Effects of potassium concentration and osmotic pressure of sea water on the cell-turgor pressure of Chaetomorpha linum

The unicellular littoral alga Chaetomorpha linum is especially capable of maintaining its cell-turgor constant by regulation of the internal osmotic pressure, when the salinity of the sea water is altered. The decrease or increase of the external potassium concentration is seen to be an important cause of this turgor regulation, as well as the alteration of the external osmotic pressure, which was already known to be an important factor. This has been shown by experiments in artificial sea water with reduced osmolality and variable potassium concentration (1 to 50 mMol/l).     

Population structure and breeding biology of Pisidia longicornis and Porcellana platycheles

Quantitative sampling of littoral and sublittoral populations of Pisidia longicornis (L.) and a littoral population of Porcellana platycheles (Pennant) has been carried out over a 2 year period (1970/1972). On the shore, P. longicornis were most abundant at mean low water spring (MLWS), whereas P. platycheles were found mostly at lower mid-shore level. Differences were found between littoral and sublittoral P. longicornis. Sublittoral females are gravid for a slightly shorter period (April to September), carry more eggs, and are smaller than littoral females. It is suggested that their lifespan may also be shorter than that of littoral individuals. Young of both species settle in autumn and grow until December, when growth ceases until the following spring. Numbers decline in the summer months due to the mortality of older individuals. Many sublittoral and some littoral P. longicornis are sexually mature towards the end of their first year on the shore, and may copulate after the early spring moult and carry eggs in the subsequent summer months. P. platycheles appear to follow a similar basic pattern, but results are less clear.     

Buoyancy in Porcellana platycheles

Intermoult Porcellana platycheles (Pennant) are slow moving, crawling crabs, incapable of swimming upwards from the substrate. Newly moulted porcelain crabs, however, are capable of effective upward swimming. An increased efficiency of tail-fan propulsion in newly moulted crabs was thought unlikely, and it was suspected that newly moulted P. platycheles were more buoyant than intermoult crabs. Results are presented to show that this is the case, and calculations were performed which indicate that fluid uptake at moult is insufficient to produce the observed increase in buoyancy. It is concluded that loss of heavy minerals and scleroprotein at moult cause the increased buoyancy. The adaptive significance of the enhanced swimming ability in newly moulted crabs is thought to lie in an improved ability to escape predation by intertidal fish and crabs.     

Physiological capacity and environmental tolerance in two sandhopper species with contrasting geographical ranges: Talitrus saltator and Talorchestia ugolinii

We investigated the physiological plasticity and environmental tolerance of two phylogenetically closely related, ecologically similar and co-occurring species of supralittoral amphipods differing drastically in the size of their geographical ranges. A series of physiological traits were characterised for the Corsican-endemic Talorchestia ugolinii Bella-Santini and Ruffo and the widespread Talitrus saltator Montagu. The effect of body mass, temperature and salinity on heart rate (used as proxy for metabolic activity and stress), the effect of temperature on oxygen consumption and the tolerance to salinity exposure were investigated in both species, together with the characterisation of haemolymph osmoregulation in T. ugolinii. Our results showed that there is a clear difference in the resting metabolic rates and physiological capacity, as well as environmental tolerance, between T. saltator and T. ugolinii, with T. saltator overall showing a broader physiological niche. Although T. ugolinii showed a relatively good ability to regulate its haemolymph osmotic concentration (similar to that previously described for T. saltator), it demonstrated a lower tolerance to exposure to hypo-osmotic stress. In addition, a consistent picture emerged between the ability to control the cardiac function and the capacity to actively respond to osmotic stress. The physiological findings are discussed in relation to the known ecology and geographical distribution of T. ugolinii.     

Adaptations to osmotic stress in the fresh-water euryhaline teleost Tilapia mossambica. IV. Changes in blood glucose,liver glycogen and muscle glycogen levels

Some aspects of osmoregulation energetics have been studied in the euryhaline teleost Tilapia mossambica (Peters) acclimated to media of different salinities. In stress media (75 and 100% sea water) the blood glucose of the fish increases significantly, accompanied by a corresponding increase in oxygen consumption and cytochrome-oxidase activity, suggesting that oxidative degradation of blood glucose is the predominant energy source for osmotic work in these stress media. It is likely that the variations in the blood-glucose level as a function of acclimation to the heterosmotic media —except the natural fresh-water medium — are governed by the combined effects of salinity of the medium and blood-medium osmotic gradient, rather than by the effect of any one of them separately. Perhaps, metabolic homoeostasis is in operation in the natural fresh-water medium. Depletion of muscle glycogen at significant levels is noticed only in stress media. Presumably, there is an augmentation of oxidative metabolism with glycolysis to meet the exacting energy demands for heavy osmotic work in high-stress media. Prior acclimation to 75% sea water (24.375 S) facilitates subsequent acclimation to 100% sea water (32.50 S) with less energy cost —an instance of facilitation acclimation. Smaller individuals of T. mossambica osmoregulate with less energy expenditure than larger ones. Thus, smaller individuals are osmotically more efficient.     

Some mechanisms of salinity acclimation in the euryhaline teleost,Etroplus maculatus

Mechanisms of salinity acclimation in the euryhaline freshwater living teleost, Etroplus maculatus (Bleeker), were studied. Brain water content decreased steadily with increasing salinity of the medium. Liver and muscle water content increased initially in 30% sea-water but in higher salinities decreased markedly, approximating control levels. Osmotic pressure of tissue fluid increased by 33% from 0.45% NaCl equivalent in freshwater to 0.60% NaCl equivalent in 100% sea-water. K and Mg decreased in dilute saline media but in 100% sea-water increased markedly approximating control levels; this is especially so in regard to Mg. Na decreased steadily and Ca increased with increasing salinity. Cl increased in 60% sea-water but decreased in 100% sea-water; however, its concentration in 100% sea-water exceeded that in freshwater. Free amíno acids increased by 11% in 30% sea-water but decreased in 60% and 100% sea-water, such that their concentration was lower by 28% in 60% sea-water and by 34% in 100% sea-water relative to the control level. Ascorbic acid decreased by 37% in 30% sea-water but in 60% and 100% sea-water increased steadily approximating the control level. The significance of these changes in the physiology of salinity acclimation is discussed and a correlation suggested between acclimation to salinity and to high temperature.     

Water balance systems of Rangia cuneata: ionic and amino acid regulation in changing salinities

In order to study the interaction of the extracellular and intracellular osmoregulatory systems of the bivalve Rangia cuneata, we have measured blood osmotic and ionic concentrations together with intracellular free amino acid concentrations and total tissue water under identical salinity conditions. Like freshwater bivalves, the blood of R. cuneata is maintained hyperosmotic (50 mOsm) to the environment in salinities below 110 mosm by the regulation of Na⁺, Cl^-, K⁺ and Ca²⁺ concentrations. On the other hand in company with marine bivalves, R. cuneata also regulates intracellular free amino acids (FAA) as a mechanism to control cellular volume during osmotic stress over the entire non-lethal salinity range (3 to 620 mOsm). Alanine is the predominant intracellular osmotic effector. Thus, by utilizing the osmoregulatory mechanisms of both marine and freshwater bivalves, R. cuneata is able to tolerate salinities ranging from freshwater to 25 ppt and to traverse the faunal salinity boundary, known as the horohalinicum (5 to 8 ppt), controlling cell volume throughout.Please address requests for reprints to Dr. S. K. Pierce     

Regulation of body dimethylsulfoniopropionate (DMSP) content by the copepod Temora longicornis: a test of four mechanisms

Copepods contain dimethylsulfoniopropionate (DMSP) in their tissues in addition to the DMSP in their gut contents and therefore constitute an additional pool of DMSP in the ocean. In the estuarine copepod Temora longicornis Müller, this dynamic pool of DMSP is reduced when external salinity decreases. In the present study the mechanism(s) used by T. longicornis to adjust its DMSP content were examined. Four possible mechanisms were tested in experiments conducted between July 1997 and May 1999: (1) DMSP cleavage, (2) demethylation, (3) conversion to methionine, and (4) release from the body. Tissue extracts of T. longicornis did not exhibit the ability to cleave or demethylate DMSP. In incubation experiments, when external salinity decreased, T. longicornis individuals reduced their DMSP content without increasing their methionine content. The total amount of DMSP in the incubation vials was conserved regardless of salinity. The copepods retained most of the DMSP in their tissues in 30‰ water, but when salinity was reduced to 20‰, the copepods released 41 to 55% of their body DMSP to the surrounding medium; 89 to 91% of the total DMSP was recovered. This suggests that estuarine copepods represent a reservoir of DMSP, which is released as dissolved DMSP upon salinity decline. Therefore, while osmoregulation by these copepods may have little impact on the chemical properties of DMSP, it will affect the partitioning of DMSP between particulate and dissolved phases in the water column. Received: 20 August 1999 / Accepted: 28 March 2000     

Effects of tidal fluctuations of salinity on hemolymph composition of the southern oyster drill Thais haemastoma

Effects of varying the amplitude and duration of tidal fluctuations of salinity upon the hemolymph osmotic and ionic composition of the southern oyster drill Thais haemastoma were studied. The composition of diluent was varied during one experiment to approximate Mississippi River water. Snails were also subjected to a diurnal 20-10-20%. S fluctuation pattern for two weeks and hemolymph was collected twice daily. Amplitude of hemolymph osmolality and ion fluctuation during tidal fluctuations of salinity was directly related to the amplitude of ambient salinity fluctuation and inversely related to the rate of fluctuation. The rate of hemolymph osmolality and ion change was directly related to the rate of ambient salinity change. Dilution of seawater with simulated river water instead of deionized water resulted in a reduced amplitude of fluctuation of hemolymph osmolality and ion concentration. Most of the hemolymph osmolality fluctuation was due to solute movement and not to shifts in body water. Hemolymph, sodium and chloride level changed in a similar manner throughout all of the experiments except the 10-5-10% S-simulated river-water experiment in which chloride changed much less than sodium. Hemolymph ninhydrinpositive substance (NPS) levels cycled inversely with ambient salinity during the 30-10-30%. and 20-10-20%. S diurnal and the 30-10-30%. S semidiurnal experiments, but did not change during the 10-5-10%. S deionized water or simulated river-water experiment. Snails fed for most of the 2-week 20-10-2%. S diurnal cycle fluctuation experiment and no mortality occurred. Drills were hyperosmotic to the ambient water at all but two sampling periods, when they were isosmotic. Hemolymph NPS levels tended to be higher during low-salinity slack water than during high-salinity slack water. Even small fluctuations of ambient salinity result in fluctuations of hemolymph osmolality and ionic composition which may affect rate functions within the zone of capacity adaptation of the southern oyster drill.     

Tolerance of Mediterranean seagrasses (<Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis>) to hypersaline stress: water relations and osmolyte concentrations

The present study examines for the first time the effects of increased salinity on water relations and osmolyte (carbohydrates and amino acids) concentrations in two Mediterranean seagrass species, Posidonia oceanica and Cymodocea nodosa, which are adapted to growth in environments with contrasting salinity and have a known differential sensitivity to alterations in ambient salinity. The specific aim was to obtain insights into their respective capacities to cope with natural or anthropogenically induced (e.g. desalination plants) hypersaline stress and its ecological implications. To this end, large plant fragments of both seagrass species were maintained for 47 days in a laboratory mesocosm system under ambient salinity (37 psu; control) and three chronic hypersaline conditions (39, 41 and 43 psu). Analyses of leaf-tissue osmolality indicated that both species followed a dehydration avoidance strategy, decreasing their leaf water potential (Ψ_w) as the external salinity increased, but using different physiological mechanisms: whereas P. oceanica leaves exhibited a reduction in osmotic potential (Ψ_π), C. nodosa leaves maintained osmotic stability through a decrease in turgor pressure (Ψ_p) probably mediated through cell-hardening processes. Accordingly, the concentrations of soluble sugars and some amino acids (mainly Pro and Gly) suggested the activation of osmoregulatory processes in P. oceanica leaves, but not in C. nodosa leaves. Osmotic adjustments probably interfered with leaf growth and shoot survival of P. oceanica under hypersaline stress, whereas C. nodosa showed a more efficient physiological capacity to maintain plant performance under the same experimental conditions. These results are consistent with the more euryhaline ecological behaviour of C. nodosa and contribute to understanding the high vulnerability shown by P. oceanica to even mild increments in seawater salinity.     

Salzgehalt und Photosynthese bei marinen Pflanzen

Marine plants — phanerogams as well as algae — show a proportional decrease of photosynthetic rate, when the salinity is lowered by dilution with distilled water. Using natural water instead of distilled water, it is absolutely indispensable to take into consideration the carbon supply. Normally, the photosynthetic rate decreases in freshwater with low alcalinity, in comparison to marine water. On the other hand, using water with high contents of bicarbonates, assimilation rate is higher than in marine water. These results show that salinity may indirectly affect photosynthesis due to differences in carbon supply. The direct influence is caused by exosmosis in hypotonic media and is irreversible. — The author cannot but agree with the statement by Ogata and Matsui (1965): “It may generally be said that the changes in salinity, osmotic pressure, pH, and also carbon dioxide supply, particularly in natural sea-water, are rather inseparably associated.”     

Implications of salinity fluctuation for growth and nitrogen metabolism of the marine diatom Ditylum brightwellii in comparison with Skeletonema costatum

In 1987 effects of salinity fluctuations on growth of Ditylum brightwellii (West) Grunow, isolated from the Eastern Scheldt estuary (SW Netherlands) in 1981, were studied. D. brightwellii was grown in a 12 h light: dark cycle at constant salinity in brackish media. Ammonium-limited cultures were subjected to a salinity fluctuation. By decreasing the salinity to 4.8 photosynthesis and cell division were inhibited; cells were deformed. Protein and carbohydrate contents increased slightly, dark respiration was stimulated and cellular levels of glucose decreased at low salinity; this indicated a possible role of sugars in osmoregulation. Ammonium was accumulated in cultures, amino acids may have been stored; the role of the vacuole as a storage compartment was discussed. Both the ammonium uptake capacity and the affinity for ammonium decreased. Nitrogen limitation was relieved in the transient state. [With the activity of the nitrogen assimilation enzymes glutamine synthetase (GS) and glutamate synthase (GOGAT) being uninhibited by lower salinity.] Recovery from hypo-osmotic stress during a salinity increase was initiated by stimulated photosynthesis; chlorophyll a increased, but persistant contractions of cytoplasm (with chloroplasts) may have delayed cell growth. The glutamate dehydrogenase (GDH) activity decreased further whereas the cellular level of alanine increased in the presence of large ammonium pools; this may indicate a temporary activity of ADH (alanine dehydrogenase). Skeletonema costatum (Greville) Cleve, recovered faster from hypoosmotic stress than did D. brightwellii. Due to an osmotic shock from 13.6 to 7.1 S both species excreted amino acids and glucose; S. costatum accumulated more glucose, D. brightwellii accumulated more amino acids. S. costatum may with the competition for nitrogen in waters with an unstable salinity; it will replace D. brightwellii.Contribution no. 427 Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Kinetics of osmoregulation in the crab Carcinus maenas

When shore crabs Carcinus maenas are transferred from 11 to 38 S at 11°C, new constant levels of hemolymph freezing points and of concentrations of Na, K, Ca, and Mg in the hemolymph are accomplished within 24h. From a decrease in serum protein and in serum free amino acids and an increase in the relative amounts of individual essential free amino acids in the body fluids of whole crabs, a participation of proteolytic activities and a transport of amino acids from hemolymph to cell is deduced; the stationary concentration of total intracellular free amino acids increases up to a nearly two-fold value, compared with the concentration in crabs remaining in diluted sea water. Also, the low molecular neutral sugars increase in whole crabs after high salinity stress, reaching values of more than two-fold initial concentrations within a period of 10 days. This increase is fully accounted for by a 6.7-fold increase in trehalose levels. The processes of increasing the concentrations of low molecular organic material seem to be slower than those of establishing new osmotic pressures in the hemolymph. The oxygen consumption decreases by 30 to 45% to new constant values within 8 to 12 h after the salinity change from 11 to 38, reflecting similar kinetics as the establishment of new osmotic pressures in the hemolymph.     

Effects of in situ tidal salinity fluctuations on osmotic and lonic composition of body fluid in Southeastern Alaska Rocky intertidal fauna

Tidal fluctuations in salinity and temperature were monitored at a location on Lynn Canal, north of Juneau, Alaska. Organisms were collected from 4 tidal levels during each slack water over a period of 24 h. Body-fluid milliosmolality as well as sodium, potassium, calcium, magnesium and chloride levels were determined. Mytilus edulis were collected at the +1.0 m tidal level, where ambient salinity ranged from 7.9 to 25.1% and body fluids from 356±62 to 730±17 mOsm/kg water. Cucumaria vegea and Katherina tunicata were collected at the +0.6 m level, where ambient salinity ranged from 13.5 to 24.9 and body fluids from 461±27 to 662±50 and 443±31 to 616±38 mOsm/kg water, respectively. Eupentacta quinquesemita and Evasterias troschelii were collected at the -0.9 m level, where ambient salinity ranged from 11.0 to 28.2 and body fluids from 504±32 to 632±51 and 316±31 to 664±37 mOsm/kg water, respectively. Strongylocentrotus droebachiensis were collected subtidally at the -3.7 m level, where ambient salinity ranged from 14.1 to 28.0 and perivisceral fluids from 448±35 to 661±51 mOsm/kg water. Ion levels of the perivisceral fluid of c. vegae were not determined. Potassium appeared to be regulated by all animals except Evasterias troschelii. Little regulation occurred for calcium, magnesium, sodium and chloride by the organisms. Eupentacta quinquesemita appeared to be regulating all ions determined, but this may have been due to its microhabitat. Water temperature cycled inversely to salinity at all tidal levels. Ambient-water sodium, chloride and calcium levels deviated from levels that would be expected upon dilution of seawater with deionized water. There was not indication that body-fluid osmoconcentration of the species collected intertidally increased on exposure to air.Communicated by J.S. Pearse, Santa Cruz     

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities. Received: 23 June 1998 / Accepted: 19 November 1998     

Physiological responses of horseshoe crab (<Emphasis Type="Italic">Limulus polyphemus</Emphasis>) embryos to osmotic stress and a possible role for stress proteins (HSPs)

We tested the effects of osmotic stress on survival, developmental rate, and level of HSPs on American horseshoe crab (Limulus polyphemus) embryos. Animals were maintained in the laboratory at an ambient salinity of 20 ppt and then exposed to 4-h osmotic shocks at salinities of 10, 30, 40, 50, and 60 ppt, with a control group at 20 ppt. Horseshoe crab embryos had 100% developmental success (defined as individuals reaching the first instar or trilobite larval stage) at all salinities. However, osmotic stresses, especially hyperosmotic conditions, slowed the rate of development. Embryos subjected to osmotic stress showed higher levels of HSP70 and HSP90 than control animals kept at a salinity of 20 ppt. HSPs are of value to horseshoe crab embryos in surviving the fluctuating salinities that are typical of estuarine beach habitats.     

Effects of tidal fluctuations of salinity on pericardial fluid composition of the American oyster Crassostrea virginica

Crassostrea virginica Gmelin were subjected to simulated tidal fluctuations of salinity, and the subsequent effects on osmotic and ionic composition of the pericardial fluid, body water and valve movements were investigated. Ambient salinity fluctuation patterns of 20-10-20, 15-10-15 and 10-5-10 were simulated during 24.8-h periods. An additional 10-5-10 S experiment was performed using a dilution water approximating the ionic composition of Mississippi River water with regard to Mg⁺⁺, Ca⁺⁺ and SO ₄ ⁼ , instead of deionized water. Finally the effects of a 2-week diurnal fluctuation pattern between 20 and 10 S were investigated with respect to pericardial fluid composition. Pericardial fluid osmolality, concentrations of Cl^-, Na⁺, Mg⁺⁺, K⁺, Ca⁺⁺ and ninhydrin-positive substances (NPS) were analyzed periodically throughout all experiments. Pericardial fluid osmolality was slightly hyperosmotic as ambient water salinity decreased during a cycle, and then became slightly hyposmotic as ambient salinity increased. In the 2-week experiment, pericardial fluid osmolality tracked ambient seawater closely through Day 5, but became more intermediate between high and low seawater values as the experiment progressed. Similar patterns during fluctuations of salinity were observed for Na⁺, Cl^-, Mg⁺⁺ and Ca⁺⁺. Pericardial fluid K⁺ levels did not track ambient seawater as closely as did other ions. The ionic composition of dilution water had little effect on the osmotic or ionic response of the oyster's pericardial fluid. Pericardial fluid NPS level varied inversely with salinity during the 20-10-20 cycle. During the longterm fluctuation experiment, NPS values gradually decreased over the 2-week period compared to constant salinity control values. Percent body water also varied inversely with ambient salinity. Solute movement accounted for most of the change in pericardial fluid osmolality during the simulated cycles with water movement responsible for 1 to 11%. Water movement contributed more to the change of pericardial fluid osmolality during the decreasing salinity phase than the increasing phase of a given cycle. During 20-10-20 S cycles, oyster valves remained open 56% of the time (n=23). In contrast, when salinity was abruptly changed from 20 to 10 within 5 min, valve closure occurred in 4.8±0.3 min (n=20). Valves did not reopen for 19.3±1.2 h (n=15).     

Vertical distribution of early and competent larvae of Concholepas concholepas in two systems of Chilean inland seas

The transport of marine invertebrate larvae is strongly influenced by their distribution in the water column, which could be affected by the biological features of the larvae and environmental variables. Larvae can modify their swimming behavior throughout their planktonic cycle, thereby changing the observed distributional patterns. This ability, coupled with oceanographic features, could induce landward or seaward transport. We studied the vertical distribution of C. concholepas larval stages in two differently stratified systems in Chilean inland seas; Refugio Channel (a strongly stratified channel, where previously has been described as a frontal system) and Guaitecas (a gently stratified system). Combinations of 12–24 h larval collection experiments were done simultaneously with fixed temperature and salinity profiles; meteorological data were also obtained. The results suggest that both salinity and day period influence the C. concholepas distributional patterns in the water column. Early veliger and competent larvae are concentrated in different parts of the water column, probably related to their transport capacities. The upper layer of the water column at the Refugio site, unlike the Guaitecas site, showed a stratified regime, which could affect larval density and larval length between the two sites. Finally, our results suggest that Refugio may be a sink habitat for C. concholepas.     

Naked Dictyocha speculum — a new type of phytoplankton bloom in the Western Baltic

Chlamydomonas pulsatilla Wollenweber, a euryhaline, marine flagellate (isolated from rockpool at St. Andrew's, New Brunswick, Canada in 1980 by J. A. Hellebust), shows decreasing rates of activity of its four contractile vacuoles in the salinity range of 0 to 15% artificial seawater (ASW). Electron microscopy shows that the contractile vacuole complex persists as a spongiome (collection of small vesicles or tubules) in cells grown at salinities above the range for operation of contractile vacuoles. From calculations of rates of water expulsion, based on size and frequency of contraction of individual vacuoles determined by light microscopy, the time necessary to empty one cell volume increased from ca 20 min at 1% ASW to ca 600 min at 15% ASW. Analysis of inorganic and organic solute contents of cells grown at 1 and 5% ASW allowed the calculation of internal osmotic pressures. Estimates of hydraulic conductivities based on rates of water expulsion via contractile vacuoles and differences in internal and external osmotic pressures resulted in values ranging from 1.1 to 1.4×10^-14 m s^-1 Pa^-1 for individual cells. Growth experiments at low photon flux densities over a salinity range of 1 to 15% ASW, over which contractile vacuole activity varied by a factor of 30, showed little difference in growth rates. This indicates that the cost for operation of contractile vacuoles must be very low. The secretion of large molecular weight organic substances does not appear to be associated with the functioning of contractile vacuoles.     

Osmoregulation of the grass shrimp Palaemonetes pugio exposed to polychlorinated biphenyls (PCBs). I. Effecton chloride and osmotic concentrations and chloride-and water-exchange kinetics

Grass shrimp, Palaemonetes pugio, were capable of hypo- and hyper-osmotic regulation of body fluids. Hemolymph chloride and osmotic concentrations were maintained at relatively stable levels over a wide salinity range. Following an abrupt transfer from intermediate (14 and 17) to high (31 and 35) or low (1 and 2) salinities, hemolymph chloride levels exhibited initial overshoot and undershoot, respectively, of new steady-state levels. Osmotic concentrations exhibited an initial undershoot at low, but not overshoot at high salinity. Chloride space in salinity-acclimated shrimp was relatively stable at salinities from 1 to 35. Changes in chloride space following salinity transfer paralleled those of hemolymph chloride levels, and are discussed in the light of alterations in intracellular sodium concentrations reported earlier. Rate constants for chloride turnover indicated independent exchanges of sodium and chloride ions. Water-turnover measurements showed that permeability of P. pugio was greatest at the isosmotic salinity (17) and reduced at salinities which were associated with active osmoregulation. Exposure to sublethal and 96-h LC₅₀ levels of Aroclor^® 1254 did not seriously alter hemolymph chloride and osmotic concentrations, chloride space or chloride-exchange kinetics in adult shrimp. Disruption of hemolymph chloride regulation in juvenile shrimp was associated with large mortalities not observed in adults. Shrimp exposed to Aroclor 1254 at 17 S exhibited reduced water permeability similar to levels previously observed in controls at high and low salinities in response to osmotic or ionic gradients. Exposure to PCBs did not result in further reduction in permeability at the latter salinities.