Ultrastructural changes in the gill epithelium of the green crab Carcinus maenas in relation to the external salinity

Two faces of one coin: Beneficial and deleterious effects of reactive oxygen species during short-term acclimation to hypo-osmotic stress in a decapod crab

Exposure to environmental changes often results in the production of reactive oxygen species (ROS), which, if uncontrolled, leads to loss of cellular homeostasis and oxidative distress. However, at physiological levels these same ROS are known to be key players in cellular signaling and the regulation of key biological activities (oxidative eustress). While ROS are known to mediate salinity tolerance in plants, little is known for the animal kingdom. In this study, we use the Mediterranean crab Carcinus aestuarii, highly tolerant to salinity changes in its environment, as a model to test the healthy or pathological role of ROS due to exposure to diluted seawater (dSW). Crabs were injected either with an antioxidant [N-acetylcysteine (NAC), 150 mg·kg-1] or phosphate buffered saline (PBS). One hour after the first injection, animals were either maintained in seawater (SW) or transferred to dSW and injections were carried out at 12-h intervals. After ≈48 h of salinity change, all animals were sacrificed and gills dissected for analysis. NAC injections successfully inhibited ROS formation occurring due to dSW transfer. However, this induced 55% crab mortality, as well as an inhibition of the enhanced catalase defenses and mitochondrial biogenesis that occur with decreased salinity. Crab osmoregulatory capacity under dSW condition was not affected by NAC, although it induced in anterior (non-osmoregulatory) gills a 146-fold increase in Na+/K+/2Cl- expression levels, reaching values typically observed in osmoregulatory tissues. We discuss how ROS influences the physiology of anterior and posterior gills, which have two different physiological functions and strategies during hyper-osmoregulation in dSW.

Short-term exposure to high pCO2 leads to decreased branchial cytochrome C oxidase activity in the presence of octopamine in a decapod

In a recent mechanistic study, octopamine was shown to promote proton transport over the branchial epithelium in green crabs, Carcinus maenas. Here, we follow up on this finding by investigating the involvement of octopamine in an environmental and physiological context that challenges acid-base homeostasis, the response to short-term high pCO2 exposure (400 Pa) in a brackish water environment. We show that hyperregulating green crabs experienced a respiratory acidosis as early as 6 h of exposure to hypercapnia, with a rise in hemolymph pCO2 accompanied by a simultaneous drop of hemolymph pH. The slightly delayed increase in hemolymph HCO3- observed after 24 h helped to restore hemolymph pH to initial values by 48 h. Circulating levels of the biogenic amine octopamine were significantly higher in short-term high pCO2 exposed crabs compared to control crabs after 48 h. Whole animal metabolic rates, intracellular levels of octopamine and cAMP, as well as branchial mitochondrial enzyme activities for complex I + III and citrate synthase were unchanged in posterior gill #7 after 48 h of hypercapnia. However, application of octopamine in gill respirometry experiments suppressed branchial metabolic rate in posterior gills of short-term high pCO2 exposed animals. Furthermore, branchial enzyme activity of cytochrome C oxidase decreased in high pCO2 exposed crabs after 48 h. Our results indicate that hyperregulating green crabs are capable of quickly counteracting a hypercapnia-induced respiratory acidosis. The role of octopamine in the acclimation of green crabs to short-term hypercapnia seems to entail the alteration of branchial metabolic pathways, possibly targeting mitochondrial cytochrome C in the gill. Our findings help advancing our current limited understanding of endocrine components in hypercapnia acclimation. SUMMARY STATEMENT: Acid-base compensation upon short-term high pCO2 exposure in hyperregulating green crabs started after 6 h and was accomplished by 48 h with the involvement of the biogenic amine octopamine, accumulation of hemolymph HCO3-, and regulation of mitochondrial complex IV (cytochrome C oxidase).

Effects of different salinity reduction intervals on osmoregulation, anti-oxidation and apoptosis of Eriocheir sinensis megalopa

Eriocheir sinensis megalopa has a special life history of migrating from seawater to freshwater. In order to investigate how the megalopa adapt themselves to the freshwater environment, we designed an experiment to reduce the salinity of water from 30 ppt to 0 at rates of 30 ppt, 15 ppt, 10 ppt, and 5 ppt per 24 h to evaluate the effects of different degrees of hyposaline stress on the osmotic regulation ability and antioxidant system of the megalopa. Experimental results related to osmotic pressure regulation show that the gill tissue of megalopa in the treatment group of 30 ppt/24 h rapid reduction of salinity was damaged, while in the treatment group of 5 ppt/24 h it was intact. At the same time, the experiment also found that in each treatment group with different salinity reduction rates, compared with the control salinity, the NKA activity of megalopa increased significantly after the salinity was reduced to 20 ppt (p < 0.05). In addition, two genes involved in chloride ion transmembrane absorption have different expression patterns in the treatment groups with different salinity reduction rates. Among them, Clcn2 was significantly highly expressed only in the rapid salinity reduction intervals of 30 ppt/24 h and 15 ppt/24 h (p < 0.05). Slc26a6 was significantly highly expressed only in the slow salinity reduction intervals of 10 ppt/24 h and 5 ppt/24 h (p < 0.05). On the other hand, the results of antioxidant and apoptosis related experiments showed that in all treatment groups with different rates of salinity reduction, the activities of T-AOC, GSH-PX, and CAT basically increased significantly after salinity reduction compared to the control salinity. Moreover, the activities of T-AOC and CAT were significantly higher in the 10 ppt/24 h and 5 ppt/24 h treatment groups than in the 30 ppt/24 h and 15 ppt/24 h treatment groups. Finally, the experimental results related to apoptosis showed that the expression trends of Capase3 and Bax-2 were basically the same in the treatment groups with different salinity reduction rates, and their expressions were significantly higher in the 10 ppt/24 h and 5 ppt/24 h treatment groups than in the 30 ppt/24 h and 15 ppt/24 h treatment groups. In summary, the present study found that megalopa had strong hyposaline tolerance and were able to regulate osmolality at different rates of salinity reduction, but the antioxidant capacity differed significantly between treatment groups, with rapid salinity reduction leading to oxidative damage in the anterior gills and reduced antioxidant enzyme activity and apoptosis levels.

The role of octopamine and crustacean hyperglycemic hormone (CHH) in branchial acid-base regulation in the European green crab, Carcinus maenas

Crustaceans' endocrinology is a vastly understudied area of research. The major focus of the studies on this topic to date has been on the molting cycle (and in particular, the role of crustacean hyperglycemic hormone (CHH)), as well as the role of other hormones in facilitating physiological phenotypic adjustments to salinity changes. Additionally, while many recent studies have been conducted on the acclimation and adaptation capacity of crustaceans to a changing environment, only few have investigated internal hormonal balance especially with respect to an endocrine response to environmental challenges. Consequently, our study aimed to identify and characterize endocrine components of acid-base regulation in the European green crab, Carcinus maenas. We show that both the biogenic amine octopamine (OCT) and the CHH are regulatory components of branchial acid-base regulation. While OCT suppressed branchial proton excretion, CHH seemed to promote it. Both hormones were also capable of enhancing branchial ammonia excretion. Furthermore, mRNA abundance for branchial receptors (OCT-R), or G-protein receptor activated soluble guanylate cyclase (sGC1b), are affected by environmental change such as elevated pCO2 (hypercapnia) and high environmental ammonia (HEA). Our findings support a role for both OCT and CHH in the general maintenance of steady-state acid-base maintenance in the gill, as well as regulating the acid-base response to environmental challenges that C. maenas encounters on a regular basis in the habitats it dwells in and more so in the future ocean.

Identification of different physiological functions within the gills and epipodites of the American lobster: Differences in metabolism, transbranchial transport, and mRNA expression

Transbranchial transport processes are responsible for the homeostatic regulation of most essential physiological functions in aquatic crustaceans. Due to their widespread use as laboratory models, brachyuran crabs are commonly used to predict how other decapod crustaceans respond to environmental stressors including ocean acidification and warming waters. Non-brachyuran species such as the economically-valuable American lobster, Homarus americanus, possess trichobranchiate gills and epipodites that are known to be anatomically distinct from the phyllobranchiate gills of brachyurans; however, studies have yet to define their potential physiological differences. Our results indicate that the pleuro-, arthro-, and podobranch gills of the lobster are functionally homogenous and similar to the respiratory gills of brachyurans as indicated by equivalent rates of H⁺_Eq., CO₂, HCO₃^-, and ammonia transport and mRNA expression of related transporters and enzymes. The epipodites were found to be functionally distinct, being capable of greater individual rates of H⁺_Eq., CO₂, and ammonia transport despite mRNA transcript levels of related transporters and enzymes being only a fraction found in the gills. Collectively, mathematical estimates infer that the gills are responsible for 91% of the lobster's branchial HCO₃^- accumulation whereas the epipodites are responsible for 66% of branchial ammonia excretion suggesting different mechanisms exist in these tissues. Furthermore, the greater metabolic rate and amino acid catabolism in the epipodites suggest that the tissue much of the CO₂ and ammonia excreted by this tissue originates intracellularly rather than systemically. These results provide evidence that non-brachyuran species must be carefully compared to brachyuran models.

Characterization of 3 different types of aquaporins in Carcinus maenas and their potential role in osmoregulation

Intertidal crustaceans like Carcinus maenas shift between an osmoconforming and osmoregulating state when inhabiting full-strength seawater and dilute environments, respectively. While the bodily fluids and environment of marine osmoconformers are approximately isosmotic, osmoregulating crabs inhabiting dilute environments maintain their bodily fluid osmolality above that of their environment by actively absorbing and retaining osmolytes (e.g., Na⁺, Cl^-, urea) while eliminating excess water. Few studies have investigated the role of aquaporins (AQPs) in the osmoregulatory organs of crustaceans, especially within brachyuran species. In the current study, three different aquaporins were identified within a transcriptome of C. maenas, including a classical AQP (CmAQP1), an aquaglyceroporin (CmGLP1), and a big-brain protein (CmBIB1), all of which are expressed in the gills and the antennal glands. Functional expression of these aquaporins confirmed water transport capabilities for CmAQP1, CmGLP1, but not for CmBIB1, while CmGLP1 also transported urea. Higher relative CmAQP1 mRNA expression within tissues of osmoconforming crabs suggests the apical/sub-apically localized channel attenuates osmotic gradients created by non-osmoregulatory processes while its downregulation in dilute media reduces the water permeability of tissues to facilitate osmoregulation. Although hemolymph urea concentrations rose upon exposure to brackish water, urea was not detected in the final urine. Due to its urea-transport capabilities, CmGLP1 is hypothesized to be involved in a urea retention mechanism believed to be involved in the production of diluted urine. Overall, these results suggest that AQPs are involved in osmoregulation and provide a basis for future mechanistic studies investigating the role of AQPs in volume regulation in crustaceans.

The epidermis cells of mandible teeth in the terrestrial isopod Porcellio scaber: Differentiations for mineralisation with calcium phosphate and carbonate

Generally, the mineralisation of the crustacean cuticle occurs when the cuticle has expanded after moulting. However, in the partes incisivae of Porcellio scaber, cuticle mineralisation with calcium phosphate already occurs before the moult. We investigated the ultrastructure and distribution of organelles within the epidermis cells and searched for calcium-containing organelles using EDX and EFTEM analysis. We found two different cell types. Calcium carbonate-secreting C-cells, which resemble the epithelial cells of the general integument, and the P-cells, which, as an unusual feature, have cell extensions up to 400 μm long. During secretion of the partes incisivae, these extensions end at the unmineralised tip and the phosphate-containing middle region. Their cell bodies contain most of the mitochondria located in basal folds and a high amount of endoplasmic reticulum. The cell extensions contain many microtubules, endoplasmic reticulum, large and small vesicles and densely stained rod-shaped cisternae. The rod-shaped cisternae and the endoplasmic reticulum contain calcium. During cuticle mineralisation, vesicles, which probably belong to the endo-lysosomal system, contain calcium and phosphorus. They occur at some distance and close to the cuticle. The mineral in these vesicles has a similar composition to that within the cuticle, suggesting that they play a role in cuticle mineralisation.

Ion regulation in a freshwater crab, Potamonautes warreni: The effects of trace metal exposure

Crustaceans inhabiting metal-contaminated freshwaters are susceptible to toxic insult to their osmoregulatory systems. The main osmoregulatory organs of decapod crustaceans, the gills, are continually bathed in freshwater and are therefore at risk from trace metal impacts. The effects of chronic (21 d) exposure to raised dissolved concentrations of Zn, Cd, Cu and Pb on aspects of hydromineral balance were investigated in Potamonautes warreni, a freshwater crab endemic to rivers in South Africa at potential risk from trace metal contamination from mining operations. Generally, hydromineral balance of P. warreni was tolerant to chronic metal exposures although sublethal cadmium exposure of 860 µg.l^-1 for 21 days resulted in a reduced sodium concentration in the haemolymph. A chronic exposure to 43 µg.l^-1 cadmium produced an elevated maximum unidirectional sodium uptake, possibly resulting from acclimation to the metal exposure. Branchial Na⁺/K⁺-ATPase and V-Type H⁺-ATPase activity were not affected by chronic in vivo Cd (43 µg.l^-1) and Zn (500 µg.l^-1) exposures. An important aspect of ameliorating metal toxicity may be through antioxidants and therefore the effects of applying a reducing agent were tested following in vitro metal treatment. Inhibition of Na⁺/K⁺-ATPase could be prevented by pre-incubation with a reducing agent, indicating the importance of antioxidants in reducing metal toxicity in this species. Although this study demonstrates the physiological resilience of P. warreni to dissolved trace metal impacts, the energetic consequences of long-term exposure are as yet not known.

Exploring the versatility of the perfused crustacean gill as a model for transbranchial transport processes

The study of transbranchial ion and gas transport of water-breathing animals has long been a useful means of modeling transport processes of higher vertebrate organs through comparative physiology. The molecular era of biological research has brought forward valuable information detailing shifts in gene expression related to environmental stress and the sub-cellular localization of transporters; however, purely molecular studies can cause hypothetical transport mechanisms and hypotheses to be accepted without any direct physiological proof. Isolated perfused gill experiments are useful for testing most of these hypotheses and can sometimes be used outright to develop a well-supported working model for transport processes relating to an animal's osmoregulation, acid-base balance, nitrogen excretion, and respiratory gas exchange as well as their sensitivity to pollutants and environmental stress. The technique allows full control of internal hemolymph-like saline as well as the ambient environmental fluid compositions and can measure the electrophysiological properties of the gill as well as the transport rates of ions and gases as they traverse the gill epithelium. Additives such as pharmaceuticals or peptides as well as the exclusion of ions from the media are commonly used to identify the importance of specific transporters to transport mechanisms. The technique can also be used to identify the penetrance, retention, and localization of pollutants within the gill epithelium or to explore the uptake and metabolism of nutrients directly from the ambient environment. While this technique can be applied to virtually any isolatable organ, the anatomy and rigidity of the decapod crustacean gill make it an ideal candidate for most experimental designs.

Hypoxia attenuate ionic transport in the isolated gill epithelium of Carcinus maenas

The gills are osmorespiratory organs of aquatic organisms and the prime target of environmentally induced hypoxia. We have studied the impact of severe hypoxia (0.5 mg O₂/L) on the ionic transport across posterior gills of Carcinus maenas acclimated to 12 ppt seawater (DSW). The short-circuit current (Isc) across hemilamellae from gills i.e. active ion transport was studied in micro Ussing chambers. Hypoxia induced by deoxygenation of the basolateral side, and not the apical side, resulted in time-dependent inhibition of Isc and full recovery of Isc after reoxygenation. Exposure of the crabs to severe 7 h hypoxia decreased the specific activity of Na⁺,K⁺-ATPase in the gills by 36%. Full recovery of enzyme activity occurred in fasted crabs after 3 days of reoxygenation. The intensity of Western blotting bands was not different in the gills of oxygenated, hypoxic and reoxygenated crabs. The reversible, nonspecific blocker of K⁺ channels Cs and hypoxia inhibited over 90% of Isc which is after reoxygenation fully recovered. The specific blocker of Cl^- channels NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid] blocked Isc by 68.5%. Only the rest of not inhibited Isc in aerated saline was blocked by hypoxia and recovered after reoxygenation. The activity of the antioxidant enzyme catalase was not changed during hypoxia and reoxygenation kept the high enzyme activity in the gills at the level of crabs acclimated to DSW. As a response to hypoxia the presence of 2 mM H₂O₂ induce an initial slight transient decrease of Isc followed by a rise and after reoxygenation fully recovered Isc. Incubation of hemilamellae with the antioxidant derivative Trolox did not affect the inhibition of Isc by hypoxia.

Biochemical and morphological responses in Ucides cordatus (Crustacea, Decapoda) as indicators of contamination status in mangroves and port areas from northern Brazil

The aims of this study were to analyze biochemical and morphological responses (glutathione S-transferase activity and branchial lesions) in Ucides cordatus (crabs) and to verify how the species is responding to environmental contamination in a port (potentially contaminated area) and mangrove (reference area; Amazon Coast, Maranhão, Brazil). Adult males were captured bimonthly for a period of 1 year. Higher GST activity (1.03 ± 0.07 μmol min^-1 mg protein^-1) was observed in crabs in the port when compared with those in the reference area (p < 0.05). The greatest number of branchial lesions (serious alterations) was recorded in crabs only in the port area. The GST activity increased until serious lesions appeared; after this limit, GST activity decreased dramatically to very low levels, thus resulting in irreversible lesions (lamella collapse). The mathematical model based on the two parameters evaluated in U. cordatus showed that the port area experienced substantial contamination impact, while the mangroves (reference area) presented moderate environmental quality.

Salinity Variation in a Mangrove Ecosystem: A Physiological Investigation to Assess Potential Consequences of Salinity Disturbances on Mangrove Crabs

Dimitri Theuerkauff, Georgina A. Rivera-Ingraham, Jonathan A.C. Roques, Laurence Azzopardi, Marine Bertini, Mathilde Lejeune, Emilie Farcy, Jehan-Hervé Lignot, and Elliott Sucré (2018) Salinity is one of the main environmental factors determining coastal species distribution. However, in the specific case of mangrove crabs, salinity selection cannot be understood through ecological approaches alone. Yet understanding this issue is crucial in the context of mangrove conservation, since this ecosystem is often used as biofilter of (low- salinity) wastewater. Crabs are keystone species in this mangrove ecosystem and are differentially affected by salinity. We hypothesize that crab salinity selection may be partly explained by specific salinity-induced physiological constraints associated with osmoregulation, energy and redox homeostasis. To test this, the response to salinity variation was analysed in two landward mangrove crabs: the fiddler crab Tubuca urvillei, which inhabits low-salinity areas of the mangrove, and the red mangrove crab Neosarmatium meinerti, which lives in areas with higher salinity. Results confirm that both species are strong hypo-/hyper-osmoregulators that deal easily with large salinity variations. Such shifts in salinity do not induce changes in energy expenditure (measured as oxygen consumption) or in the production of reactive oxygen species. However, T. urvillei is physiologically suited to habitats with brackish water, since it presents i) high hemolymph osmolalities over a wider range of salinities and lower osmoregulatory capacity in seawater, ii) high Na+/K+-ATPase (NKA) activity in the posterior osmoregulatory gills and iii) a thicker osmoregulatory epithelium along the posterior gill lamellae. Therefore, while environmental salinity alone cannot directly explain fiddler and red mangrove crab distributions, our data suggest that salinity selection is indeed influenced by specific physiological adjustments.

Cadmium bioaccumulates after acute exposure but has no effect on locomotion or shelter-seeking behaviour in the invasive green shore crab (Carcinus maenas)

Cadmium (Cd²⁺) is a non-essential metal ubiquitous in the environment due to industrial processes. However, little is known regarding the ability of Cd²⁺ to impact the behaviour of aquatic animals in receiving environments. Green shore crabs (Carcinus maenas) were exposed to waterborne Cd²⁺ [control (no Cd²⁺), low (0.30 μmol/L), medium (3.3 μmol/L) and high (63 μmol/L)], for 24 h, then, crabs were placed in an open field and shelter test to determine potential changes in locomotion and preference for shelter. Tissues (gill, haemolymph, stomatogastric ganglion) were taken for bioaccumulation analysis of Cd²⁺ and ion content. Behavioural testing was recorded with a motion-tracking software system and showed no impact of Cd²⁺ on any variable in either of the tests used. All three tissues accumulated Cd²⁺ in a concentration-dependent manner. Crabs exposed to low Cd²⁺ showed a small but significant decrease in haemolymph Ca²⁺, however, this effect was not present at higher Cd²⁺ exposures. Overall, the results indicate that short-term Cd²⁺ exposure, and the resulting Cd²⁺ accumulation, had no effect on locomotor and anxiety-related behaviour of green shore crabs.

The effects of hypoxia on active ionic transport processes in the gill epithelium of hyperregulating crab, Carcinus maneas

Effects of hypoxia on the osmorespiratory functions of the posterior gills of the shore crab Carcinus maenas acclimated to 12ppt seawater (DSW) were studied. Short-circuit current (Isc) across the hemilamella (one epithelium layer supported by cuticle) was substantially reduced under exposure to 1.6, 2.0, or 2.5mg O₂/L hypoxic saline (both sides of epithelium) and fully recovered after reoxygenation. Isc was reduced equally in the epithelium exposed to 1.6mg O₂/L on both sides and when the apical side was oxygenated and the basolateral side solely exposed to hypoxia. Under 1.6mg O₂/L, at the level of maximum inhibition of Isc, conductance was decreased from 40.0mScm^-2 to 34.7mScm^-2 and fully recovered after reoxygenation. Isc inhibition under hypoxia and reduced ⁸⁶Rb⁺ (K⁺) fluxes across apically located K⁺ channels were caused preferentially by reversible inhibition of basolaterally located and ouabain sensitive Na⁺,K⁺-ATPase mediated electrogenic transport. Reversible inhibition of Isc is discussed as decline in active transport energy supply down regulating metabolic processes and saving energy during oxygen deprivation. In response to a 4day exposure of Carcinus to 2.0mg O₂/L, hemolymph Na⁺ and Cl^- concentration decreased, i.e. hyperosmoregulation was weakened. Variations of the oxygen concentration level and exposure time to hypoxia lead to an increase of the surface of mitochondria per epithelium area and might in part compensate for the decrease in oxygen availability under hypoxic conditions.

Mechanisms of acid–base regulation in seawater-acclimated green crabs (Carcinus maenas)

The present study investigated acid–base regulatory mechanisms in seawater-acclimated green crabs (Carcinus maenas (L., 1758)). In full-strength seawater, green crabs are osmoconformers so that the majority of the observed responses were attributed to ion fluxes based on acid–base compensatory responses alone. Similar to observations in brackish-water-acclimated C. maenas, seawater-acclimated green crabs exposed to hypercapnia rapidly accumulated HCO3− in their hemolymph, compensating for the respiratory acidosis caused by excess hemolymph pCO2. A full recovery from the decreased hemolymph pH after 48 h, however, was not observed. Gill perfusion experiments on anterior gill No. 5 indicated the involvement of all investigated genes (i.e., bicarbonate transporters, V-(H+)-ATPase, Na+/K+-ATPase, K+-channels, Na+/H+-exchanger, and carbonic anhydrase) in the excretion of acid–base equivalents. The most significant effects were observed when targeting a potentially cytoplasmic and (or) basolaterally localized V-(H+)-ATPase, as well as potentially basolaterally localized bicarbonate transporter (likely a Na+/HCO3−-cotransporter). In both cases, H+ accumulated in the hemolymph and CO2 excretion across the gill epithelium was significantly reduced or even reversed when blocking bicarbonate transporters. Based on the findings in this study, a working model for acid–base regulatory mechanisms and their link to ammonia excretion in the gill epithelium of C. maenas has been developed.

Osmoregulation and salinity-induced oxidative stress: is oxidative adaptation determined by gill function?

Osmoregulating decapods such as the Mediterranean green crab Carcinus aestuarii possess two groups of spatially segregated gills: anterior gills serve mainly respiratory purposes, while posterior gills contain osmoregulatory structures. The co-existence of similar tissues serving different functions allows the study of differential adaptation, in terms of free radical metabolism, upon salinity change. Crabs were immersed for 2 weeks in seawater (SW, 37 ppt), diluted SW (dSW, 10 ppt) and concentrated SW (cSW, 45 ppt). Exposure to dSW was the most challenging condition, elevating respiration rates of whole animals and free radical formation in hemolymph (assessed fluorometrically using C-H2DFFDA). Further analyses considered anterior and posterior gills separately, and the results showed that posterior gills are the main tissues fueling osmoregulatory-related processes because their respiration rates in dSW were 3.2-fold higher than those of anterior gills, and this was accompanied by an increase in mitochondrial density (citrate synthase activity) and increased levels of reactive oxygen species (ROS) formation (1.4-fold greater, measured through electron paramagnetic resonance). Paradoxically, these posterior gills showed undisturbed caspase 3/7 activity, used here as a marker for apoptosis. This may only be due to the high antioxidant protection that posterior gills benefit from [superoxide dismutase (SOD) in posterior gills was over 6 times higher than in anterior gills]. In conclusion, osmoregulating posterior gills are better adapted to dSW exposure than respiratory anterior gills because they are capable of controlling the deleterious effects of the ROS production resulting from this salinity-induced stress.

Making sense of nickel accumulation and sub-lethal toxic effects in saline waters: Fate and effects of nickel in the green crab, Carcinus maenas

In freshwater, invertebrates nickel (Ni) is considered an ionoregulatory toxicant, but its mechanism of toxicity in marine settings, and how this varies with salinity, is poorly understood. This study investigated Ni accumulation and physiological mechanisms of sub-lethal Ni toxicity in the euryhaline green crab Carcinus maenas. Male crabs were exposed to 8.2μg/L (the US EPA chronic criterion concentration for salt waters) of waterborne Ni (radiolabelled with (63)Ni) at three different salinities, 20%, 60% and 100% SW for 24h. Whole body Ni accumulation in 20% SW was 3-5 fold greater than in 60% or 100% SW, and >80% of accumulated Ni was in the carapace at all salinities. Ni also accumulated in posterior gill 8, which showed a higher accumulation in 20% SW than in other salinities, a pattern also seen at higher exposure concentrations of Ni (500 and 3000μg/L). Gill perfusion experiments revealed that Ni was taken up by both anterior and posterior gills, but in 20% SW the posterior gill 8, which performs ionoregulatory functions, accumulated more Ni than the anterior gill 5, which primarily has a respiratory function. The sub-lethal consequences of Ni exposure were investigated by placing crabs in Ni concentrations of 8.2, 500, and 3000μg/L at 20, 60 or 100% SW for 24h. In 20% SW, haemolymph Ca levels were significantly decreased by exposure to Ni concentrations of 8.2μg/L or higher, whereas Na concentrations were depressed only at 3000μg/L. Na(+)/K(+)-ATPase activity was inhibited at both 500 and 3000μg/L in gill 8, but only in 20% SW. Haemolymph K, Mg, and osmolality were unaffected throughout, though all varied with salinity in the expected fashion. These data suggest that Ni impacts ionoregulatory function in the green crab, in a gill- and salinity-dependent manner.

Gene expression profiling in gills of the great spider crab Hyas araneus in response to ocean acidification and warming

BACKGROUND

Hypercapnia and elevated temperatures resulting from climate change may have adverse consequences for many marine organisms. While diverse physiological and ecological effects have been identified, changes in those molecular mechanisms, which shape the physiological phenotype of a species and limit its capacity to compensate, remain poorly understood. Here, we use global gene expression profiling through RNA-Sequencing to study the transcriptional responses to ocean acidification and warming in gills of the boreal spider crab Hyas araneus exposed medium-term (10 weeks) to intermediate (1,120 μatm) and high (1,960 μatm) PCO2 at different temperatures (5°C and 10°C).

RESULTS

The analyses reveal shifts in steady state gene expression from control to intermediate and from intermediate to high CO2 exposures. At 5°C acid-base, energy metabolism and stress response related genes were upregulated at intermediate PCO2, whereas high PCO2 induced a relative reduction in expression to levels closer to controls. A similar pattern was found at elevated temperature (10°C). There was a strong coordination between acid-base, metabolic and stress-related processes. Hemolymph parameters at intermediate PCO2 indicate enhanced capacity in acid-base compensation potentially supported by upregulation of a V-ATPase. The likely enhanced energy demand might be met by the upregulation of the electron transport system (ETS), but may lead to increased oxidative stress reflected in upregulated antioxidant defense transcripts. These mechanisms were attenuated by high PCO2, possibly as a result of limited acid-base compensation and metabolic down-regulation.

CONCLUSION

Our findings indicate a PCO2 dependent threshold beyond which compensation by acclimation fails progressively. They also indicate a limited ability of this stenoecious crustacean to compensate for the effects of ocean acidification with and without concomitant warming.

Osmoregulation in the Hawaiian anchialine shrimp Halocaridina rubra (Crustacea: Atyidae): expression of ion transporters, mitochondria-rich cell proliferation, and hemolymph osmolality during salinity transfers

Studies of euryhaline crustaceans have identified conserved osmoregulatory adaptions allowing hyper-osmoregulation in dilute waters. However, previous studies have mainly examined decapod brachyurans with marine ancestries inhabiting estuaries or tidal creeks on a seasonal basis. Here, we describe osmoregulation in the atyid Halocaridina rubra, an endemic Hawaiian shrimp of freshwater ancestry from the islands' anchialine ecosystem (coastal ponds with subsurface fresh water and seawater connections) that encounters near-continuous spatial and temporal salinity changes. Given this, survival and osmoregulatory responses were examined over a wide salinity range. In the laboratory, H. rubra tolerated salinities of ~0-56‰, acting as both a hyper- and hypo-osmoregulator and maintaining a maximum osmotic gradient of ~868 mOsm/kg H2O in freshwater. Furthermore, hemolymph osmolality was more stable during salinity transfers relative to other crustaceans. Silver nitrate and vital mitochondria-rich cell staining suggest all gills are osmoregulatory, with a large proportion of each individual gill functioning in ion transport (including when H. rubra acts as an osmoconformer in seawater). Additionally, expression of ion transporters and supporting enzymes that typically undergo up-regulation during salinity transfer in osmoregulatory gills (i.e., Na+/K+-ATPase, carbonic anhydrase, Na+/K+/2Cl- cotransporter, V-type H+-ATPase, and arginine kinase) were generally unaltered in H. rubra during similar transfers. These results suggest H. rubra (and possibly other anchialine species) maintains high, constitutive levels of gene expression and ion transport capability in the gills as a means of potentially coping with the fluctuating salinities that are encountered in anchialine habitats. Thus, anchialine taxa represent an interesting avenue for future physiological research.

Multiple functions of the crustacean gill: osmotic/ionic regulation, acid-base balance, ammonia excretion, and bioaccumulation of toxic metals

The crustacean gill is a multi-functional organ, and it is the site of a number of physiological processes, including ion transport, which is the basis for hemolymph osmoregulation; acid-base balance; and ammonia excretion. The gill is also the site by which many toxic metals are taken up by aquatic crustaceans, and thus it plays an important role in the toxicology of these species. This review provides a comprehensive overview of the ecology, physiology, biochemistry, and molecular biology of the mechanisms of osmotic and ionic regulation performed by the gill. The current concepts of the mechanisms of ion transport, the structural, biochemical, and molecular bases of systemic physiology, and the history of their development are discussed. The relationship between branchial ion transport and hemolymph acid-base regulation is also treated. In addition, the mechanisms of ammonia transport and excretion across the gill are discussed. And finally, the toxicology of heavy metal accumulation via the gill is reviewed in detail.

Effects of elevated seawater pCO(2) on gene expression patterns in the gills of the green crab, Carcinus maenas

Background

The green crab Carcinus maenas is known for its high acclimation potential to varying environmental abiotic conditions. A high ability for ion and acid-base regulation is mainly based on an efficient regulation apparatus located in gill epithelia. However, at present it is neither known which ion transport proteins play a key role in the acid-base compensation response nor how gill epithelia respond to elevated seawater pCO₂ as predicted for the future. In order to promote our understanding of the responses of green crab acid-base regulatory epithelia to high pCO₂, Baltic Sea green crabs were exposed to a pCO₂ of 400 Pa. Gills were screened for differentially expressed gene transcripts using a 4,462-feature microarray and quantitative real-time PCR.

Results

Crabs responded mainly through fine scale adjustment of gene expression to elevated pCO₂. However, 2% of all investigated transcripts were significantly regulated 1.3 to 2.2-fold upon one-week exposure to CO₂ stress. Most of the genes known to code for proteins involved in osmo- and acid-base regulation, as well as cellular stress response, were were not impacted by elevated pCO₂. However, after one week of exposure, significant changes were detected in a calcium-activated chloride channel, a hyperpolarization activated nucleotide-gated potassium channel, a tetraspanin, and an integrin. Furthermore, a putative syntaxin-binding protein, a protein of the transmembrane 9 superfamily, and a Cl^-/HCO₃^- exchanger of the SLC 4 family were differentially regulated. These genes were also affected in a previously published hypoosmotic acclimation response study.

Conclusions

The moderate, but specific response of C. maenas gill gene expression indicates that (1) seawater acidification does not act as a strong stressor on the cellular level in gill epithelia; (2) the response to hypercapnia is to some degree comparable to a hypoosmotic acclimation response; (3) the specialization of each of the posterior gill arches might go beyond what has been demonstrated up to date; and (4) a re-configuration of gill epithelia might occur in response to hypercapnia.

Changes in Na+/K+-ATPase activity, unsaturated fatty acids and metallothioneins in gills of the shore crab Carcinus aestuarii after dilute seawater acclimation

We have assessed the activity of Na(+)/K(+)-ATPase, cAMP, free fatty acids (FFA) and metallothionein (MT) in the posterior gills of the brackish water shore crab Carcinus aestuarii during acclimation to 10 ppt dilute seawater (DSW). Following 3-18 days acclimation in DSW specific activity of Na(+)/K(+)-ATPase in native gill homogenates and partially purified membrane vesicles was progressively increased, from 1.7- to 3.9-fold. After short-term acclimation of crabs in DSW with added sucrose to make media isosmotic with the haemolymph the specific Na(+)/K(+)-ATPase activity in homogenates was not increased, relative to SW enzyme activity. Moreover, hyposmotic conditions led to depletion of cAMP in gills. In partially purified membrane vesicles isolated from posterior gills, fatty acids with compositions 16:0, 18:0, 18:1, 20:4 and 20:5 dominated in both SW- and DSW-acclimated Carcinus. During a year in which the metabolic activity of crabs was increased, the arachidonic/linoleic acids ratio (ARA/LA) for DSW-acclimated crabs was markedly increased relative to that in SW. Increased Na(+)+K(+)-ATPase activity under hyposmotic stress may be modulated at least partially by the changed proportion of fatty acids in the purified membranes of posterior gills. Long-term acclimation of shore crabs to DSW resulted in a 2.6-fold increase in cytosolic metallothionein (MT) content in posterior gills over those in SW crabs. Assuming an antioxidant role of MT associated with intracellular zinc partitioning, the observed MT induction in posterior gills may be considered an adaptive response of C. aestuarii to hyposmotic stress.

Salinity-stimulated changes in expression and activity of two carbonic anhydrase isoforms in the blue crabCallinectes sapidus

Two isoforms of the enzyme carbonic anhydrase (CA) in the blue crab gill,CasCAg and CasCAc, were identified, sequenced, and found to match the membrane-associated and cytoplasmic isoforms, respectively. The membrane-associated isoform is present in much higher levels of mRNA expression in both anterior and posterior gills in crabs acclimated to high salinity (35 p.p.t.), but expression of the cytoplasmic isoform in the posterior gill undergoes a significantly greater degree of up-regulation after exposure to low salinity (15 p.p.t.). CasCAc has the largest scope of induction (100-fold) reported for any transport-related protein in the gill,and this may be necessary to overcome diffusion limitations between gill cytoplasm and the apical boundary layer. Furthermore, the timing of the changes in expression of CasCAc corresponds to the timing of the induction of protein-specific CA activity and CA protein concentration. No changes in CA mRNA expression or activity occur in the anterior gills. The pattern of up-regulation of expression of mRNA of the α-subunit of the Na+/K+-ATPase is similar to that for CasCAc, and both precede the establishment of the new acclimated physiological state of the crab in low salinity. A putative `housekeeping' gene, arginine kinase, also showed about a threefold increase in expression in response to low salinity,but only in the posterior gills. These results suggest that for studies of expression in crustacean gill tissue, a control tissue, such as the anterior gill, be used until an adequate control gene is identified.

Effects of long-term exposure to different salinities on the location and activity of Na+–K+-ATPase in the gills of juvenile mitten crab, Eriocheir sinensis

The euryhalinity of mitten crab, Eriocheir sinensis, is based on osmoregulation, and thus on the activity of Na(+)-K(+)-ATPase. We studied location and activity of this enzyme in gills of juvenile crabs exposed to 5 per thousand, 25 per thousand, and 40 per thousand salinity. The posterior gills showed always a high number of immunopositive cells (IPC), staining with fluorescent antibody against Na(+)-K(+)-ATPase, covering at 5 per thousand the entire lamellae. At 25 per thousand, they showed fewer IPC which occurred only at the bases of the lamellae. Enzyme activity was consistently higher in posterior than in anterior gills. Low salinity stimulated the activity only in posterior gills. Both histochemical and enzymatic results are consistent with previous ultrastructural observations showing that the epithelial cells of the posterior, but not the anterior gills exhibit typical traits of ionocytes. While an increase in Na(+)-K(+)-ATPase activity at a reduced salinity is consistent with a strong hyper-osmoregulatory capacity in juvenile crabs, a low activity at an enhanced salinity suggests a physiological response, directed towards a reduction of Na(+) uptake. The activity increase of ion-transporting enzymes is directly related to spatial changes in their distribution along the osmoregulatory tissue, i.e. an enhanced number of IPC scattered along the entire lamellae. In juveniles, this allows for successful development and growth at reduced salinities.

A structure-function analysis of ion transport in crustacean gills and excretory organs

Osmotic and ionic regulation in the Crustacea is mostly accomplished by the multifunctional gills, together with the excretory organs. In addition to their role in gas exchange, the gills constitute organs of active, transepithelial, ion transport, an activity of major importance that underlies many essential physiological functions like osmoregulation, calcium homeostasis, ammonium excretion and extracellular pH regulation. This review focuses on structure-function relationships in crustacean gills and excretory effectors, from the organ to molecular levels of organization. We address the diversity of structural architectures encountered in different crustacean gill types, and in constituent cell types, before examining the physiological mechanisms of Na(+), Cl(-), Ca(2+) and NH(4)(+) transport, and of acid-base equivalents, based on findings obtained over the last two decades employing advanced techniques. The antennal and maxillary glands constitute the principal crustacean excretory organs, which have received less attention in functional studies. We examine the diversity present in antennal and maxillary gland architecture, highlighting the structural similarities between both organ types, and we analyze the functions ascribed to each glandular segment. Emphasis is given to volume and osmoregulatory functions, capacity to produce dilute urine in freshwater crustaceans, and the effect of acclimation salinity on urine volume and composition. The microanatomy and diversity of function ascribed to gills and excretory organs are appraised from an evolutionary perspective, and suggestions made as to future avenues of investigation that may elucidate evolutionary and adaptive trends underpinning the invasion and exploitation of novel habitats.

Osmoregulation, Immunolocalization of Na+/K+‐ATPase, and Ultrastructure of Branchial Epithelia in the Developing Brown Shrimp,Crangon crangon(Decapoda, Caridea)

Aspects of osmoregulation including salinity tolerance, osmoregulatory capacity, location of transporting epithelia, and the expression of the enzyme Na+/K+-ATPase were investigated in the developing brown shrimp, Crangon crangon (L.), from the North Sea. Early developmental stages and large juveniles were exposed to a wide range of salinities for measurement of hemolymph osmolality and survival rates. In media ranging from 17.0 per thousand to 32.2 per thousand, salinity tolerance was generally high (survival rates: 70%-100%) in all developmental stages, but it decreased in media <10.2 per thousand. Zoeal stages and decapodids slightly hyperregulated at 17.0 per thousand and osmoconformed in media > or =25.5 per thousand. At 10.2 per thousand, these stages showed high mortality, and only juveniles survived at 5.3 per thousand. Juveniles hyperregulated at 10.2 per thousand and 17.0 per thousand, osmoconformed at 25.5 per thousand, and hyporegulated in media > or =32.2 per thousand. Large juveniles hyperregulated also at 5.3 per thousand. Expression of the Na+/K+-ATPase and ion-transporting cells was located through immunofluorescence microscopy and transmission electron microscopy. In zoeae I and VI, a strong immunoreactivity was observed in cells of the inner epithelia of the branchiostegites and in epithelial cells lining the pleurae. Their ultrastructure showed typical features of ion-transporting cells. In decapodids and juveniles, ionocytes and expression of Na+/K+-ATPase remained located in the branchiostegite epithelium, but they disappeared from the pleurae and appeared in the epipodites. In large juveniles, the cells of the gill shaft showed positive immunolabeling and ultrastructural features of ionocytes. In summary, the adult pattern of osmoregulation in C. crangon is accomplished after metamorphosis from a moderately hyperosmoconforming decapodid to an effectively hyper-/hyporegulating juvenile stage. Salinity tolerance and osmoregulatory capacity are closely correlated with the development of ion-transporting cells and the expression of Na+/K+-ATPase.

Branchial chamber tissues in two caridean shrimps: the epibenthic Palaemon adspersus and the deep-sea hydrothermal Rimicaris exoculata

The structure of the epithelia of the branchial chamber organs (gills, branchiostegites, epipodites) and the localization of the Na(+),K(+)-ATPase were investigated in two caridean shrimps, the epibenthic Palaemon adspersus and the deep-sea hydrothermal Rimicaris exoculata. The general organization of the phyllobranchiate gills, branchiostegites and epipodites is similar in P. adspersus and in R. exoculata. The gill filaments are formed by a single axial epithelium made of H-shaped cells with thin lateral expansions and a basal lamina limiting hemolymph lacunae. In P. adspersus, numerous ionocytes are present in the epipodites and in the inner-side of the branchiostegites; immunofluorescence reveals their high content in Na(+),K(+)-ATPase. In R. exoculata, typical ionocytes displaying a strong Na(+),K(+)-ATPase specific fluorescence are observed in the epipodites only. While the epipodites and the branchiostegites appear as the main site of osmoregulation in P. adspersus, only the epipodites might be involved in ion exchanges in R. exoculata. In both species, the gill filaments are mainly devoted to respiration.

Hyper-osmoregulatory capacity of the Chinese mitten crab (Eriocheir sinensis) exposed to cadmium; acclimation during chronic exposure

The aim of this study was to evaluate the effects of waterborne cadmium on hyper-osmoregulatory capacity of the Chinese mitten crab Eriocheir sinensis acclimated to freshwater. For this purpose, crabs were submitted to acute (0.5 mg Cd L(-1) for 1, 2 or 3 days), chronic (10 or 50 microg Cd L(-1) for 30 days) or chronic, immediately followed by acute, exposure. While no effect was observed after 1 or 2 days, hemolymph osmolality, Na(+) and Cl(-) concentrations were significantly reduced after 3 days of acute exposure. Under this latter condition, the respiratory anterior gill ultrastructure, Na(+)/K(+)-ATPase and cytochrome c oxidase activities were significantly impaired. In contrast, the osmoregulatory posterior gill was unaffected for all treatments. As a consequence, we suggest that the observed hyper-osmoregulatory capacity impairment is the result of increased dissipative flow of ions and/or water through anterior gills. In contrast to acute exposure, chronic exposure did not induce any observable effect. However, crabs submitted to a known deleterious acute condition (0.5 mg Cd L(-1) for 3 days) directly after chronic exposure to 50 microg Cd L(-1) for 30 days showed normal hyper-osmoregulatory capacity with no change in gill Na(+)/K(+)-ATPase activity, and only little disturbance of anterior gill ultrastructure. These results demonstrate that a chronic cadmium exposure can induce acclimation mechanisms related to osmoregulation in this euryhaline decapod crustacean.

Ontogeny of osmoregulatory structures and functions in the green crabCarcinus maenas(Crustacea, Decapoda)

The ontogeny of osmoregulation, the development of branchial transporting epithelia and the expression of the enzyme Na+/K+-ATPase were studied in Carcinus maenas (L.) obtained from the North Sea,Germany. Laboratory-reared zoea larvae, megalopae and young crabs were exposed to a wide range of salinities, and hemolymph osmolality was measured after 24 h exposure time (72 h in juveniles). Zoea I larvae slightly hyper-regulated in dilute media (10.2‰ and 17.0‰) and osmoconformed at&gt;17‰. All later zoeal stages (II-IV) osmoconformed in salinities from 10.2‰ to 44.3‰. The megalopa hyper-regulated at salinities from 10.2 to 25.5‰. Young crabs hyperregulated at salinities from 5.3‰ to 25.5‰, showing an increase in their osmoregulatory capacity. The development of transporting epithelia and the expression of Na+/K+-ATPase were investigated by means of transmission electron microscopy and immunofluorescence microscopy. In the zoea IV, only a very light fluorescence staining was observed in gill buds. Epithelial cells were rather undifferentiated, without showing any features of ionocytes. Gills were present in the megalopa, where Na+/K+-ATPase was located in basal filaments of the posterior gills. In crab I juveniles and adults, Na+/K+-ATPase was noted in the three most posterior pairs of gills, but lacking in anterior gills. Ionocytes could first be recognized in filaments of megalopal posterior gills, persisting through subsequent stages at the same location. Thus, the development of the gills and the expression of Na+/K+-ATPase are closely correlated with the ontogeny of osmoregulatory abilities. The morphological two-step metamorphosis of C. maenas can also be regarded as an osmo-physiological metamorphosis, (i) from the osmoconforming zoeal stages to the weakly regulating megalopa, and (ii) to the effectively hyper-regulating juvenile and adult crabs.

Uptake of cadmium through isolated perfused gills of the Chinese mitten crab, Eriocheir sinensis

Using the perfusion method, we compared cadmium accumulation and influx across the gills of the euryhaline Chinese mitten crab Eriocheir sinensis, exposed to 4.8 microM cadmium in the incubation medium (OUT). Cadmium influx was not observed across posterior gills while it ranged from 0.15 to 6.82 nmol Cd g(-1) gill w.w. h(-1) across anterior ones. For these respiratory gills, a strong increase (40 times) was observed when calcium was removed in both incubation and perfusion media while the lack of sodium in the perfusion medium resulted in a 46 times decrease. For crabs acclimated 15 days to artificial seawater, cadmium influx across anterior gills showed a 21 times decrease when compared with freshwater acclimated ones. On the other hand, after 3 h of perfusion, we detected cadmium accumulation in both types of gills, ranging from 3.8 to 68 nmol Cd g(-1) gill w.w. in anterior gills and from 2.1 to 39 nmol Cd g(-1) gill w.w. in posterior ones. Such accumulations represent between 61.3 and 100% of the total uptake of cadmium through the gills. From these results, we suggest that cadmium can penetrate more easily into the hemolymph space through the 'respiratory' type epithelium present in the anterior gills but absent in the posterior ones. This metal uptake is likely to occur at least in part through the same pathways as calcium. On the contrary, cadmium seems to be sequestered inside the posterior gills, perhaps in the cuticle of the salt-transporting type epithelium.

Na(+)+K(+)-ATPase in gills of aquatic crustacea

The sodium pump, or Na(+)+K(+)-ATPase, provides at least part of the driving force for transepithelial movement of monovalent ions across the gills and other transporting tissues in many aquatic animals including the Crustacea. The crustacean Na(+)+K(+)-ATPase, like that in all animal cells, is composed of a catalytic alpha-subunit and an accompanying beta-subunit. The amino acid sequence of the crustacean alpha-subunit is 71-74% identical to vertebrate alpha-subunit sequences. In brachyuran Crustacea, the Na(+)+K(+)-ATPase is more highly expressed in posterior gills compared with anterior and is found predominantly in mitochondria-rich cells that are morphologically and biochemically specialized to mediate NaCl uptake from the medium. When the external salinity is lowered from that of normal seawater, producing conditions in which many euryhaline Crustacea hyperosmo regulate their hemolymph, both the enzymatic activity of the Na(+)+K(+)-ATPase and the gene expression of the alpha-subunit are increased in these tissues. Although the precise regulatory mechanism is not known, evidence suggests that crustacean hyperglycemic hormone may be responsible for the induction of Na(+)+K(+)-ATPase activity. Whether it also plays a role in activation of gene transcription is not known. A comparison of a range of aquatic Crustacea suggests that the level of Na(+)+K(+)-ATPase function in transporting tissues may be correlated with their ability to invade estuarine habitats.

Does sulphide detoxication occur in the gills of the hydrothermal vent shrimp, Rimicaris exoculata?

Ultrastructural observations of the gills of the hydrothermal vent shrimp Rimicaris exoculata reveal that the epithelial cells contain numerous mitochondria clustered around unusual organelles (diameter of 0.7 to 2.5 microns) containing membrane stacks. These organelles were termed sulphide-oxidising bodies (SOBs) by structural analogy with organelles observed in the tissues of species adapted to sulphide-rich environments. Moreover, in the gills of R. exoculata, mitochondria display numerous electron-dense granules in their stroma. Such ultrastructural features suggest that sulphide detoxication may probably occur in the gills of R. exoculata. Comparable structures were also described in the gills of other hydrothermal vent species, as the alvinellid Pompeii worms that, as R. exoculata, are housing ectosymbiotic bacteria.

Hyperosmoregulation in the red freshwater crab Dilocarcinus pagei (Brachyura, Trichodactylidae): structural and functional asymmetries of the posterior gills

The osmotic and ionic status of the haemolymph and the structural and ion-transport characteristics of the posterior gills of Dilocarcinus pagei, a hololimnetic crab, were investigated. Haemolymph osmolality was 386±18 mosmol kg–1, while [Na+] and [Cl–] were 190±13 and 206±12 mmol l–1, respectively; [K+], [Ca2+] and [Mg2+] were 9.7±0.7, 10.2±0.5 and 2.8±0.4 mmol l–1, respectively (means ± s.e.m., N=12–17). The gill lamellae possess a central, osmiophilic area, which exhibits a marked structural asymmetry. The thick (18–20 μm) proximal epithelium is characterised by basal invaginations and a few apical vesicles, while the thin (3–10 μm) distal epithelium consists of apical pillar cell flanges populated by vesicles and membrane invaginations. Isolated gills, bathed and perfused with NaCl saline, spontaneously generate a negative transbranchial potential difference (Vte), which stabilises at positive or negative values. Ouabain shifts Vte to more positive values. When mounted in an Ussing chamber, distal split lamellae generate a negative, Cl–-dependent short-circuit current (Isc). Na+ substitution leads to more negative values of Isc. Internal ouabain is without effect, while diphenylamine-2-carboxylate and acetazolamide abolish Isc. Proximal split lamellae show a positive, Na+-dependent Isc, which decreases after internal application of ouabain. These data suggest that the thin epithelium actively absorbs Cl–, while the thick epithelium actively absorbs Na+.

Transepithelial potential differences and Na+ flux in isolated perfused gills of the crabChasmagnathus granulatus(Grapsidae) acclimated to hyper- and hypo-salinity

We studied the transepithelial potential difference (TEPD) and 22Na flux across isolated perfused gills (anterior pair 5 and posterior pairs 6–8) of the crab Chasmagnathus granulatus acclimated to either hypo- or hyper-osmotic conditions.

The gills of crabs acclimated to low salinity, perfused and bathed with 10 ‰ saline solutions, produced the following TEPDs (hemolymph side with respect to bath side): 0.4±0.7, –10.2±1.6, –10.8±1.3 and –6.7±1.3 mV for gills 5, 6, 7 and 8, respectively. Gills 6, 7 and 8 did not differ significantly. Reducing the saline concentration of bath and perfusate from 30 ‰ to 20 ‰ or 10 ‰ increased significantly the TEPDs of these gills. TEPDs of gill 6 (representative of posterior gills) were reduced by 69±5 % and 60±5 % after perfusion with ouabain or BaCl2 (5 mmol l–1 each), respectively. The same gill showed a net ouabain-sensitive Na+ influx of 1150±290 μequiv g–1 h–1.

Gill 6 of crabs acclimated to high salinity produced TEPDs of –1.5±0.1 and –1.3±0.09 mV after perfusion with 30 ‰ or 40 ‰ salines, respectively. Perfusion with ouabain or BaCl2 reduced TEPDs by 76±7 % and 86±4 %, respectively. A net ouabain-sensitive Na+ efflux of 2282±337 μequiv g–1 h–1 was recorded in gill 6 perfused with 38 ‰ saline.

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.

Environmentally mediated carbonic anhydrase induction in the gills of euryhaline crustaceans

The enzyme carbonic anhydrase appears to be a central molecular component in the suite of physiological and biochemical adaptations to low salinity found in euryhaline crustaceans. It is present in high activities in the organs responsible for osmotic and ionic regulation, the gills, and more specifically, the individual gills that are specialized for active ion uptake from dilute sea water. Within those gills carbonic anhydrase is distributed among different subcellular pools, the cytoplasm, mitochondria and microsomes. The cytoplasmic pool represents the largest subcellular fraction of carbonic anhydrase activity, and it is this fraction that undergoes a tenfold induction during acclimation to low salinity. Carbonic anhydrase activity is present in excess of that needed to support the general ion-transport processes, and so it is doubtful that carbonic anhydrase activity itself is a point of short-term regulation in response to salinity changes. Rather, upregulation of carbonic anhydrase appears to be a result of selective gene expression, representing a permanent response to long-term adaptation to low salinity. The exact signal that initiates the induction of carbonic anhydrase, and the pathway through which that signal is transduced to the activation of the carbonic anhydrase gene, are unknown, but two promising avenues of research exist. First, induction of carbonic anhydrase is immediately preceded by hemodilution and subsequent cell swelling, a potential initiating event in the process. Second, recent work indicates that expression of carbonic anhydrase is under the control of a repressor substance, located in the eyestalk, whose effect is removed upon exposure to low salinity.

Bimodal breathing in the estuarine crab Chasmagnathus granulatus Dana 1851--physiological and morphological studies

Chasmagnathus granulatus is an estuarine crab which actively moves from subtidal to supratidal areas. To elucidate the possible existence of extrabranchial sites for aerial gas exchange, we measured respiratory and acid-base variables in animals with and without branchial water (controls and experimental crabs, respectively) during air exposure. An histological study of the branchiostegite was also performed. Throughout 4 h of emergence C. granulatus did not suffer venous hypoxia, even without branchial water. The rate of oxygen uptake (M(O(2))) was similar in both groups. The rate of carbon dioxide excretion (M(CO(2))) and the gas exchange ratio (R) significantly decreased during emergence in both groups, with R significantly lower for experimental crabs. Consequently, CO(2) was accumulated in the hemolymph. This variable stabilized after 90 min in control animals, but experimental crabs continued accumulating CO(2). Histological study of the branchiostegites demonstrated the presence of an attenuated and greatly perfused epithelium facing the branchial chamber lumen, with a shortest diffusion distance of 0.5 microm. Simple folds and lobulated projections increase the respiratory surface area. These results suggest that C. granulatus is a bimodal breathing crab, active both in water and air. When emerged, this species extract oxygen directly from air through branchiostegal lungs, but relies on branchial exchange to eliminate carbon dioxide.

The ability to feed in hypoxia follows a seasonally dependent pattern in shore crab Carcinus maenas

The ability of the adult shore crab Carcinus maenas, native to the Bay of Arcachon (SW France), to feed in hypoxia was determined at various seasons. Crabs previously kept at field temperature were fed after a 5-day fasting period at 15 degrees C. Their blood oxygenation and pH regulation strategy and also their gill anatomy were analysed. From May to October, C. maenas feed at levels of O(2) partial pressure (p(O(2))) in the water, pwO(2)=2 kPa (1 mg l(-1)), without switching to their anaerobic metabolism. In March-April, before the main moulting period, the same food intake at pwO(2)=4 kPa induced a systematic blood lactate increase associated with some mortality. An analysis performed at pwO(2)=4 kPa at that time showed that in intermoult crabs the development of a coating of foreign material over the gill cuticle interfered with O(2)-supply, preventing the small arterial p(O(2)) increases (from 0.7 to 1 kPa) which occurred at other seasons. This led to a cellular hypoxia despite a systematic postprandial blood-pH alkalinisation which favoured O(2)-loading at gill level and increased arterial O(2) concentration. In March-April, alkalinisation appeared at pwO(2) values >/=6 kPa and from May to at least July at pwO(2)>/=2 kPa. Results are discussed in terms of season-related physiological performance, as hypoxic events mainly occur during the hot season.

Active osmoregulatory ion uptake across the pleopods of the isopod Idotea baltica (Pallas): electrophysiological measurements on isolated split endo- and exopodites mounted in a micro-ussing chamber

The mechanism of active, osmoregulatory ion uptake was investigated in the pleopods of the marine isopod Idotea baltica (Pallas). Using isolated split half-podites of isopods acclimated to brackish water (20 salinity) mounted in a micro-Ussing chamber and symmetrically superfused with identical haemolymph-like salines, a mean short-circuit current I(sc) of −445 microA cm(−)(2) was measured in endopodites 3–5, corresponding to an inwardly directed transcellular movement of negative charge. Application of ouabain (5 mmol l(−)(1)) to the basolateral superfusate resulted in the almost total abolition of the I(sc) (reduced from −531 to −47 microA cm(−)(2)), suggesting that the Na(+)/K(+)-ATPase is the driving force for active, electrogenic uptake of NaCl. In contrast, mean I(sc) values close to zero were found in preparations of all exopodites and in endopodites 1 and 2. The specific activities of Na(+)/K(+)-ATPase corresponded with these results. Specific activities were highest in posterior endopodites 3–5 and depended on ambient salinity. In all other rami, the activities were much lower and independent of ambient salinity. Activities in posterior endopodites 3–5 were lowest in isopods acclimated to 30 salinity (2–4 micromol P(i)mg(−)(1)protein h(−)(1)), increased in individuals kept in 20 salinity (8.4 micromol P(i)mg(−)(1)protein h(−)(1)) and were highest in isopods acclimated to 15 salinity (18.2 micromol P(i)mg(−)(1)protein h(−)(1)). When specimens were transferred from 30 to 40 salinity, Na(+)/K(+)-ATPase activity increased in the posterior endopodites. The electrophysiological and Na(+)/K(+)-ATPase activity measurements show that active electrogenic ion transport in this species occurs almost exclusively in posterior endopodites 3–5. The endopodite of the fifth pleopod of I. baltica exhibited a microscopic structure remarkably similar to that described for the lamellae of the phyllobranchiae of brachyurans. It is composed of two opposed epithelial monolayers of ionocytes, each covered by cuticle. Bundles of pillar cells are located within the ionocyte layers, which are separated by a fenestrated lamellar septum of connective tissue. The results obtained in this study indicate that endopodites 3–5 play the main role in osmoregulatory ion uptake of the isopod I. baltica. Moreover, the Na(+)/K(+)-ATPase is the only driving force behind active electrogenic ion uptake across the epithelial cells.

Cadmium influx and efflux across perfused gills of the shore crab, Carcinus maenas

Cadmium influx across perfused gills of the shore crab Carcinus maenas exposed to 9.0 µM cadmium in the external medium ranged from 0.6 to 2.0 nmol Cd(2+) g(-1) gill wet wt. per h. Cadmium efflux across perfused gills exposed to 9.0 µM cadmium in the internal medium ranged from 0.3 to 0.9 nmol Cd(2+) g(-1) gill wet wt. per h. There was no significant difference between cadmium influx and efflux across the gills. Cadmium influx across the gills was not detectable after exposure to lanthanum in the external medium, whereas cadmium efflux was unaffected by external as well as internal lanthanum. Cadmium influx was not affected when the internal medium was changed to Na-free medium, whereas efflux showed a fourfold increase when the gills were perfused with Na-free medium. Low pH in the external medium did not exert any significant effect on cadmium influx and efflux.

Na(+) and Ca(2+) pumps in the gills, epipodites and branchiostegites of the european lobster Homarus gammarus: effects of dilute sea water

Crude homogenates and plasma-membrane-enriched fractions were prepared from the epithelium of the gills, epipodites and branchiostegites of intermoult European lobsters Homarus gammarus, and Na(+)/K(+)-ATPase, Ca(2+)-ATPase and Na(+)/Ca(2+) exchange activities were quantified in these tissues. Lobsters were kept in sea water (salinity 35) or were adapted to dilute sea water (22.1). The lobster hyperregulates haemolymph osmolarity and Ca(2+) levels in both media. Homogenates of the podobranchs, arthrobranchs and pleurobranchs had comparable Na(+)/K(+)-ATPase specific activities, and mean activities increased significantly for all three types of gills when the animals were kept in dilute sea water. In the epipodites and branchiostegites, Na(+)/K(+)-ATPase specific activities exceeded those in the gills, and exposure to dilute sea water greatly enhanced these activities. In sea water, 80 % of the total Na(+)/K(+)-ATPase activity is associated with the gills and epipodites (each tissue containing 40 %) and 20 % with the branchiostegites; in dilute sea water, the gills contained approximately 25 %, the epipodites 40 % and the branchiostegites approximately 35 % of the total activity, indicating the relative importance of the epipodites and branchiostegites for ionic hyperregulation in dilute media. In plasma membrane vesicles isolated from the gills, epipodites and branchiostegites, Ca(2+) transport driven by ATP and by a Na(+)gradient was demonstrated. Exposure to dilute sea water enhanced Na(+)/Ca(2+)exchange and Ca(2+)-ATPase activities in the epipodites and branchiostegites; in the gills, however, Ca(2+) transport activities decreased. The role of these tissues and enzymes in Na(+) and Ca(2+) handling by the lobster is discussed.

Ultracytochemical location of Na(+)/K(+)-atpase activity and effect of high salinity acclimation in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii (Crustacea, Decapoda)

Accumulation sites of lead phosphate reaction product consequent to Na(+)/K(+)-ATPase activity in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii were located ultracytochemically by para-nitrophenyl-phosphate hydrolysis and lead precipitation, and quantified per unit membrane area and cytoplasmic volume. In shrimps in freshwater (<0.5 per thousand S, 20 mOsm/kg H(2)O, 0.7 mEq Na(+)/liter), numerous sites of electron-dense, Na(+)/K(+)-ATPase reaction product accumulation were demonstrated in the membrane invaginations of the mitochondria-rich, intralamellar septal cells (12.5 +/- 1.7 sites/microm(2) membrane, 179 +/- 22 sites/microm(3) cytoplasm, mean+/- SEM, N </= 7) and in the basal region of the medial renal tubules (19.8 +/- 1.8 sites/microm(2) membrane, 437 +/- 53 sites/microm(3) cytoplasm), but not in the pillar cells whose apical flanges form the primary interface with the external medium. A putative, ouabain-insensitive Na(+)- or H(+)-ATPase was found in the apical microvilli of the medial renal tubules (17.4 +/- 1.7 sites/microm(2) membrane, 629 +/- 101 sites/microm(3) cytoplasm). This restricted location of Na(+)/K(+)-ATPase activity within the gill epithelium suggests that during uptake, Na(+) moves across the apical pillar cell membrane, passes through specialized, basolateral coupling junctions into the septal cell cytoplasm and is pumped into the hemolymph via the Na(+)/K(+)-ATPase in the invagination membranes. In shrimps acclimated to a high-salinity medium (21 per thousand S, 630 mOsm/kg H(2)O, 280 mEq Na(+)/liter) for 2 and 5 days, the mean number of sites of para-nitrophenylphosphatase activity/microm(2) membrane and /microm(3) cytoplasm for both epithelia increases markedly by 83 and 163%, respectively. However, after 10 days acclimation, the number of sites declines dramatically, attaining values far below those for shrimps in freshwater. These acclimation-induced alterations in numerical density/microm(3) cytoplasm cannot be accounted for by corresponding changes in membrane surface density (microm(2) membrane/microm(3) cytoplasm) and reflect a real alteration in the number of Na(+)/K(+)-ATPase reaction product sites/unit membrane area. These data suggest that neither the gill nor the renal Na(+)/K(+)-ATPase systems function at maximal activity in shrimps in freshwater, possibly due to the low Na(+) concentration, and are initially stimulated by the increase in external ionic concentration. However, these powerful Na(+) transport systems respond to salt loading by a notable reduction in the number of hydrolysis sites, possibly through the incorporation of the Na(+)/K(+)-ATPase into isolated membrane vesicles in the basal invaginations of the medial renal tubules, together with ultrastructural alterations like the spatial isolation of the mitochondria by multiple membrane stacks in the intralamellar septal cells. J. Exp. Zool. 284:617-628, 1999.

Membrane particle distribution in the sternal epithelia of the terrestrial isopod Porcellio scaber latr. (Crustacea, oniscidea) during CaCO(3) deposit formation and resorption, a freeze-etch analysis

The anterior sternal epithelium of terrestrial isopods transports cuticular Ca(2+) to and from large sternal CaCO(3) deposits. We analyzed the anterior and posterior sternal epithelium by the means of the freeze-etch technique and measured the size distribution and density of intramembrane particles (IMPs) during three different molting stages. At least three IMP size classes around 4.5, 7.7, and 9.4 nm can be distinguished on the P-face of the apical and basolateral plasma membrane. An additional size class of around 12.8 nm is restricted to the apical compartment. In the anterior sternal epithelium, the density of these large particles changes by a factor of 1.9 during the molt cycle, suggesting a role in CaCO(3) formation and/or resorption. The density of the smaller IMPs rises transiently by a factor of 1.3 in the posterior sternal epithelium only. The IMP density of the basolateral plasma membrane increases significantly by a factor of 1.4 and 1.3 in the anterior and posterior sternal epithelia, respectively. The results indicate that increases in the IMP density contribute to the differentiation to an increased transport activity during the cyclic enlargements of the plasma membrane surface area in the anterior sternal epithelium.