and no pathological changes took place under the conditions of artificial rearing in freshwater. According to light and electron microscopic investigations the morphology and motility of the spermatozoa of males kept in freshwater proved to be similar to those in seawater. The authors suppose that freshwater rearing of males is not a barrier factor in the artificial propagation of European eels.     

Development of Anguillicola crassus (Dracunculoidea, Anguillicolidae) in experimentally infected Balearic congers Ariosoma balearicum (Anguilloidea, Congridae).

The development of Anguillicola crassus in experimentally infected Ariosoma balearicum (Anguilloidea, Congridae) kept in seawater was studied in the laboratory. In parallel trials the effect of water salinity on the development of larval A. crassus in European eels Anguilla anguilla was also investigated using eels kept in seawater of a salinity of 34 per thousand. Both eel species were orally inoculated with L3 larvae of A. crassus and then maintained for up to 3 mo at 18 degrees C in seawater. 110 d post infection, no adult but larval (L3 and L4) stages of A. crassus were detected in the swimbladder wall of Balearic congers, although this period of time was sufficient for the parasites to develop to the adult stage in European eel kept in seawater. The results presented suggest that the definitive host specificity of A. crassus comprises species of the family Anguillidae (i.e. the genus Anguilla), but not members of the Congridae. Theoretically however, A. balearicum might serve as a metaparatenic host. Factors determining the definitive host range of A. crassus remain to be elucidated. Water salinity does not seem to act as a factor affecting definitive host specificity once the parasite has become ingested by the eel.     

Electroneutral cation-Cl- cotransporters NKCC2β and NCCβ expressed in the intestinal tract of Japanese eel Anguilla japonica

In the present study, we aimed to elucidate the mechanisms of intestinal Na(+) and Cl(-) absorption in Japanese eel, focusing on electroneutral cation-Cl(-) cotransporters, NKCC2β and NCCβ, expressed in the intestinal tract. First, we cloned cDNAs encoding NKCC2β and NCCβ from the intestinal tract of Japanese eel. In both freshwater- and seawater-acclimated eels, quantitative PCR analysis showed that NKCC2β was predominantly expressed in the anterior and posterior intestines, and that NCCβ expression was specifically high in the rectum. According to immunohistochemistry with anti-eel NKCC2β (reacting with NKCC2β but not with NCCβ) and T4 antibody (reacting with both NKCC2β and NCCβ), NKCC2β was localized in the apical surface of the epithelial cells in the anterior and posterior intestines, whereas NCCβ was likely to be distributed to that in the rectum. Furthermore, a specific NCC inhibitor, hydrochlorothiazide, inhibited of Na(+) and Cl(-) absorption, as well as water absorption, in the rectal sac preparations from seawater eel, indicating the involvement of NCCβ in ion absorption in the rectum. Our findings indicate that NKCC2β expressed in the anterior and posterior intestines and NCCβ in the rectum are importantly involved in ion absorption to reduce osmolality of ingested seawater prior to water absorption in seawater-acclimated eel.     

Compensation for hypercapnia by a euryhaline elasmobranch: effect of salinity and roles of gills and kidneys in fresh water

Specimens of the euryhaline elasmobranch, Dasyatis sabina were acclimated to seawater and fresh water, and exposed to normocapnic (air) and hypercapnic (1% CO2 in air) environmental water. Blood pH, PCO2, and [HCO3-], as well as whole-animal net-acid excretion, were measured for up to 24 h of hypercapnia. In a separate experimental series, urine was collected from freshwater acclimated stingrays during 8 h of normocapnia and hypercapnia. Stingrays in both salinities at least partially compensated for the respiratory acidosis by accumulating HCO3- in their extracellular spaces. The degree of compensation for blood pH was 88.5% in seawater, but only 31.0% in fresh water after 24 h of hypercapnia. Whole-animal net-acid excretion was also greater in seawater than in fresh water, as was the increase in extracellular fluid [HCO3-]. Mean urinary net-acid excretion rates were slightly negative, and never increased above normocapnic control rates during hypercapnia. Since whole-animal net-acid excretion rates increased with blood [HCO3-], and urinary excretion was always negative, the gills were probably the primary organ responsible for compensation from environmental hypercapnia. The faster, and more complete, compensation for hypercapnia in seawater than in fresh water for this euryhaline elasmobranch is consistent with data for euryhaline teleosts, and probably reflects Na+-dependent mechanisms of branchial acid excretion.     

Influence of aquatic microbiota on the survival in water of the human and eel pathogen Vibrio vulnificus serovar E

The eel and human pathogen Vibrio vulnificus serovar E (biotype 2) is seldom isolated from natural waters, although it can survive in sterilized artificial seawater microcosms for years. The main objective of the present study was to investigate whether aquatic microbiota can limit its survival and recovery from water samples. A set of preliminary experiments of survival in microcosms containing natural seawater and water from eel farms showed that the persistence of this pathogen was mainly controlled by grazing, and secondarily by bacterial competition. The bacterial competition was further analysed in artificial seawater microcosms co-inoculated with selected virulent serovar E (VSE) strains and potential competitors. Competitors included V. vulnificus biotype 1 isolates and strains of selected species that can grow on the selective media designed for V. vulnificus isolation from water samples. Evidences of bacterial competition that was detrimental for VSE recovery were recorded. Thus, some species produced a deleterious effect on VSE strains under starvation, and others were able to use the resources more efficiently under nutrient input. These results suggest that an overgrowth of more efficient competitor bacteria in conventional media used for isolation of V. vulnificus could mask the recovery of VSE strains and explain the scarcity of reports on the isolation of this human and eel pathogen from natural waters.     

Expression of sodium/hydrogen exchanger 3 and cation-chloride cotransporters in the kidney of Japanese eel acclimated to a wide range of salinities

Reabsorption of monovalent ions in the kidney is essential for adaptation to freshwater and seawater in teleosts. To assess a possible role of Na⁺/H⁺ exchanger 3 (NHE3) in renal osmoregulation, we first identified a partial sequence of cDNA encoding NHE3 from the Japanese eel kidney. For comparison, we also identified cDNAs encoding kidney specific Na⁺–K⁺–2Cl^? cotransporter 2 (NKCC2α) and Na⁺–Cl^? cotransporter (NCCα). In eels acclimated to a wide range of salinities from deionized freshwater to full-strength seawater, the expression of NHE3 in the kidney was the highest in eel acclimated to full-strength seawater. Meanwhile, the NCCα expression exhibited a tendency to increase as the environmental salinity decreased, whereas the NKCC2α expression was not significantly different among the experimental groups. Immunohistochemical studies showed that NHE3 was localized to the apical membrane of epithelial cells composing the second segments of the proximal renal tubule in seawater-acclimated eel. Meanwhile, the apical membranes of epithelial cells in the distal renal tubule and collecting duct showed more intense immunoreactions of NKCC2α and NCCα, respectively, in freshwater eel than in seawater eel. These findings suggest that renal monovalent-ion reabsorption is mainly mediated by NKCC2α and NCCα in freshwater eel and by NHE3 in seawater eel.     

Diversity of microorganisms in solar salterns of Tamil Nadu,India

In this study, the diversity of prokaryotes inhabiting crystallizer ponds of three solar salterns, located along Bengal Bay in Tamil Nadu, India was examined. Unlike other salterns studied the Tamil Nadu salterns are fed by hypersaline spring water mixed with seawater and led to the ponds from bore wells. In addition, prokaryotic community development is restricted as salterns operate only during the arid part of the year. Both culture-based and culture-independent polymerase chain reaction 16S rRNA molecular phylogenetic approaches were employed. Representatives of the family Halobacteriaceae dominated in cultivable portion of diversity encountered with members of genera Haloferax, Halorubrum, Haloarcula, Halobacterium and Halogeometricum recovered in pure culture. In contrast, members of Bacteria were recovered from only one sampling site and were represented by members of genera Salinibacter, Cytophaga and Marinococcus. Based on culture-independent sampling, the predominant members of the haloarchaeal crystallizer community belonged to the genus Natrinema.     

The effects of calcium and magnesium on water and ionic permeabilities in the sea water adapted eel,Anguilla anguilla L.

Summary The permeability characteristics to water and ions, of the seawater adapted eel, have been studied in the absence and presence of external calcium and magnesium.The absence of these divalent ions from the external medium produces an increase in the unidirectional and net water fluxes as well as a decrease in temperature coefficients. Similarly, an increase in the effluxes of sodium and chloride and a large increase in the passive loss of these ions was observed when fish were transferred to deionized water. The stimulating effect of externally added (10 mM) potassium on the sodium and chloride effluxes, when fish are transferred to fresh water is abolished in the absence of calcium and magnesium (Fig. 1). These kinetic changes are accompanied by ultrastructural changes concerned principally with the chloride cells (Figs. 2 and 3).The results are discussed in terms of the possible role of calcium and magnesium in the maintenance of the hydro-mineral equilibrium of the eel.     

Water balance in the european eel (Anguilla anguilla L.). Role of the oesophagus in the utilisation of drinking water and a study of the osmotic permeability of the gills (author's transl)]

10 New experimental devices are described which allow chonic measurements of drinking rate and osmotic gill permeability in the eel. 20 The oesophagus of the seawater (SW) silver eel plays a role in osmoregulation. It decreases the concentration of Cl- and Na+ of the ingested SW without losing water in the serosal to mucosal direction. This allows for immediate water absorption in the intestine and decreases the quantity of ions actively absorbed by the intestine. In the freshwater (FW) silver eel, the oesophagus is impermeable to water, Cl- and Na+. The ionic impermeability exists only in the serosal to mucosal direction. A mucosal to serosal permeability to Cl- and Na+ exists in the FW oesophagus receiving hypertonic drinking water, this promotes seawater adaptation. 30 The osmotic gill permeability, measured in vivo in the silver eel, is very low in FW and decreases slightly in SW. Thus, the silver eel has an osmotic gill permeability preadapted to SW life. The kinetics of FW to SW adaptation are described.     

Dehydration and drinking responses in a pelagic sea snake

Recent investigations of water balance in sea snakes demonstrated that amphibious sea kraits (Laticauda spp.) dehydrate in seawater and require fresh water to restore deficits in body water. Here, we report similar findings for Pelamis platurus, a viviparous, pelagic, entirely marine species of hydrophiine ("true") sea snake. We sampled snakes at Golfo de Papagayo, Guanacaste, Costa Rica and demonstrated they do not drink seawater but fresh water at variable deficits of body water incurred by dehydration. The threshold dehydration at which snakes first drink fresh water is -18.3 ± 1.1 % (mean ± SE) loss of body mass, which is roughly twice the magnitude of mass deficit at which sea kraits drink fresh water. Compared to sea kraits, Pelamis drink relatively larger volumes of water and make up a larger percentage of the dehydration deficit. Some dehydrated Pelamis also were shown to drink brackish water up to 50% seawater, but most drank at lower brackish values and 20% of the snakes tested did not drink at all. Like sea kraits, Pelamis dehydrate when kept in seawater in the laboratory. Moreover, some individuals drank fresh water immediately following capture, providing preliminary evidence that Pelamis dehydrate at sea. Thus, this widely distributed pelagic species remains subject to dehydration in marine environments where it retains a capacity to sense and to drink fresh water. In comparison with sea kraits, however, Pelamis represents a more advanced stage in the evolutionary transition to a fully marine life and appears to be less dependent on fresh water.     

Histological and enzyme histochemical changes found in the renal tubules of eels transferred from freshwater to seawater

Renal tubules of the eel,Anguilla japonica, transferred from freshwater to seawater were examined histologically and histochemically. The epithelial cells of renal tubules in eels adapted to seawater for 10 days were less interdigitated in comparison with those in freshwater. The poor interdigitation of epithelial cells seems to be a structure relating to high water permeability in the renal tubules of eels in seawater. Newly formed nephrons and degenerated ones were especially recognized in eels adapted to seawater for 2 and 4 days. These findings indicate that some of the nephrons equipped with poorly interdigitated epithelial cells are produced newly within a few days after transfer from freshwater to seawater. No significant difference was encountered in the activities and localizations of all enzymes examined between the renal tubules of freshwater and seawater eels. The enzyme activities and localizations did not reflect the physiological changes in the renal tubules of eels adapted from freshwater to seawater.     

Influence of modern irrigation,drainage system and water management on spawning migration of mud loach, <Emphasis Type="Italic">Misgurnus anguillicaudatus</Emphasis> C.

We investigated the influence of human activities on the spawning migration of the mud loach, Misgurnus anguillicaudatus. Mud loaches migrate from rivers to paddy fields for spawning. In this study, the number of adult loaches in paddy fields showed a peak in late May and rapidly decreased in July. In early July, water is discharged from paddy fields in order to allow the fields to dry for a few weeks, and water is again supplied later. The spawning season of the loaches probably spans from mid May to August; this was concluded by observing physiological indicators such as the gonad somatic index, oocyte maturity and serum levels of testosterone, estradiol-17β and 17α, 20β-dihydroxy-4-pregnen-3-one. When the loaches mature, they migrated to paddy fields if they could swim upward through the drainage ditches. However, the drainage ditches of modern paddy fields were reconstructed to deepen the water levels. The artificial drought of the rice culture management led to a discharge of water and loaches from the paddy fields during their spawning season. These results indicate that modern rice cultivation causes obstruction of the migration route and loss of spawning grounds; consequently, loaches use limited space and period of the paddy field for their spawning.     

Plasma and pituitary prolactin levels in rainbow trout during adaptation to different salinities

The development of a highly specific radioimmunoassay for salmonid prolactin (PRL) using chinook salmon PRL allowed us to study plasma and pituitary PRL profiles in large sedentary rainbow trout (Salmo gairdneri) transferred from fresh water to seawater and vice versa. Plasma osmotic pressure and chloride levels were also measured for 3 weeks following change of salinity. Within 1 day after transfer to full seawater we observed a plasma PRL decrease, which stayed significantly lower (3-5 ng/ml) than the fresh water control group (10-15 ng/ml) during the entire experiment. Pituitary PRL content showed an initial abrupt increase, but after 3 weeks in seawater pituitary PRL content had decreased to the same level as in the fresh water control group. On the contrary, transfer from seawater to fresh water was followed within 1 day by a rise in plasma PRL levels, which stayed high (10-15 ng/ml) after 3 weeks in fresh water. Simultaneously, pituitary PRL content decreased significantly. These results may indicate an important role of PRL in fresh water adaptation of sedentary rainbow trout.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

Factors influencing otolith strontium/calcium ratios in Anguilla japonica elvers

The effects of temperature and salinity on the concentration ratios of strontium (Sr) to calcium (Ca) within the sagittal otoliths of elvers of the Japanese eel, Anguilla japonica, were studied by spot analysis using a wavelength dispersive X-ray electron microprobe. A total of 340 elvers were used: 100 elvers were reared for 15 days under various salinity conditions (freshwater, one-third seawater, two-thirds seawater and pure seawater at 22 °C; 240 elvers were reared for 58 days under various water temperature conditions (12, 17, 22, 27 °C) in either freshwater or pure seawater. Otolith Sr/Ca ratios were found to be positively correlated with water salinity. On the other hand, the Sr/Ca ratios were not found to be significantly different among the various temperature groups. The above results strongly suggest that the physiological mechanism of incorporation of Sr and Ca within the otolith of an eurythermal fish, Japanese eel, does not change within this range of temperatures (12–27 °C).