Haemolymph Osmotic Pressure Research Articles

Osmosis as nature’s method for establishing optical alignment

For eyes to maintain optimal focus, precise coordination is required between lens optics and retina position, a mechanism that in vertebrates is governed by genetics, visual feedback, and possibly intraocular pressure (IOP).1 While the underlying processes have been intensely studied in vertebrates, they remain elusive in arthropods,though visual feedback may be unimportant.2 How do arthropod eyes remain functional while undergoing substantial growth? Here, we test whether a common physiological process, osmoregulation,3 could regulate growth in the sophisticated camera-type eyes of the predatory larvae of Thermonectus marmoratus diving beetles. Upon molting, their eye tubes elongate in less than an hour, and osmotic pressure measurements reveal that this growth is preceded by a transient increase in hemolymph osmotic pressure. Histological evaluation of support cells that determine the lens-to-retina spacing reveals swelling rather than the addition of new cells. In addition, as expected, treating larvae with hyperosmotic media post-molt leads to far-sighted (hyperopic) eyes due to a failure of proper lengthening of the eye tube and results in impaired hunting success. This study suggests that osmoregulation could be of ubiquitous importance for properly focused eyes.

Managing fuels and fluids: Network integration of osmoregulatory and metabolic hormonal circuits in the polymodal control of homeostasis in insects.

Osmoregulation in insects is an essential process whereby changes in hemolymph osmotic pressure induce the release of diuretic or antidiuretic hormones to recruit individual osmoregulatory responses in a manner that optimizes overall homeostasis. However, the mechanisms by which different osmoregulatory circuits interact with other homeostatic networks to implement the correct homeostatic program remain largely unexplored. Surprisingly, recent advances in insect genetics have revealed several important metabolic functions are regulated by classic osmoregulatory pathways, suggesting that internal cues related to osmotic and metabolic perturbations are integrated by the same hormonal networks. Here, we review our current knowledge on the network mechanisms that underpin systemic osmoregulation and discuss the remarkable parallels between the hormonal networks that regulate body fluid balance and those involved in energy homeostasis to provide a framework for understanding the polymodal optimization of homeostasis in insects.

High dietary lipid level promotes low salinity adaptation in the marine euryhaline crab (Scylla paramamosain).

The physiological processes involved in adaptation to osmotic pressure in euryhaline crustaceans are highly energy demanding, but the effects of dietary lipids (fat) on low salinity adaptations have not been well evaluated. In the present study, a total of 120 mud crabs (Scylla paramamosain, BW=17.87±1.49g) were fed control and high-fat (HF) diets, at both medium salinity (23‰) and low salinity (4‰) for 6wk, and each treatment had 3 replicates with each replicate containing 10 crabs. The results indicated that a HF diet significantly mitigated the reduction in survival rate, percent weight gain and feed efficiency induced by low salinity (P<0.05). Low salinity lowered lipogenesis and activated lipolysis resulting in lipid depletion in the hepatopancreas of mud crabs (P<0.05). Thus, HF diets enhanced the process of lipolysis to supply more energy. In the gills, low salinity and the HF diet increased the levels of mitochondrial biogenesis markers, the activity of mitochondrial complexes, and the expression levels of genes related to energy metabolism (P<0.05). Consequently, the positive effects of the HF diet on energy metabolism in mud crabs at low salinity promoted osmotic pressure regulation. Specifically, significantly higher haemolymph osmotic pressure and inorganic ion content, as well as higher osmotic pressure regulatory enzyme activity in gills, and gene and protein expression levels of NaK-ATPase were observed in crabs fed the HF diet at low salinity (P<0.05). In summary, high dietary lipid levels improved energy provision to facilitate mitochondrial biogenesis, which increased ATP provision for osmotic pressure regulation of mud crabs. This study also illustrates the importance of dietary lipid nutrition supplementation for low salinity adaptations in mud crabs.

Physiological responses and adaptive strategies to acute low-salinity environmental stress of the euryhaline marine fish black seabream (Acanthopagrus schlegelii)

The physiological metabolism of marine animals farmed in coastal areas is influenced greatly by extreme weather and frequent aqueous layer exchange, with short or long periods of heavy rain reducing seawater salinity. Black seabream (Acanthopagrus schlegelii) is a euryhaline fish, that are farmed in China under both normal/ambient salinity in seawater cages and low salinity conditions in pond culture. To explore the effects of an acute change in salinity on physiological metabolism, juvenile black seabream (weight 19.80 ± 0.37 g) was subjected to low salinity (4–5 psu) over five time points including 1 h (H1), 6 h (H6), 12 h (H12), 24 h (H24) and one week (W), with 0 h (initial, natural salinity of 22 psu) as control group. Results indicated that acute exposure to low-salinity resulted in various responses related to oxidative stress (OS) and endoplasmic reticulum stress (ERS). The key markers of OS including reactive oxygen species and malonaldehyde were significantly increased with acute salinity stress up to 6 h, then decreased to return to normal levels. A similar pattern was recorded for total antioxidant capacity, which showed significantly higher activity in the H6 group. All parameters related to ERS increased with duration of the low-salinity stress, which stimulated expression of key genes including nuclear factor kappa B (nf-κb) and c-Jun N-terminal kinase (jnk), triggering inflammation and apoptosis. However, total apoptotic cell rate decreased markedly as the duration of salinity stress increased up to one week. Gill expression of genes of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis was up-regulated by low salinity stress with levels of docosahexaenoic (DHA), eicosapentaenoic (EPA) and arachidonic acids (ARA) being highest in the H12 group, which positively contributed to enhanced ability of osmoregulation. In conclusion, acute low-salinity stress stimulated various stress responses but, as the duration of salinity stress increased, the fish showed signs of physiological adaptation to the environmental change. During the adaptive process to maintain hemolymph osmotic pressure regulation, LC-PUFA were rapidly enriched in gill, which might be a crucial strategy allowing A. schlegelii to adapt to low-salinity stress. These findings provided further insight and understanding of the physiological responses and underlying adaptive strategy of euryhaline fish to salinity stress, and confirmed time-dependent effects.

A Comparative Study on Low and High Salinity Tolerance of Two Strains of Pinctada fucata

This study compares salinity tolerance between red and black shell Pinctada fucata salinity stress of 20 and 50‰, while 35‰ was used as a control. The hemolymph osmotic pressure, inorganic ion concentration, the activities of Na+-K+ -ATPase, respiratory metabolism related enzymes and liver tissue antioxidant related enzymes were measured at 12 and 24 h after salinity stress. The osmotic pressure and inorganic ion concentration of hemolymph of two strains P. fucata increased significantly with the increase of salinity. The activity of Na+-K+ -ATPase of red P. fucata only decreased under low salinity at 24 h, and was significantly higher than that the control under low salinity at 12 h and high salinity at 12 and 24 h. The succinate dehydrogenase (SDH) activities of the P. fucata treatment groups were significantly higher than those the control at 12 h. The lactate dehydrogenase (LDH) activity increased significantly with salinity at 12 h. and the black P. fucata LDH activity was significantly higher than the control at 24 h, while the LDH activity of red P. fucata was significantly lower than that the control in 50‰ salinity. The superoxide dismutase (SOD) activity of black P. fucata was significantly lower than that the control, while that of red P. fucata was significantly higher than that the control within 24. At 12 h, the catalase (CAT) activity of red P. fucata increased significantly with salinity, but decreased significantly with salinity at 24 h. The CAT activity of black P. fucata was highest at 24 h under low salinity. Glutathione peroxidase (GSH-Px) and alkaline phosphatase (AKP) activities of red P. fucata were significantly higher than those the control under low or high salinity. At high salinity for 24 h, the GSH-Px activity was lowest in black P. fucata, AKP activity was highest. The present study indicates that the physical responses of P. fucata to the salinity stress vary with shell colors. The red P. fucata can quickly respond positively to the change of environmental salinity and reduce the damage caused by the change of environmental salinity.

The effect of the insecticide diazinon on the osmoregulation and the avoidance response of the white leg shrimp (Penaeus vannamei) is salinity dependent

Diazinon is one of the insecticides that represent a high risk for Costa Rican estuarine environments due to its widespread use in pineapple plantations. In estuaries, organisms are frequently submitted to stress caused by natural factors (e.g., continuous changes in salinity levels) and, additionally, to stress due to contamination. Therefore, the driving question of this study was: will organisms be more susceptible to suffer the deleterious effects caused by diazinon because of the stress resulting from the salinity changes? The estuarine shrimp Penaeus vannamei was used as the model organism and two responses were measured: osmoregulation (the physiological effect after a forced and continuous 24 h-exposure) and avoidance [the behavioural effect after a short (3 h) non-forced, multi-compartmented exposure]. Juveniles were exposed to diazinon (0.1, 1, 10 and 100 μg/L) at three different salinities (10, 20 and 30). Disruption in the capacity to regulate the haemolymph osmotic pressure was observed at a salinity of 30 in individuals exposed to diazinon and methanol (used as vehicle). At that salinity, the ability of shrimps to detect and avoid the highest diazinon concentrations was impaired. P. vannamei juveniles inhabit environments with a high variation in salinity, but with an optimum osmotic point close to a salinity of 20; therefore, the higher the salinity, the greater the vulnerability of shrimps to the effects of diazinon. From an ecological point of view, this combined effect of salinity and contamination might also limit the spatial distribution of the organisms.

Increased salinity affects survival and osmotic response of rusty crayfish Orconectes rusticus Girard, 1852 and northern clearwater crayfish O. propinquus Girard, 1852 (Decapoda: Astacoidea: Cambaridae) as salinity increases: the potential for estuarine invasions

The potential colonization of the rusty crayfish, Orconectes rusticus (Girard, 1852), has previously focused on lakes and streams of northeastern United States and southeastern Canada, but estuaries have not been considered. Rusty crayfish have recently been reported in tributaries of the Chesapeake Bay and the potential exists for crayfishes to invade and spread into the estuary. We determined survival, hemolymph osmotic pressure, and identified possible mechanisms of osmoregulation of the invasive O. rusticus and northern clearwater crayfish, Orconectes propinquus (Girard, 1852), in salinities of 0.5, 5, 15, and 30 ppt. Survival of both crayfishes (>83%) was unaffected at salinities of up to 15 ppt but declined significantly at 30 ppt to 17% for O. rusticus ( P O. propinquus ( P > 0 . 05 ). Hemolymph osmotic pressure in both species was higher than the external medium up to 15 ppt, and then became nearly isosmotic with salinities at 30 ppt with a significant species by treatment interaction ( F 3 , 55 = 4 . 4627 , P = 0 . 0071 ). The primary osmoregulatory mechanisms in crayfishes involve ions (Na + and Cl − ) and free amino acids (FAAs) in muscle tissues. Concentrations of Na + and Cl − increased with increasing salinity in both species and accounted for 74-91% of the total hemolymph osmotic pressure with a significant species (Pillai’s Trace = 0.9565, P 0 . 05 ). FAA in tail muscle tissue increased significantly with increasing salinity with a significant species by treatment interaction ( F 3 , 20 = 5 . 1911 , P = 0 . 0082 ). Both O. rusticus and O. propinquus were active in salinities of up to 15 ppt and were able to osmoregulate. Based on these laboratory experiments, the potential exists for both of these species to spread through large areas of estuaries into new rivers potentially threatening previously isolated watersheds.

The combined effect of temperature and salinity changes on osmoregulation and haemocyanin concentration in Saduria entomon (Linnaeus, 1758)

ABSTRACTGlobal warming is having an impact on the temperature and salinity of Baltic Sea waters. Therefore, it is important to determine the conditions in which animals can exist and how these changes may influence their functioning. Hence, the purpose of this research was to determine the broad tolerance limits of temperature and salinity of the glacial relict Saduria entomon by studying its behaviour, osmoregulatory ability and haemocyanin concentration. This effect of temperature was confirmed in the laboratory for individuals acclimated to different salinity and temperature regimes. Changes in the physiological parameters of S. entomon at various temperatures (5.5–21.5°C) and salinity levels (1–15 PSU) were recorded. There were statistically significant differences in haemolymph osmotic pressure under the influence of salinity and temperature. The mean haemolymph osmotic pressures were the lowest at 1 PSU at all the temperatures examined and the highest at 15 PSU and high temperatures 16.5 and 21.5°C. The haemocyanin concentration decreased significantly with increasing temperature at 1 PSU. There was a significant difference in haemocyanin concentration due to salinity at temperatures of 5.5 and 10.0°C (the haemocyanin concentration decreased with increasing salinity). The results showed that, although S. entomon is classified as a cold-water animal, it can survive at high temperatures above 16.5°C at least for a short time, as it is capable of osmoregulation. The tolerance to temperature changes was better than expected.

RNA interference against gut osmoregulatory genes in phloem-feeding insects

In planta RNAi (i.e. plants engineered to synthesize active RNAi molecules) has great potential as a strategy to control insect crop pests. This study investigated the impact of RNAi against osmoregulatory genes expressed in the gut of two phloem-feeding species, the green peach aphid Myzus persicae and the potato/tomato psyllid Bactericera cockerelli. The target genes comprising candidate gut sucrase, aquaporin and sugar transporter genes were identified by mining insect genomic and transcriptomic datasets for genes orthologous to empirically-tested osmoregulatory genes of the pea aphid Acyrthosiphon pisum. Insects feeding on plants with RNAi against the target genes exhibited elevated hemolymph osmotic pressure (a predicted effect of perturbed osmotic function) and some reduction in performance, especially offspring production in M. persicae and mortality in B. cockerelli, associated with up to 50% reduction in mean expression of the target genes. The effects were particularly pronounced for insects treated with RNAi against multiple osmoregulatory genes, i.e. combinatorial RNAi, suggesting that the partial silencing of multiple genes with related roles can yield greater functional impairment than RNAi against a single gene. These results demonstrate the potential of RNAi against osmoregulatory genes, but further advances to improve the efficacy of RNAi in phloem-feeding insects are required to achieve effective pest control.

Combined effect of temperature and salinity on the Thermotolerance and osmotic pressure of juvenile white shrimp litopenaeus vannamei (Boone)

Thermotolerance (CTMax) was determined in L. vannamei in three salinities and five acclimation temperatures 20, 23, 26, 29 and 32°C. In white shrimp, the CTMax was not significantly affected by salinity (P>0.05). A direct relationship was obtained between CTMax and acclimation temperature. The end point of the CTMax in L. vannamei exposed to different combinations of temperature and salinity was defined as the loss of the righting response (LRR). The acclimation response ratio (ARR) for the juveniles of white shrimp ranged from 0.42 to 0.49; values in agreement with other crustaceans from tropical and sub tropical climates. The osmotic pressure of the hemolymph was measured in control organisms and in organisms exposed to CTMax; significant differences were found in organisms maintained in 10 and 40psu, but there were no significant differences in hemolymph osmotic pressure in those that were acclimated to 26psu.

Effects of serotonergic agents on survival and hemolymph composition of the larval mosquitoAedes aegypti(Diptera: Culicidae, L.)in vivo: does serotonin regulate hemolymph acid–base homeostasis?

The role of serotonin in the regulation of larval Aedes aegypti hemolymph composition was investigated in vivo using two reuptake inhibitors (SSRIs), alaproclate HCl and 6-nitroquipazine maleate, and the receptor antagonist methiothepin mesylate. Larvae were placed in media differing in pH and salinity in the presence and absence of serotonergic agents. The toxicity of each agent was strongly influenced by ambient pH. For each agent, toxicity was negligible in acidic media, intermediate in neutral media and greatest in alkaline media. By contrast, toxicity of all agents was independent of salinity. No effects on mass-specific body water or hemolymph volume were observed whereas hemolymph osmotic pressure, Na(+) concentrations and pH differed significantly among treatments. 6-nitroquipazine caused a decrease in Na(+) from 115+/-1.7 to 103+/-0.9 mmol l(-1), and alaproclate caused alkalosis of the hemolymph from pH 7.55+/-0.026 to pH 7.72+/-0.044. Methiothepin decreased hemolymph osmotic pressure from 329+/-9.9 to 304+/-8.8 and showed the greatest overall toxicity. Control larvae excreted net base in pH 4 media (1.4 micromol g(-1) h(-1)) and net acid in pH 7 (1.2 micromol g(-1) h(-1)) and pH 11 (5.1 micromol g(-1) h(-1)) media. In pH 4 media, alaproclate and methiothepin caused a shift to net H(+) excretion (1.1 and 1.5 micromol g(-1) h(1), respectively) whereas these agents did not influence acid excretion rates in pH 7 or pH 11 media. The hypothesis that serotonin is involved in hemolymph acid-base balance is discussed.

Effects of Salinity on Biogenic Amines, Hemolymph Osmotic Pressure, and Activity of Gill’s Na+/K+‐ATPase in Charybdis japonica (Crustacea, Decapoda)

Abstract The effects of salinity on hemolymph osmotic pressure, gill Na+/K+‐ATPase activity and dopamine (DA), norepinephrine (NE), serotonin (5‐HT) in the gills, and hemolymph of the adult Charybdis japonica were studied. DA levels increased significantly (P &lt; 0.05), while the NE and 5‐HT revealed contrary change in hemolymph and gills. The iso‐osmotic point of C. japonica (911.4 mOsm/kg) was at salinity of 27.87 ppt. The Na+/K+‐ATPase activity of gill showed negative correlation with salinity in the hypotonic environment (&lt;27.87 ppt). The results of this experiment indicated that C. japonica had great capability to acclimate to low salinity.

Injection of biogenic amines modulates osmoregulation of Litopenaeus vannamei: Response of hemolymph osmotic pressure, ion concentration and osmolality effectors

In this paper, we compared systematically the temporal and dose response relationship and physiological significance among biogenic amines injection, changes of ion concentration, FAA concentrations and composition and protein in context of osmoregulatory ability in marine euryhaline shrimp: Litopenaeus vannamei. The dopamine (DA) and 5-hydroxytryptamine (5-HT) injection all had transient effects on hemolymph osmolality, ion concentrations but which occurred in different time and were dose-dependent. The highest concentrations of FAAs in hemolymph of L. vannamei were alanine, glycine, argnine, proline, lysine which were considered to be specific osmotic effectors. Contrary to the reduction of hemocyanin, injection of DA 10 − 6 mol shrimp − 1 and 5-HT 10 − 6 mol shrimp − 1 induced notable protein increase respectively, which led to the rapid reduction of hemocyanin/protein ratio in range of 63.2% to 78.3%. The increase of hemolymph FAAs might come from the new amino acid synthesis or degradation of muscle protein to FAAs or denovo synthesis of FAAs. Our study showed that dopamine plays an important role in neurotransmission and causes osmoregulation response modulation and 5-HT has different activation mechanism on osmoregulation.

Life on the edge: physiological problems in penaeid prawns Litopenaeus stylirostris, living on the low side of their thermopreferendum

The underlying physiological mechanisms explaining why the adult penaeid prawn Litopenaeus stylirostris cannot successfully face heavy stressful events on the low edge of its thermopreferendum (20–22°C) were studied during the austral winter. Prawns were studied during recovery from net fishing and rapid transfer from outdoor earthen ponds into indoor facilities. This was assimilated to a predator–prey interaction. O2-consumption, hemolymph osmotic pressure (OPh), arterial O2 partial pressure (PO2), a–v O2-capacitance and mortality rates were analysed. Data were compared to similar challenges performed at 28°C during the austral summer. At 20–22°C, mortality of up to 70% was observed after 2 days whereas at 28°C, maximum mortality was 3–5%. Mortality occurred when OPh shifted towards equilibrium with seawater, the resting O2-consumption, the a–v O2-capacitance and the arterial PO2 went down to minimal values. These events can be counterbalanced by transiently hyper-oxygenating the hemolymph or by blocking the OPh shift in isosmotic water (Wabete et al. in Aquaculture 260:181–193, 2006): both led to a dramatic decrease in mortality. It is concluded that in penaeid prawns L. stylirostris, a mismatch between O2-demand and O2-supply contributes to setting the geographical limits for this animal species through an impairment of their hemolymph O2-carrying capacity during heavy stressful events like chasing by predators.

Effect of salinity on hemolymph osmotic pressure, sodium concentration and Na+-K+-ATPase activity of gill of Chinese crab, Eriocheir sinensis

The effects of salinity on hemolymph osmotic pressure, Na+ concentration and Na+-K+-ATPase activity of gill of Chinese crab Eriocheir sinensis were studied. The results showed that hemolymph osmotic pressure and Na+ concentration increased significantly (P 0.05); However, the protein concentration decreased gradually with the increase of salinity from 0.25 d to 1 d, and then tended to be stable from day 1 to day 15.

Sweet problems: insect traits defining the limits to dietary sugar utilisation by the pea aphid,Acyrthosiphon pisum

Plant phloem sap is an extreme diet for animals, partly because of its high and variable sugar content. The physiological and feeding traits of the pea aphid Acyrthosiphon pisum that define the upper and lower limits to the range of dietary sucrose concentrations utilised by this insect were determined principally using chemically defined diets containing 0.125-1.5 mol l(-1) sucrose. On the diets with 0.125 mol l(-1) and 1.5 mol l(-1) sucrose, the aphids died as larvae within 8 and 14 days of birth, respectively. On the other diets, 60-96% of aphids developed to adulthood, and the 0.5 mol l(-1) and 0.75 mol l(-1) diets supported the highest fecundity. The diet with 0.125 mol l(-1) sucrose was ingested at 36% of the rate of the 0.25 mol l(-1) sucrose diet, but >90% of ingested sucrose-carbon was assimilated on both diets. This suggests that the lower limit is dictated by the aphid feeding response, specifically, a requirement for a minimal concentration of sucrose for sustained feeding. The haemolymph osmotic pressure of aphids on diets with 0.125-1.5 mol l(-1) sucrose was up to 68% higher than on 0.125-1.0 mol l(-1) sucrose diets, but diet consumption and sucrose-carbon assimilation was not reduced on the very high sucrose diets relative to 1.0 mol l(-1) sucrose. This suggests that failure of the osmoregulatory capacity of the insects on high sucrose diets may define the upper limit to the range of dietary sucrose utilised by the aphids. The mean haemolymph osmotic pressure of aphids on plants with phloem sap containing 0.37-0.97 mol l(-1) sucrose was 1.61+/-0.063 MPa (mean +/- s.e.m.), not significantly different from that (1.47+/-0.059 MPa) on diets with 0.25-1.0 mol l(-1) sucrose. It is concluded that the osmoregulatory response of aphids to diets and plants are comparable, and, more generally, that the feeding and osmoregulatory capabilities of the aphids are compatible with the phloem sugar levels commonly encountered by aphids feeding on plants.

Haemolymph amino acid, sugar and glycerol levels in the Namib Desert tenebrionid Physadesmia globosa (Coleoptera: Tenebrionidae) during dehydration and rehydration

Haemolymph levels of amino acids, sugars and glycerol were investigated in the tenebrionid Physadesmia globosa during dehydration and rehydration. The absolute amount of amino acid decreases during dehydration and increases during rehydration, indicating active regulation of this solute (the osmolal contribution of amino acids is large - approx. 25%). Changes in the amino acid content of the haemolymph during dehydration are not the result of interchange with soluble protein; the possibility exists during rehydration (between 1 h and 48 h). Trehalose and glucose are the only sugars found in appreciable quantity in the haemolymph of this species. Their osmolal contributions (total sugar: 2.6%), and contributions to osmoregulation, are not great. Glycerol is a minor osmolar effector in the haemolymph of Physadesmia, and changes in its levels do not contribute importantly to the regulation of haemolymph osmotic pressure.

The significance of gut sucrase activity for osmoregulation in the pea aphid, Acyrthosiphon pisum

The osmotic pressure of the body fluids of aphids is lower than in their diet of plant phloem sap. It is hypothesised that aphids reduce the osmotic pressure of ingested food by sucrase-mediated hydrolysis of dietary sucrose to glucose and fructose, and the polymerisation of glucose into oligosaccharides of low osmotic pressure per hexose unit. To test this hypothesis, the impact of the α-glucosidase inhibitor acarbose on the sugar relations and osmoregulation of aphids was explored. Acarbose inhibited sucrase activity in gut homogenates and the production of monosaccharides and oligosaccharides in the honeydew of live aphids. Acarbose caused an increase in the haemolymph osmotic pressure for aphids reared on a diet (containing 0.75 M sucrose) hyperosmotic to the haemolymph and not on the isoosmotic diet containing 0.2 M sucrose. It did not affect aphid feeding rate over 2 days, except at high concentrations on 0.75 M sucrose diet, and this may have been a secondary consequence of osmotic dysfunction. Acarbose-treated aphids died prematurely. With 5 μM dietary acarbose, mean survivorship on 0.2 M sucrose diet was 4.2 days, not significantly different from starved aphids, indicating that, although these aphids fed, they were deprived of utilisable carbon; and on 0.75 M sucrose diet, mean survivorship was just 2.8 days, probably as a consequence of osmotic failure. It is concluded that the aphid gut sucrase activity is essential for osmoregulation of aphids ingesting food hyperosmotic to their body fluids.

Effect of low salinity on cadmium accumulation and calcium homeostasis in the shore crab (Carcinus maenas) at fixed free Cd2+ concentrations.

Increased Cd toxicity at low salinity has been attributed to increased free Cd2+ ion concentration ([Cd2+]sw), but transfer to dilute seawater also stimulates physiological ionic regulation in crabs. In this study, Cd accumulation and Ca homeostasis in the shore crab (Carcinus maenas) were explored at fixed [Cd2+]sw to reveal the physiological events during sublethal Cd exposure. Crabs were exposed to 3.4 or 34 microg/L [Cd2+], in both 100% seawater (SW) and 33% SW for up to 10 d and sampled for hemolymph composition as well as gill and hepatopancreas Ca, Cd, and Ca-ATPase activity. Cadmium exposure ameliorated the expected fall in hemolymph osmotic pressure and NaCl at low salinity and generally protected tissue Ca from decline. Cadmium exposure alone (within salinity) inhibited Ca-ATPase, but this was offset by stimulation of Ca-ATPase at low salinity. The Ca-ATPase activity in the anterior and posterior gills showed different responses to Cd/low salinity stress. Crabs were more sensitive to a 10-fold increase in [Cd2+]sw at low salinity. Overall, we conclude that exposure to a fixed sublethal [Cd2+]sw reveals a compensatory physiological response that is driven primarily by salinity rather than Cd2+ free ion concentration. Physiological responses are therefore important during low-level Cd exposure in dilute seawater.

Osmoregulation in Onymacris rugatipennis, a free-ranging tenebrionid beetle from the Namib Desert

Haemolymph levels of organic and inorganic constituents were investigated in the tenebrionid Onymacris rugatipennis during dehydration and rehydration. The major osmolar effectors are sodium (26%), chloride (24%), amino acids (18%), and sugars (11%); regulation of haemolymph osmotic pressure (OP) during dehydration is effected largely by a reduction in the haemolymph content of these constituents. Changes in amino acid levels are not the result of interchanges with soluble protein. During rehydration, the main contributors to osmoregulation are sodium (26%), chloride (24%), and an increase in haemolymph solute(s) not measured in this study (31%). Of the sodium removed from the haemolymph during dehydration, 21.2% was excreted. Faecal losses of potassium during dehydration far exceeded the amounts removed from the haemolymph; however, haemolymph potassium levels were strongly regulated during rehydration. Regulatory efficiency increases as desiccation proceeds, and is greatest only when this species is severely challenged.