Ion transport in the gill epithelium of aquatic crustacea

A novel modulation of physiological regulation in cultured Chinese mitten crab (Eriocheir japonica sinensis) in response to consistent salinity changes

The life history of the Chinese mitten crab (Eriocheir japonica sinensis) includes two migrations: a feeding migration and a reproductive migration. Ambient salinity is one of the most critical factors during migration. In this study, the salinity adaptation mechanism of Chinese mitten crabs was simulated using continuous salinity changes. The expression of six key genes [Na⁺/K⁺-ATPase α subunit (NAK-α), V-type H⁺-ATPase subunit A (VHA-A), Zinc transporter (ZnT), Cl^- channel protein 2 (CLCN2), ubiquitin/ribosomal S27 fusionprotein (S27), and glutathione S-transferase (GST)] and the activities of three enzymes [Na⁺/K⁺-ATPase (NAK), V-type H⁺-ATPase (VHA), and glutathione S-transferase (GST)] were evaluated in ten groups exposed to a range of salinity changes during mariculture based on the transcriptome data obtained from short term salinity-induced crabs (ES) compared to control group in freshwater crabs (EF). The results revealed that different genes exhibited different roles in physiological regulation. In total, 3,599 unigenes were significantly and differentially expressed in a comparison between the EF and ES treatments. A novel modulation of gene expression and the corresponding enzyme activity of NAK and VHA exhibited similar patterns. As genes related to osmoregulation, NAK and VHA showed similar patterns of both gene expression and enzyme activity in mariculture. During the gradual change in salinity from 0‰ to 25‰ and back to 0‰, the gene expression and enzyme activities of NAK and VHA initially increased (0‰ → 10‰), weakened (10‰ → 20‰) and then increased again (20‰ → 25‰ → 0‰). S27 could serve as a reference gene in the expression analysis of Chinese mitten crabs under salinity stress. ZnT and CLCN2 were involved in osmoregulation as functional proteins. Our findings provide insights into the regulation mechanisms employed during the migration of the Chinese mitten crab.

Differential effects of water loss and temperature increase on the physiology of fiddler crabs from distinct habitats

Temperature is one of the main environmental constraints to organism distribution, affecting physiology and survival. Organisms that inhabit the intertidal zone are exposed to temperature variation and, with climate change, they should face different conditions which include higher temperatures, leading to higher rates of water loss through evaporation and then fitness reduction or mortality. Here we tested the effects of desiccation and increased temperature in two fiddler crabs species that occupy distinct habitats in regard to vegetation cover and position on the intertidal zone and thus may respond differently to these stressors. Leptuca thayeri, which is restricted to the mid-tide zone and vegetated areas, had higher desiccation and mortality rates than Minuca rapax, a generalist species, when exposed to desiccation for 120 min. Also, compared to M. rapax, L. thayeri had a more permeable carapace. Temperature elevation of 10 °C and 20 °C for 72 h caused no mortality in either species. However, there were changes in hemolymph osmolality and muscle hydration in both species. Leptuca thayeri osmolality was low in the intermediate temperature, suggesting that at this temperature this species has a better salt secretion capability. Minuca rapax, however, had an increase in hemolymph osmolality at the highest temperatures with no LDH increase, which indicates that osmotic control in this species is more sensitive to temperature increase. Our results show that L. thayeri suffers more from desiccation, due to a more permeable carapace. However, because of this higher permeability L. thayeri is capable of lowering its temperature more than M. rapax. As temperature elevation produces great physiological changes in M. rapax, a reduced ability to keep a low temperature can be an issue for this species if temperature increases. However, higher water loss to keep body temperature low may decrease L. thayeri survivability in the same scenario.

Induction of branchial ion transporter mRNA expression during acclimation to salinity change in the euryhaline crab Chasmagnathus granulatus

Using quantitative real-time PCR, the expression of mRNAs encoding three transport-related proteins and one putative housekeeping protein was analyzed in anterior and posterior gills of the euryhaline crab Chasmagnathus granulatus following transfer from isosmotic conditions (30 per thousand salinity) to either dilute (2 per thousand) or concentrated (45 per thousand) seawater. Modest changes were observed in the abundance of mRNAs encoding the housekeeping protein arginine kinase and the vacuolar-type H(+)-ATPase B-subunit, both of which were highly expressed under all conditions. By contrast, the expression of Na(+)/K(+)-ATPase alpha-subunit mRNA and Na(+)/K(+)/2Cl(-) cotransporter mRNA was strongly responsive to external salinity. During acclimation to dilute seawater, cotransporter mRNA increased 10-20-fold in posterior gills within the first 24 h while Na(+)/K(+)-ATPase alpha-subunit mRNA increased 35-55-fold. During acclimation to concentrated seawater, cotransporter mRNA increased 60-fold by 96 h and Na(+)/K(+)-ATPase alpha-subunit increased approximately 25-fold in posterior gills. Our results indicate a complex pattern of transcriptional regulation dependent upon the direction of salinity change and the developmental background of the gills.

Adaptation to hypoosmotic challenge in brachyuran crabs: a microanatomical and electrophysiological characterization of the intestinal epithelia

Besides its role in digestion and nutrient absorption, the crustacean gut participates in osmo/ionic regulation. We investigate microanatomy, ionic permeability and transepithelial electrophysiological parameters in the mid- and hindguts of three hyperosmoregulating crabs that inhabit estuarine waters (Chasmagnathus granulata), brackish mangrove swamp (Sesarma rectum) or freshwater (Dilocarcinus pagei). The abdominal hindguts are cuticle lined, the single-layered epithelia consisting of narrow, columnar cells exhibiting apically dense, unvesiculated cytoplasm. In the saltwater species, the thoracic midgut epithelium consists of tall, narrow, columnar cells displaying numerous, apical microvilli above dense apical cytoplasm. However, the corresponding gut segment in the hololimnetic species, D. pagei, consists of squat cells lacking apical microvilli, overlain by a heavy cuticle, constituting a thoracic or anterior hindgut. The midgut/thoracic hindgut epithelia in all three crabs, and abdominal (posterior) hindgut of D. pagei, exhibit similar, small, lumen-negative voltages when perfused symmetrically with hemolymph-like salines. The hindguts of the saltwater species show similar, small, lumen-positive voltages. Small short-circuit currents are detectable after voltage clamping. Washout and/or addition of luminal glucose or amino acids do not alter current or conductance, suggesting the absence of active, electrogenic nutrient absorption. Ion substitution did not disclose active, electrogenic absorption or secretion of Na+ and/or Cl-. The midguts of the saltwater species exhibit similar conductances, greater than in D. pagei, but no ion selectivity; hindgut conductance is low, the epithelia showing moderate anion selectivity. The thoracic (anterior) and abdominal (posterior) hindgut epithelia of D. pagei, the freshwater species, exhibit similar, low conductances, and are ion selective. These findings reveal that active, electrogenic, salt and nutrient transport is undetectably low or absent. The reduced transepithelial conductances and notable ion selectivities in the abdominal and thoracic hindguts of D. pagei may reduce passive salt losses in fresh water, contributing to osmotic and ionic regulation.

Effects of environmental anoxia and different periods of reoxygenation on oxidative balance in gills of the estuarine crab Chasmagnathus granulata

We investigated the effects of anoxia (8 h) and different periods of reoxygenation (20 and 40 min) on the oxidative balance in anterior and posterior gills of the crab Chasmagnathus granulata. Enzyme activity of catalase and GST was increased in the gills of the animals submitted to anoxia, and SOD activity was decreased. These enzymes returned approximately to control levels during the anoxia recovery time. These results demonstrated enzyme activities change with variations in environmental oxygen levels. The posterior gills showed a higher antioxidant enzyme activity than anterior gills. In the gills, there were no changes in the non-enzymatic antioxidant system (TRAP) during anoxia. On the other hand, during anoxia recovery, an increase of TRAP in both gills was observed. Anoxia does not change lipid peroxidation (TBARS) in the gills. During anoxia recuperation, an increase in levels of TBARS was observed. Thus the results demonstrate that C. granulata has a similar strategy of preparation for oxidative stress as observed in other intertidal species, enabling the crabs to survive in an environment with extreme variations in physical and chemical characteristics, such as salt marshes.

Na++K+-ATPase in gills of the blue crabCallinectes sapidus: cDNA sequencing and salinity-related expression of α-subunit mRNA and protein

Many studies have shown that hyperosmoregulation in euryhaline crabs is accompanied by enhanced Na++K+-ATPase activity in the posterior gills, but it remains unclear whether the response is due to regulation of pre-existing enzyme or to increased gene transcription and mRNA translation. To address this question, the complete open reading frame and 3′ and 5′ untranslated regions of the mRNA coding for the α-subunit of Na++K+-ATPase from the blue crab Callinectes sapidus were amplified by reverse transcriptase/polymerase chain reaction (RT-PCR) and sequenced. The resulting 3828-nucleotide cDNA encodes a putative 1039-amino-acid protein with a predicted molecular mass of 115.6 kDa. Hydrophobicity analysis of the amino acid sequence indicated eight membrane-spanning regions, in agreement with previously suggested topologies. The α-subunit amino acid sequence is highly conserved among species, with the blue crab sequence showing 81–83 % identity to those of other arthropods and 74–77 % identity to those of vertebrate species. Quantitative RT-PCR analysis showed high levels of α-subunit mRNA in posterior gills 6–8 compared with anterior gills 3–5. Western blots of gill plasma membranes revealed a single Na++K+-ATPase α-subunit protein band of the expected size. The posterior gills contained a much higher level of α-subunit protein than the anterior gills, in agreement with previous measurements of enzyme activity. Immunocytochemical analysis showed that the Na++K+-ATPase α-subunit protein detected by α5 antibody is localized to the basolateral membrane region of gill epithelial cells. Transfer of blue crabs from 35 to 5 ‰ salinity was not accompanied by notable differences in the relative proportions of α-subunit mRNA and protein in the posterior gills, suggesting that the enhanced Na++K+-ATPase enzyme activity that accompanies the hyperosmoregulatory response may result from post-translational regulatory processes.

Effects of water salinity on acid-base balance in decapod crustaceans

Extracellular acid-base balance in decapod crustaceans is influenced by water salinity, although the nature of this relationship is unclear. In euryhaline crabs, a decrease in salinity results in a metabolic alkalosis in the haemolymph and an increase in salinity results in a metabolic acidosis. Alterations in acid-base status by external changes in salinity are thought to be secondary to the adjustments required for ionic and osmotic regulation. In the present study, acid-base adjustments in the haemolymph of Eriocheir sinensis after transfer to 30 % sea water accompanied alterations in muscle pH and [HCO(3)(−)], as an initial acidosis coincided with an alkalosis in the leg muscle. By 48 h transfer, haemolymph pH increased as muscle pH and HCO(3)(−) declined. Haemolymph [Cl(−)] decreased significantly 3 h after transfer to a new steady state but haemolymph [Na(+)] and muscle [Na(+)] and [Cl(−)] remained unchanged. Muscle free amino acid concentration increased twofold 6 h after transfer, followed by a 2.5-fold increase in the haemolymph after 24 h. In contrast, 30 % sea water had no effect on haemolymph acid-base adjustments in the osmoconforming crab, Necora puber, which lacks ion and osmo-regulatory mechansims. Collectively these observations support the view that salinity-induced alterations in acid-base status are caused by adjustments consistent with cell volume regulation.

Early carbonic anhydrase induction in the gills of the blue crab,Callinectes sapidus, during low salinity acclimation is independent of ornithine decarboxylase activity

Carbonic anhydrase (CA) induction in the gills of the euryhaline blue crab, Callinectes sapidus, was measured in response to lowered environmental salinity. Simultaneous measurements of ornithine decarboxylase (ODC) activity were made in gills and nonbranchial tissues to determine whether ODC activity and the resultant synthesis of polyamines played a role in the initiation and regulation of CA induction. CA induction in the seventh gill pair (G7) was proportional to the decrease in ambient salinity, but activity in the third gill pair (G3) remained unchanged. Induction began by 24 hr after low salinity transfer, much earlier than previously reported, and peaked after 4 days. The magnitude of salinity change affected the magnitude of CA induction only, not the time course. A general cell volume regulatory response, as measured by the appearance of total ninhydrin-positive substances (TNPS) in the hemolymph, was initiated within 4 hr of low salinity transfer and was complete by 24 hr post-transfer. General cell swelling may be the initial signal in the pathway of CA induction. ODC activity in the gills of acclimated animals was not influenced by salinity. For crabs transferred from 35 to 25 ppt, ODC activity did not change significantly over the time course of acclimation. There was an early but transient increase in ODC activity in all tissues for crabs acclimated to 28 ppt and transferred to 15 ppt. Induction of ODC activity does not appear to be a precursor for CA induction; therefore, it does not appear that polyamines are substantially involved in the up-regulation of transport enzyme activity in low salinity. ODC, and resultant polyamine synthesis, may, however, have a role in cell volume regulation.

Expression of arginine kinase enzymatic activity and mRNA in gills of the euryhaline crabs Carcinus maenas and Callinectes sapidus

Phosphagen kinases catalyze the reversible dephosphorylation of guanidino phosphagens such as phosphocreatine and phosphoarginine, contributing to the restoration of adenosine triphosphate concentrations in cells experiencing high and variable demands on their reserves of high-energy phosphates. The major invertebrate phosphagen kinase, arginine kinase, is expressed in the gills of two species of euryhaline crabs, the blue crab Callinectes sapidus and the shore crab Carcinus maenas, in which energy-requiring functions include monovalent ion transport, acid-base balance, nitrogen excretion and gas exchange. The enzymatic activity of arginine kinase approximately doubles in the ion-transporting gills of C. sapidus, a strong osmoregulator, when the crabs are transferred from high to low salinity, but does not change in C. maenas, a more modest osmoregulator. Amplification and sequencing of arginine kinase cDNA from both species, accomplished by reverse transcription of gill mRNA and the polymerase chain reaction, revealed an open reading frame coding for a 357-amino-acid protein. The predicted amino acid sequences showed a minimum of 75 % identity with arginine kinase sequences of other arthropods. Ten of the 11 amino acid residues believed to participate in arginine binding are completely conserved among the arthropod sequences analyzed. An estimation of arginine kinase mRNA abundance indicated that acclimation salinity has no effect on arginine kinase gene transcription. Thus, the observed enhancement of enzyme activity in C. sapidus probably results from altered translation rates or direct activation of pre-existing enzyme protein.

Ion transport processes of crustacean epithelial cells

Epithelial cells of the gut, antennal glands, integument, and gills of crustaceans regulate the movements of ions into and across these structures and thereby influence the concentrations of ions in the hemolymph. Specific transport proteins serving cations and anions are found on apical and basolateral cell membranes of epithelia in these tissues. In recent years, a considerable research effort has been directed at elucidating their physiological and molecular properties and relating these characteristics to the overall biology of the organisms. Efforts to describe ion transport in crustaceans have focused on the membrane transfer properties of Na+/H+ exchange, calcium uptake as it relates to the molt cycle, heavy metal sequestration and detoxification, and anion movements into and across epithelial cells. In addition to defining the properties and mechanisms of cation movements across specific cell borders, work over the past 5 yr has also centered on defining the molecular nature of certain transport proteins such as the Na+/H+ exchanger in gill and gut tissues. Monovalent anion transport proteins of the gills and gut have received attention as they relate to osmotic and ionic balance in euryhaline species. Divalent anion secretion events of the gut have been defined relative to potential roles they may have in hyporegulation of the blood and in hepatopancreatic detoxification events involving complexation with cationic metals.

Effects of ions substitutions and of inhibitors on transepithelial potential difference and sodium fluxes in perfused gills of the crab Pachygrapsus marmoratus

With the same saline on both sides of the epithelium, a spontaneous inside negative transepithelial potential difference (PD) was measured in perfused posterior gills of the euryhaline crab Pachygrapsus marmoratus acclimated to dilute sea water. The origin of the PD and the transport properties of the epithelium were investigated by ionic substitutions and by application of inhibitors. Diffusion of Na+ and Cl- ions at the apical side contributes to the establishment of the PD. Sodium cyanide (10(-2) M) added to the perfusion and incubation media almost completely inhibited the transepithelial PD and considerably decreased the Na+ influx. Internally perfused ouabain (5.10(-3) M) halved the PD and the Na+ influx but had no effect on the Na+ efflux. Externally applied amiloride (10(-3) M) also reduced the Na+ influx by 27%. All inhibitions were concentration-dependent. From these results, it has been concluded that the transepithelial PD and Na+ influx are, at least partly, generated by active, metabolic energy-requiring processes. The effect of ouabain supports the existence of a Na+/K+ exchange mechanism linked to the presence of Na(+)-K+ ATPase in the basolateral membrane of posterior gills. The effect of amiloride is discussed.