Osmoregulatory Costs Research Articles

Effects of salinity acclimation on oxygen consumption in the freshwater blenny, Salaria fluviatilis, and the marine peacock blenny, S. pavo

The effect of salinity on standard metabolic rate was studied in a freshwater blenny, Salaria fluviatilis, and a marine congener, S. pavo. These blennies tolerate both fresh water (FW, <30 mOsm L −1 ) and full-strength sea water (SW, 1000 mOsm L −1 ). Fish of both species were acclimated to FW, iso- osmotic concentration (IOC, 375 mOsm L −1 ) and SW at 21 2±2°C for three months before oxygen consumption rates were measured. Slopes of the regressions relating oxygen consumption rates to body mass did not differ significantly between species and among salinity treatments. Oxygen consumption rates of S. fluviatilis in FW were significantly higher than in IOC and SW, whereas oxygen consumption rates of S. pavo were similar in IOC and SW salinity but were significantly lower in fish in FW. These results do not seem to be accounted for by different costs of osmoregulation or by different concentrations of oxygen in the water among different salinities, and they may demonstrate reduction in metabolic activities as a reaction to chronically unfavourable, sub-optimal environmental conditions.

Salinity effects on oxygen consumption, gill Na+, K+‐ATPase and ion regulation in juvenile coho salmon

The metabolic response of juvenile coho salmon Oncorhynchus kisutch to different salinities was examined, using whole‐animal oxygen consumption rates and gill Na+, K+‐ATPase activities as indicators of osmoregulatory energetics. Coho salmon smolts were acclimated to fresh water (FW), isosmotic salinity (ISO, 10‰) and sea water (SW, 28‰) and were sampled for up to 6 weeks for plasma levels of cortisol, glucose and ions (Na+, K+, Cl−), gill Na+, K+‐ATPase activity and oxygen consumption rates. Following an initial adjustment period, plasma constituents in SW fish returned to near‐FW values, indicating that the fish were acclimated to SW by day 21. Gill Na+, K+‐ATPase activities on days 21 and 42 were lowest in ISO, higher in FW and highest in SW. This result is consistent with the idea that less energy would be required to maintain ion balance in an isosmotic environment, where the ionic gradients between extracellular fluid and water would be minimal. Oxygen consumption rates of swimming fish (1 body length s−1), however, did not differ significantly between the three test salinities after 6 weeks. The results of this study suggest that the metabolic response of juvenile salmonids to changes in salinity is dependent on life‐history stage (e.g. fry v. smolt), and that oxygen consumption rates do not necessarily reflect osmoregulatory costs.

Elevated levels of trimethylamine oxide in muscles of deep-sea gadiform teleosts: A high-pressure adaptation?

Trimethylamine oxide (TMAO) as an osmolyte typically occurs at 20–70 mM in shallow-water marine teleost fishes. However, it has not been previously examined in deep-living species. We collected species from two families of benthic gadiform teleosts by otter trawl from the continental slope (1,800–2,000 m) and abyssal plain (2,850 m) off the Oregon coast. Muscle and plasma samples were analyzed for TMAO with a picric acid method. Muscle contents (below in millimoles/kilogram of wet weight) were found to be higher than previously reported for teleosts. Results from Macrouridae from 1,800–2,200 m were as follows: Albatrossia pectoralis, 83 ± 10; Coryphaenoides cinerus, 121 ± 11. Results from Macrouridae from 2,850 m were significantly higher: C. leptolepis, 158 ± 20; C. fillifer, 177 ± 8; C. armatus, 173 ± 5. Caught at both depths, Antimora microlepis (Moridae) had 211 ± 14. Plasmas had low TMAO (2–15 mM) and high Na+concentrations (227–273 mM), except A. microlepis plasma, which had the lowest Na+ (219 ± 30 mM) and highest TMAO (159 ± 46 mM). Osmotic pressures of fresh plasma (423–557 mosm) correlated highly with muscle TMAO and with plasma TMAO plus Na+ levels. These higher osmolalities may reduce osmoregulatory costs. However, as a methylamine known to stabilize protein, TMAO may counteract destabilizing effects of hydrostatic pressure on cellular proteins. With purified C. leptolepis muscle lactate dehydrogenase, 250 mM TMAO fully offset a 30% increase in NADH Km induced by 300 atm. J. Exp. Zool. 279:386–391, 1997. © 1997 Wiley-Liss, Inc.

Early development of milkfish: effects of salinity on embryonic and larval metabolism, yolk absorption and growth

During embryogenesis of Chanos chanos, more than half of the yolk was consumed and the majority of it was converted into larval tissue. Salinity affected both yolk absorption and embryonic and larval growth. Larvae hatched in 20% had larger yolk reserves but were smaller and grew more slowly than larvae in 35 and 50%. Larvae hatched in 35 and 50% had equal amounts of yolk but those from 35% were larger. Oxygen consumption rates increased during development (from 0.06 ± 0.01 μl O2 egg–1 h–1 by blastulae to 0.37 ± 0‐01 μl O2 egg–1 h–1 by prehatch embryos and 0–43 ± 0–03 μl O2 larva –1 h –1 by newly‐hatched larvae) and were significantly affected by salinity. Eggs and yolk‐sac larvae incubated in 35% consumed more oxygen than those in the low and high salinities. Salinity affected both the rate and pattern of yolk utilization but salinity‐related differences in metabolism, yolk absorption, and growth were not related directly to the osmotic gradient. Low salinity retarded yolk absorption while high salinity reduced yolk utilization efficiencies. Differences in oxygen consumption rates were probably related to variations in the relative amounts of metabolically active embryonic and larval tissue and/or higher activity levels rather than differential osmoregulatory costs. 35% is probably the most suitable salinity for incubation and larval rearing of milkfish.

Salinity effects on macroinvertebrate assemblages and waterbird food webs in shallow lakes of the Wyoming High Plains

We determined the biomass and community structure of macroinvertebrates (>500 µm) associated with macrophytes, sediments, and unvegetated open water in three oligosaline (0.8 to 8.0 mS cm−1) and three mesosaline (8.0 to 30.0 mS cm−1) lakes in the Wyoming High Plains, USA. Total biomass of epiphytic and benthic invertebrates did not change with salinity, but biomass of macroinvertebrate zooplankton in open water was significantly higher in mesosaline lakes. Community composition of invertebrates differed between the two salinity categories: large grazer/detritivores (gastropods and amphipods) were dominant in oligosaline lakes, whereas small planktivores and their insect predators were more prevalent in mesosaline lakes. Both direct physiological effects of salinity, as well as a shift in the form of primary production from macrophytes to phytoplankton, probably explain these changes in community composition. Salinity effects on invertebrate communities appear to be less important to top avian consumers than are costs of osmoregulation.

The standard metabolic rate of dolphin fish

The standard metabolic rates (SMRs) of 11 (1.395–4.125 kg) dolphin fish (mahimahi or dorado, Coryphaena hippurus) were measured at 25°± 0.5°C. Fish were prevented from swimming with neuromuscular blocking agents and force ventilated. Heart rates were determined simultaneously. SMRs (358–726 mg O2 h –1) were several times those of other similarly sized active teleosts such as salmonids, but close to those of tunas. Heart rates (84–161 beats min –1) were also high, but alike those of tunas under similar circumstances. As in tunas, the high SMR of dolphin fish may result from high osmoregulatory costs engendered by their large gill surface areas and/or other adaptations necessary for achieving exceptionally high maximum metabolic rates.

A review of temperature and oxygen tolerance studies of tunas pertinent to fisheries oceanography, movement models and stock assessments

ABSTRACTDetailed data on the physiological abilities and habitat requirements of some tuna species have now been collected. Laboratory and field studies show tunas are not strictly prisoners of their own thermoconserving mechanisms, but have rapid and extensive control over the efficacy of their vascular countercurrent heat exchangers. Skipjack tuna (Katsuwonus pelamis) are, therefore, probably not forced out of formerly suitable habitats as they grow because of the potential for overheating, as formerly thought. Because of their thermoregulatory abilities, bigeye tuna (Thunnus obe‐sus) apparently can exploit food resources well below the thermocline by minimizing rates of heat loss when in cold water, and then maximizing rates of heat gain from the environment during brief upward excursions into the warmer mixed layer.Widely cited estimates of limiting oxygen levels, based on estimated metabolic rates at minimum hydrostatic equilibrium swimming speeds, are not accurate because tunas have exceptionally high oxygen demands even at slow speeds. High metabolic rates, even at slow swimming speeds, most likely result from the high osmoregulatory costs engendered by tunas' large thin gills and/or their other adaptations for achieving exceptionally high maximum metabolic rates. Recent laboratory research and modelling efforts suggest the capacity of tunas' cardiorespiratory systems to deliver oxygen at extraordinarily high rates was evolved to allow rapid recovery from strenuous exercise, rapid digestion, and high rates of gonadal and somatic growth, not high sustained cruising speeds. The rate at which reduced ambient oxygen levels prolong the time required for tunas to recover from strenuous exercise appears to be a good index of habitat suitability, with respect to oxy‐een.

Na+-K+-adenosine triphosphatase activities in gills of marine teleost fishes: Changes with depth, size and locomotory activity level

Activities of the primary enzyme responsible for monovalent ion regulation, Na+-K+-adenosine triphosphatase (Na+-K+-ATPase), were measured in gills of marine teleost fishes with different depths of occurrence (0 to 4800 m), body weights (a range of five orders of magnitude), and locomotory capacities. Specimens were collected off the coasts of California and Oregon in 1983–1989, and at the Galapagos Spreading Center and 13°N East Pacific Rise hydrothermal vent sites in 1987 and 1988, respectively. Except for two hydrothermal vent fishes, deep-sea species had much lower Na+-K+-ATPase activities g−1 gill filament than shallow-living species, indicating that osmoregulatory costs, like total metabolic rate, are greatly reduced in most deep-living fishes. Within a species, the total branchial Na+-K+-ATPase activity per individual was dependent on size; the average allometric scaling exponent was 0.83. Using published values for oxygen consumption rates, and the total branchial Na+-K+-ATPase activities as an index of osmoregulatory costs, we estimated the maximal cost (as percent of ATP turnover) for osmoregulation in ten teleosts. Osmoregulatory costs averaged about 10% of total ATP turnover among these species, and maximal costs were no greater than about 20%. The percent costs of osmoregulation did not differ between shallow- and deep-living fishes. The reduced total ATP expenditure for osmoregulation in deep-living fishes is proposed to result from the sluggish locomotory habits of these fishes, not from selection for reduced osmotic coastper se. Thus, the reduced swimming abilities of these fishes lead to lower rates of water flow over the gills and less blood flow through the gills due to reduced demands for oxygen. Consequently, passive flux of water and ions through the gills is much lower than in more active fishes, and osmotic costs are thereby minimized. The relatively high activities of Na+-K+-ATPase in gills of the two hydrothermal vent fishes suggest that these fishes may be more active and have higher metabolic rates than other deep-sea fishes.

Influence of temperature, salinity, and phenanthrene (a petroleum derived polycyclic aromatic hydrocarbon) on the respiration of larval mud crabs, Rhithropanopeus harrisii

Zoeal stages and the megalops stage of the mud crab, Rhithropanopeus harrisii were exposed to factorial combinations of salinity, temperature and chronic, sublethal concentrations of phenanthrene. Temperatures and salinities were, respectively, 20°, 25°, or 30°C and 5 15 25 S. Phenanthrene concentrations were o (66 l acetone l −1 seawater), 37·5, 75 or 150 parts/10 9 (gl −1 ). Respiration rates were determined for second stage zoeae, fourth stage zoeae and for the megalops raised in the different factor combinations. In general, respiration rates of temperature acclimated animals were relatively temperature-independent, with Q 10 's between 1 and 2. All stages showed an increase in respiration rate at low salinities. This increase occurred for all stages tested, but was greatest in the earlier stages. When animals were given acute osmotic shock by a move from 15to 25or 5salinity, the most consistent response was an apparent increase in temperature sensitivity of respiration rate. This was particularly marked during hypo-osmotic shock at low temperatures. Exposure to high sublethal concentrations of phenanthrene usually resulted in an increase in respiration rates over control rates. However, at low phenanthrene concentrations, the respiratory rate changes were not always dose dependent. Both the direction and magnitude of change from control rates appeared to be a function of age, prevailing physical factor regime, and level of hydrocarbon exposure. The interactive effects of temperature, salinity, and phenanthrene on growth were more consistent. Mean dry weight of the megalopae ranged from less than 100 g to about 250 g. The weight of controls increased with salinity. Animals exposed to phenanthrene consistently weighed less than controls and low-salinity, high phenanthrene combinations produced the smallest megalopae. The authors hypothesize that at low salinities, decreased growth is caused by respiratory loss of carbon, and that high phenanthrene concentrations exacerbate this effect possibly by increasing the metabolic cost of osmo-regulation or increasing the rate of membrane turnover.