Immune defense reduces respiratory fitness in Callinectes sapidus, the Atlantic blue crab

Crustacean gills function in gas exchange, ion transport, and immune defense against microbial pathogens. Hemocyte aggregates that form in response to microbial pathogens become trapped in the fine vasculature of the gill, leading to the suggestion by others that respiration and ion regulation might by impaired during the course of an immune response. In the present study, injection of the pathogenic bacterium Vibrio campbellii into Callinectes sapidus, the Atlantic blue crab, caused a dramatic decline in oxygen uptake from 4.53 to 2.56 micromol g-1 h-1. This decline in oxygen uptake is associated with a large decrease in postbranchial PO2, from 16.2 (+/-0.46 SEM, n=7) to 13.1 kPa (+/-0.77 SEM, n=9), while prebranchial PO2 remains unchanged. In addition, injection of Vibrio results in the disappearance of a pH change across the gills, an indication of reduced CO2 excretion. The hemolymph hydrostatic pressure change across the gill circulation increases nearly 2-fold in Vibrio-injected crabs compared with a negligible change in pressure across the gill circulation in saline-injected, control crabs. This change, in combination with stability of heart rate and branchial chamber pressure, is indicative of a significant increase in vascular resistance across the gills that is induced by hemocyte nodule formation. A healthy, active blue crab can eliminate most invading bacteria, but the respiratory function of the gills is impaired. Thus, when blue crabs are engaged in the immune response, they are less equipped to engage in oxygen-fueled activities such as predator avoidance, prey capture, and migration. Furthermore, crabs are less fit to invade environments that are hypoxic.     

Salinity dependent Na+-K+ATPase activity in gills of the euryhaline crab Chasmagnathus granulata

The occurrence and response of Na+-K+ATPase specific activity to environmental salinity changes were studied in gill extracts of all of the gills of the euryhaline crab Chasmagnathus granulata from Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina). All of the gills exhibited a salinity dependent Na+-K+ATPase activity, although the pattern of response to environmental salinity was different among gills. As described in other euryhaline crabs highest Na+-K+ATPase specific activity was found in posterior gills (6 to 8), which, with exception of gill 6, increased upon acclimation to reduced salinity. However, a high increase of activity also occurred in anterior gills (1 to 5) in diluted media. Furthermore, both short and long term differential changes of Na+-K+ATPase activity occurred among the gills after the transfer of crabs to reduced salinity. The fact that variations of Na+-K+ATPase activity in the gills were concomitant with the transition from osmoconformity to ionoregulation suggests that this enzyme is a component of the branchial ionoregulatory mechanisms at the biochemical level in this crab.     

Distribution and abundance of a nemertean egg predator ( Carcinonemertes sp.) on a leucosiid crab,Randallia ornata

The purple globe crab, Randallia ornata (Randall, 1839) (Decapoda: Leucosiidae) is a common crustacean found on sandy beaches from Northern California to Baja California, Mexico. An undescribed species of Carcinonemertes, which shares biological features with a partially described worm from Tasmania was recovered from R. ornata. The worm reaches 10 mm in length and secretes a sturdy tube with a distinctive spiral shape. The total prevalence of the worm on R. ornatawas 70% (85% for ovigerous female crabs, 67% for post-ovigerous female crabs, and 45% for male crabs). Intensities reached 32 worms per crab. Adult worms were found in crab egg masses, regressed adults were recovered from under the abdomen of post-ovigerous females, and encysted juveniles were found on the gill lamellae of non-ovigerous female crabs and occasionally from the gill lamellae of male crabs.     

Metallothionein-like proteins in the blue crab Callinectes sapidus: Effect of water salinity and ions

The effect of water salinity and ions on metallothionein-like proteins (MTLP) concentration was evaluated in the blue crab Callinectes sapidus. MTLP concentration was measured in tissues (hepatopancreas and gills) of crabs acclimated to salinity 30 ppt and abruptly subjected to a hypo-osmotic shock (salinity 2 ppt). It was also measured in isolated gills (anterior and posterior) of crabs acclimated to salinity 30 ppt. Gills were perfused with and incubated in an isosmotic saline solution (ISS) or perfused with ISS and incubated in a hypo-osmotic saline solution (HSS). The effect of each single water ion on gill MTLP concentration was also analyzed in isolated and perfused gills through experiments of ion substitution in the incubation medium. In vivo, MTLP concentration was higher in hepatopancreas than in gills, being not affected by the hypo-osmotic shock. However, MTLP concentration in posterior and anterior gills significantly increased after 2 and 24 h of hypo-osmotic shock, respectively. In vitro, it was also increased when anterior and posterior gills were perfused with ISS and incubated in HSS. In isolated and perfused posterior gills, MTLP concentration was inversely correlated with the calcium concentration in the ISS used to incubate gills. Together, these findings indicate that an increased gill MTLP concentration in low salinity is an adaptive response of the blue crab C. sapidus to the hypo-osmotic stress. This response is mediated, at least in part, by the calcium concentration in the gill bath medium. The data also suggest that the trigger for this increase is purely branchial and not systemic.     

Salinity adaptation of HCO3--dependent ATPase activity in the gills of blue crab (Callinectes sapidus)

A bicarbonate-dependent ATPase (EC 3.6.1.3) was found in microsomal preparations from blue crab gills. When the crabs were transferred to low salinity (200 mosmolal) from seawater (1000 mosmolal), the HCO3- dependent ATPase increased in all gill pairs, reaching its new steady state in 2 weeks. The greatest increase occurred in the sixth and seventh gill pairs (approx. 2.5-fold). Maximal enzyme activity was observed at an Mg2+ concentration of 2 mM and an optimal pH of 7.8. The apparent Ka for HCO3- was found to be 8.9 mM. Kinetic analysis showed that low-salinity adaptation increased the Vmax without altering the Km for ATP. When the microsomes from high-salinity crab gills were treated with detergent or assayed at different temperatures, the total enzyme activity did not reach the activity levels after adaptation to low salinity. These results suggest that the alteration of HCO3- -ATPase activity may be due to synthesis, rather than modulation of membranes or of the existing enzyme activity.     

CO2 Excretion Across Isolated Perfused Crab Gills: Facilitation by Carbonic Anhydrase

Crab gill carbonic anhydrase is shown to facilitate the excretionof carbondioxide across isolated perfused gills. A techniquefor perfusing crab gills and assessing the metabolic viabilityof perfused gills is also described in detail. The techniqueis used to follow the disappearance of ¹⁴C label as HCO₃^–and CO₂ from internal perfusate passing through the gill. Theexcretion of the label increases with the flow rate of the externalperfusate across the outside of the gills. The addition of carbonican hydrase to the internal perfusate results in a two- to fourfoldincrease in the excretion of label while Diamox (acetazolamide)treatment decreases the excretion of label by half. It is alsosuggested that carbonic anhydrase, present in muscle tissuesof crabs, minimizes the disequilibrium of the hemolymph CO₂system as metabolically produced CO₂ leaves the tissues andenters the hemolymph. Parallels are drawn between the presenceof carbonic anhydrase in the crab gill system and the presenceof this enzyme in the respiratory organs of both aquatic andterrestrial animals.     

NaCl absorption across split gill lamellae of hyperregulating crabs: Transport mechanisms and their regulation

The method of mounting split lamellae of crab gills in modified Ussing chambers offers the advantage that active ion transport can be measured as short-circuit current and/or flux of radioactive tracers in relation to the epithelial surface. Moreover, further modern techniques like microelectrode impalements and current-noise analysis can be applied. The epithelium of posterior gills of Chinese crabs (Eriocheir sinensis) acclimated to fresh water actively absorbs Na⁺ and Cl⁻ independent of each other. The epithelium of the gills of shore crabs (Carcinus maenas) acclimated to brackish water actively absorbs NaCl in a coupled mode. The different osmotic gradients maintained by the two crab species are reflected in the characteristics of their gill epithelia. Chinese crabs, migrating to fresh water, have a tight gill epithelium. The gill epithelium of shore crabs, living in brackish water of at least 6–8‰ salinity, is an intermediate between tight and leaky. Regulation of NaCl absorption across the gill epithelium of Chinese crabs is achieved in a hormone-independent way by the haemolymph side osmolarity (autoregulation). Moreover, NaCl absorption is regulated by a hormonal factor of so far unknown chemical nature in the eyestalk extract which stimulates the transport rates via a cAMP-dependent signal transduction pathway, activating apical V-ATPase activity and increasing the number of open apical Na⁺ channels.     

Sex-specific mediation of foraging in the shore crab, Carcinus maenas

Experiments were conducted to investigate the sex-specific differences to feeding responses of the shore crab Carcinus maenas throughout the year. Results demonstrate that female shore crabs exhibit stronger feeding responses than males throughout the year with a significantly reduced feeding response in males during the summer months' reproductive season. We also studied the possible function(s) of the moulting hormone, 20-hydroxyecdysone (Crustecdysone) that has been described as a potential female-produced sex pheromone to initiate male reproductive behaviour in a number of crustaceans. We recently presented evidence that for shore crabs this is not the case and now show that the steroid is instead functioning as a sex-specific feeding deterrent protecting the moulting 'soft' female crabs. Whilst male shore crabs were deterred from prey (Mytilus edulis) and synthetic feeding stimulants glycine and taurine when these feeding stimulants were spiked with crustecdysone, intermoult female crabs were significantly less affected and rarely deterred from feeding. This sex specificity of the moulting hormone, in combination with the female sex pheromone, which has no anti-feeding properties, ensures that male crabs mate with soft-shelled, moulted females rather than engage in cannibalism, such as found frequently in cases when soft-shelled females are exposed to intermoult females.     

Effect of microcystin on ion regulation and antioxidant system in gills of the estuarine crab Chasmagnathus granulatus (Decapoda,Grapsidae)

The objective of this work was to evaluate mechanisms of microcystin toxicity on crustacean species. Adult male crabs of Chasmagnathus granulatus (13.97+/-0.35 g) acclimated to low salinity (2 per thousand ) were injected with saline (control) or Microcystis aeruginosa aqueous extract (39.2 microg/l) at 24 h intervals for 48 h. After the exposure period, the anterior and posterior gills were dissected, measuring Na(+),K(+)-ATPase and glutathione-S-transferase (GST) activity. Total oxyradical scavenging capacity (TOSC) and lipid peroxides (LPO) content were also determined. Na(+),K(+)-ATPase activity in anterior gills was significantly lower in crabs injected with toxin than in control crabs, while no significant difference in the enzyme activity was detected in posterior gills. Both sodium and chloride concentration in the hemolymph were not affected by toxin exposure. Significant changes in GST activity were detected in posterior gills, with higher values being observed in the toxin-injected crabs. Crabs exposed to microcystin also showed a significant increase in the TOSC value against peroxyl radicals, for both anterior and posterior gills. Lipid peroxides level did not change in both gill types after exposure to the toxin. The increased levels of TOSC suggest the occurrence of a crab response against oxidative stress induced by toxin injection, which prevents lipid peroxidation.     

Effects of eyestalk ablation on carbonic anhydrase activity in the euryhaline blue crab Callinectes sapidus: neuroendocrine control of enzyme expression

Carbonic anhydrase (CA) activity in the gills of the euryhaline blue crab, Callinectes sapidus, was measured in response to acute low-salinity transfer and treatment with eyestalk ablation (ESA) in an attempt to elucidate potential regulatory mechanisms of salinity-mediated CA induction. ESA alone resulted in an approximate doubling of CA activity in the posterior, ion-transporting gills of crabs acclimated to 35 ppt. Transfer of intact crabs to 28 ppt, a salinity at which the blue crab is still an osmotic and ionic conformer, had no effect on CA activity, but treatment with ESA prior to transfer resulted in a 5-fold increase. Hemolymph osmolality was unaffected by ESA. There was a 7-fold induction of CA activity in posterior gills of intact crabs transferred from 35 to 15 ppt, and this was potentiated by about 100% by ESA. Hemolymph osmolality was slightly elevated in the ESA-treated crabs. CA activity in anterior gills did not increase in response to any treatment. Hemolymph concentrations of methyl farnesoate (MF) were measured for all experimental animals. MF concentrations were undetectable in all intact crabs, regardless of salinity. Treatment with ESA resulted in elevated levels of hemolymph MF, but these levels were still relatively low and unrelated to salinity. These results suggest that CA induction is under the control of a regulatory substance located in the eyestalk. This substance appears to be a CA repressor, keeping CA expression at low levels in the gills of crabs acclimated to high salinity. Exposure to low salinity, or treatment with ESA, removes the effects of this putative repressor and allows CA induction to occur.     

Bdellovibrios in Callinectus sapidus, the Blue Crab

Bdellovibrios were recovered from the gill tissue of all of 31 crabs sampled and from all samples of epibiota obtained from the ventral shell surface of 15 crabs. The results suggest that the blue crab is a reservoir for bdellovibrios. The association with crabs may be an important factor in the ecology of the bdellovibrios.     

Effect of salinity on osmoregulatory patch epithelia in gills of the blue crab Callinectes sapidus

In euryhaline crabs, ion-transporting cells are clustered into osmoregulatory patches on the lamellae of the posterior gills. To examine changes in the branchial osmoregulatory patch in the blue crab Callinectes sapidus in response to change in salinity and to correlate these changes with other osmoregulatory responses, crabs were acclimated to a range of salinities between 10 and 35 ppt. When crabs that had been acclimated to 35 ppt were subsequently transferred to 10 ppt, both the size of the osmoregulatory patch on individual gill lamellae and the specific activity of Na+, K+-ATPase in whole-gill homogenates increased only after the first 24 h of exposure to dilute seawater. Enzyme activity and size of patch area increased gradually and reached their maxima (increasing by 200% and 60%, respectively) 6 days following transfer to 10 ppt seawater and then remained at these levels. Patch size at acclimation varied inversely with the salinity for seawater dilutions below 26 ppt (the isosmotic point of the crab), although it did not vary in salinities at or above 26 ppt. Thus, the size of the patch clearly is modulated with acclimation salinity, but it increases only in those salinities in which the crab hyperosmoregulates. An increase in the total RNA/DNA ratio in gill homogenates, the lack of mitotic figures in the lamellae, and the lack of incorporation of bromodeoxyuridine into nuclei of lamellar epithelial cells during acclimation to dilute seawater were interpreted as evidence that no cell proliferation had occurred and that increases in the size of the osmoregulatory patch occurred through differentiation of existing gas exchange cells or of undifferentiated epithelial cells into ion-transporting cells.     

Copper effects on key metabolic enzymes and mitochondrial membrane potential in gills of the estuarine crab Neohelice granulata at different salinities

The estuarine crab Neohelice granulata was exposed (96h) to a sublethal copper concentration under two different physiological conditions (hyperosmoregulating crabs: 2ppt salinity, 1mg Cu/L; isosmotic crabs: 30ppt salinity, 5mg Cu/L). After exposure, gills (anterior and posterior) were dissected and activities of enzymes involved in glycolysis (hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase), Krebs cycle (citrate synthase), and mitochondrial electron transport chain (cytochrome c oxidase) were analyzed. Membrane potential of mitochondria isolated from anterior and posterior gill cells was also evaluated. In anterior gills of crabs acclimated to 2ppt salinity, copper exposure inhibited hexokinase, phosphofructokinase, pyruvate kinase, and citrate synthase activity, increased lactate dehydrogenase activity, and reduced the mitochondrial membrane potential. In posterior gills, copper inhibited hexokinase and pyruvate kinase activity, and increased citrate synthase activity. In anterior gills of crabs acclimated to 30ppt salinity, copper exposure inhibited phosphofructokinase and citrate synthase activity, and increased hexokinase activity. In posterior gills, copper inhibited phosphofructokinase and pyruvate kinase activity, and increased hexokinase and lactate dehydrogenase activity. Copper did not affect cytochrome c oxidase activity in either anterior or posterior gills of crabs acclimated to 2 and 30ppt salinity. These findings indicate that exposure to a sublethal copper concentration affects the activity of enzymes involved in glycolysis and Krebs cycle, especially in anterior (respiratory) gills of hyperosmoregulating crabs. Changes observed indicate a switch from aerobic to anaerobic metabolism, characterizing a situation of functional hypoxia. In this case, reduced mitochondrial membrane potential would suggest a decrease in ATP production. Although gills of isosmotic crabs were also affected by copper exposure, changes observed suggest no impact in the overall tissue ATP production. Also, findings suggest that copper exposure would stimulate the pentose phosphate pathway to support the antioxidant system requirements. Although N. granulata is very tolerant to copper, acute exposure to this metal can disrupt the energy balance by affecting biochemical systems involved in carbohydrate metabolism.     

Gill area, permeability and Na+ ,K+ -ATPase activity as a function of size and salinity in the blue crab, Callinectes sapidus

Juvenile blue crabs, Callinectes sapidus, extensively utilize oligohaline and freshwater regions of the estuary. With a presumptively larger surface-area-to-body weight ratio, juvenile crabs could experience osmo- and ionoregulatory costs well in excess of that of adults. To test this hypothesis, crabs ranging over three orders of magnitude in body weight were acclimated to either sea water (1,000 mOsm) or dilute sea water (150 mOsm), and gill surface area, water and sodium permeabilities (calculated from the passive efflux of 3H2O and 22Na+), gill Na+, K+ -ATPase activity and expression were measured. Juveniles had a relatively larger gill surface area; weight-specific gill surface area decreased with body weight. Weight-specific water and sodium fluxes also decreased with weight, but not to the same extent as gill surface area; thus juveniles were able to decrease gill permeability slightly more than adults upon acclimation to dilute media. Crabs < 5 g in body weight had markedly higher activities of gill Na+ ,K+ -ATPase than crabs > 5 g in both posterior and anterior gills. Acclimation to dilute medium induced increased expression of Na+, K+ -ATPase and enzyme activity, but the increase was not as great in juveniles as in larger crabs.The increased weight-specific surface area for water gain and salt loss for small crabs in dilute media presents a challenge that is incompletely compensated by reduced permeability and increased affinity of gill Na+, K+ -ATPase for Na+. Juveniles maintain osmotic and ionic homeostasis by the expression and utilization of extremely high levels of gill Na+, K+ -ATPase, in posterior, as well as in anterior, gills.     

Response to environmental salinity of Na-KATPase activity in individual gills of the euryhaline crab Cyrtograpsus angulatus

The occurrence, localization and response to environmental salinity changes of Na⁺-K⁺ATPase activity were studied in each of the individual gills 4-8 of the euryhaline crab Cyrtograpsus angulatus from Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina). Na⁺-K⁺ATPase activity appeared to be differentially sensitive to environmental salinity among gills. Upon an abrupt change to low salinity, a differential response of Na⁺-K⁺ATPase activity occurred in each individual gill which could suggest a differential role of this enzyme in ion transport process in the different gills of C. angulatus. With the exception of gill 8, a short-term increase of Na⁺-K⁺ATPase specific activity was observed in posterior gills, which is similar to adaptative variations of this activity described in other euryhaline crabs. However, and conversely to that described in other hyperregulating crabs, the highest increase of activity occurred in anterior gills 4 by 1 day after the change to dilute media which could suggest also a role for these gills in ion transport processes in C. angulatus. The fact that variations of Na⁺-K⁺ATPase activity in anterior and posterior gills were concomitant with the transition to hyperregulation indicate that this enzyme could be a component of the branchial ionoregulatory mechanisms at the biochemical level in this crab. The results suggest a differential participation of branchial Na⁺-K⁺ATPase activity in ionoregulatory mechanisms of C. angulatus. The possible existence of functional differences as well as distinct regulation mechanisms operating in individual gills is discussed.     

On the growth of the blue king crab (<Emphasis Type="Italic">Paralithodes platypus</Emphasis>) population in the Peter the Great Bay of the sea of Japan

The causes of the appearance of large blue king crabs (Paralithodes platypus) in Peter the Great Bay for the last decade are discussed. This species is an important commercial resource in the waters of Russian Far Eastern seas, and its general concentrations are related mainly to the sublittoral and upper bathyal zones of the northwestern Bering Sea and the northern Sea of Okhotsk. Until recently, this species has been observed in areas along the continental coast of the northwestern Sea of Japan up to the Peter the Great Bay, where it incidentally showed up in red king crab (P. camtschaticus) and snow crab (Chionoecetes opilio) catches but was also commercially used. This area was considered as the southern periphery of the species range. Since the late 1990s, both male and female blue king crabs have been recorded in trawl and trap catches during research works conducted within the Peter the Great Bay. Since 2002, any commercial catches of shelf crab species are prohibited in the waters south of 47°20′ N because of a dramatic decline in their populations. Since then all the illegally caught crabs, including blue king crabs that are seized live from poachers, are released back into the water in certain places of the bay. In total, at least 29 503 blue king crabs, including egg-bearing females, were released within the period from 2002 to November 2009. At present, the overall blue king crab abundance in Peter the Great Bay, estimated based on the trap catches over an area of 7048 km², is 50500, the abundance of commercial-size males (with a carapace width over 130 mm) is 7500, and the male to female ratio is 1.00: 1.35. The increase in the blue king crab population observed in the bay is the result of the immigration of mature and viable individuals from other areas of its range. After this “uncontrolled introduction” blue king crabs adapted to new conditions, and then began breeding and spreading over the entire area of the bay.     

Hematodinium sp. infection of red Paralithodes camtschaticus and blue Paralithodes platypus king crabs from the Sea of Okhotsk, Russia

A disease caused by a parasitic dinoflagellate of the genus Hematodinium was identified in red, Paralithodes camtschaticus, and blue, Paralithodes platypus, king crabs from the north-east region of the Sea of Okhotsk, Russia, during annual stock surveys. No carapace color change was observed even in heavily infected crabs, but diseased crabs possessed creamy-yellow hemolymph, which was visible through the arthrodial membranes of the abdomen and appendages. Several stages of the parasite’s life history, including trophonts, plasmodia, sporonts and macrodinospores, were observed in tissues of infected king crabs. Numerous parasite cells were observed in the lumina of the myocardium, the gills, the connective tissue of antennal glands and the sinuses of nerve ganglia, eyestalks and gastrointestinal tract of king crabs with gross signs of infection. Based on sequencing of the 18S rDNA, it appears that the Hematodinium sp. found in red and blue king crabs is identical or closely related to Hematodinium sp. isolated from crabs of the genera Chionoecetes and Lithodes. Observed prevalences were 0.33% in sublegal male red king crabs, 0.18% in female red king crabs, 0.34% in sublegal male blue king crabs and 0.31% in female blue king crabs.     

Modulation of expression of SOD isoenzymes in mud crab (Scylla serrata): effects of inhibitors,salinity and season

Presence of several isoenzymes of superoxide dismutase (SOD) were demonstrated in tissues (abdominal muscle: 7 number, hepatopancreas: 13 number and gills: 7 number) of mud crabs (Scylla serrata) by employing specific staining of the enzyme in native-PAGE. SOD isoenzymes in tissues of mud crab were found to be thermolabile. The intensity of a major SOD band in tissues of crabs was reduced by the treatment of H₂O₂ or chloroform:ethanol. KCN treatment resulted in splitting of that major SOD band into two or more distinct bands. SDS treatment resulted in disruption of SOD bands. A sex-specific SOD isoenzyme band of higher molecular weight was observed in gills and muscle in winter and summer seasons, respectively. The observed different SOD isoenzyme pattern in tissues at altered salinities and seasons suggests separate tissue-specific antioxidant adaptation strategies of crabs against abiotic factors.     

稻田网箱养殖辽河水系中华绒螯蟹幼蟹的个体生长

本实验在室外网箱内监测辽河水系中华绒螯蟹(Eriocheir sinensis)幼蟹阶段个体生长发育的每一次蜕壳生长情况。2013年实验选取雄蟹和雌蟹各200只,记录了每个蜕壳阶段雌蟹和雄蟹的生长,包括壳长、壳宽、总重和蜕壳间隔时长(d),并且观察雌蟹和雄蟹形态特征的变化。实验共进行111 d,幼蟹共蜕壳11次。实验结束时,雄蟹剩余34只,雌蟹剩余42只;雄蟹的特定生长率为(7.176 5±0.168 4)%/d,雌蟹的特定生长率为(7.283 3±0.174 3)%/d;雌蟹生长蜕壳过程中腹部的形态变化大,腹部由三角形变成卵圆形;雄蟹在生长蜕壳过程中螯足的增长明显较雌蟹快,并且在本实验最后一次即第11次蜕壳后螯足腹面内侧出现1小撮绒毛,外侧也出现少量绒毛,但不易被发现。     

Salinity adaptation of HCO3−-dependent ATPase activity in the gills of blue crab (Callinectes sapidus)

A bicarbonate-dependent ATPase (EC 3.6.1.3) was found in microsomal preparations from blue crab gills. When the crabs were transferred to low salinity (200 mosmolal) from seawater (1000 mosmolal), the HCO₃^?-dependent ATPase increased in all gill pairs, reaching its new steady state in 2 weeks. The greatest increase occurred in the sixth and seventh gill pairs (approx. 2.5-fold). Maximal enzyme activity was observed at an Mg²⁺ concentration of 3 mM and an optimal pH of 7.8. The apparent K_a for HCO₃^? was found to be 8.9 mM. Kinetic analysis showed that low-salinity adaptation increased the V_max without altering the K_m for ATP. When the microsomes from high-salinity crab gills were treated with detergent or assayed at different temperatures, the total enzyme activity did not reach the activity levels seen after adaptation to low salinity. These results suggest that the alteration of HCO₃^?-ATPase activity may be due to synthesis, rather than modulation of membranes or of the existing enzyme activity.