Specialized adaptations allow vent-endemic crabs (Xenograpsus testudinatus) to thrive under extreme environmental hypercapnia

Low high-temperature tolerance of the hydrothermal vent crab Xenograpsus testudinatus: Thermal biochemistry, survival rate and histological analyses, and habitat investigations

Rising environmental temperatures increase stress on marine organisms, causing physiological damage or death in extreme cases. In this study, we examined the crab species Xenograpsus testudinatus from a shallow hydrothermal vent area in Kueishan Island, off northeast Taiwan. This crab species belongs to the monotypic family Xenograpsidae and is a well-known inhabitant of the shallow hydrothermal vents of Kueishan Island. The maximum temperature tolerance of X. testudinatus and the effects of temperature on this crab species remain unclear. Our laboratory experiments demonstrated the effects of elevated seawater temperatures on the survival and enzymatic function of X. testudinatus. Its mortality rate increased with the elevation of seawater temperature from 25 °C to 40 °C. Survival duration was short (<4 h) when the crabs were reared at 40 °C, and the lethal temperature (LT50; death within 7 days of exposure) was 34.74 °C. The response of six enzymes varied with temperature. Significant differences of enzyme activities were noted among various temperature groups (25 °C, 30 °C, 30 °C, and 40 °C) in the gill, but no significant difference was noted in the hepatopancreas. A univariate analysis indicated the temperature level and exposure duration as key factors driving the variations in enzyme activity. A field investigation on the temperature of the seabed where the crabs aggregated revealed that the preferred temperature was <28 °C. Therefore, X. testudinatus does not prefer elevated temperatures. An expected combined effects of temperature tolerance and acidic, sulfuric-rich vent water on X. testudinatus is warrant further investigation.

Pathological and biochemical effects of polyethylene microplastic exposure in hydrothermal vent crab, Xenograpsus testudinatus

Microplastic pollution significantly threatens marine ecosystems, including those with unique adaptations. This study evaluates the implications of polyethylene microplastics (PE-MPs) on the hydrothermal vent crab, Xenograpsus testudinatus. Crabs were exposed to varying fluorescent green polyethylene microspheres (FGPE) concentrations for 7 days. Histological analysis revealed severe damage to the hepatopancreas and gills at higher FGPE concentrations. Antioxidant enzyme activities (SOD, CAT, GR, GST, GPx, LPO) and gene expression (sod, cat) were assessed to evaluate oxidative stress responses. Results indicated significant upregulation of SOD and CAT activities at lower FGPE concentrations, suggesting an initial antioxidant response. However, GR and GST activities were inhibited at higher concentrations, and oxidative stress markers increased. These findings proved that PE-MPs induce molecular oxidative damage and lead to possible oxidative responses. Despite their resilience to extreme environments, HV crabs are vulnerable to pollutant-induced stressors, which pose ecological risks to species interactions and population dynamics within vent ecosystems.

Effects of elevated CO2 on the critical oxygen tension (Pcrit) and aerobic metabolism of two oxygen minimum zone (OMZ) hypoxia tolerant squat lobster species

Marine invertebrates living in oxygen minimum zones (OMZ), where low pO2 and high pCO2 conditions co-occur, display physiological mechanisms allowing them to deal with these coupled stressors. We measured aerobic metabolic rate (MRa) and the critical oxygen tension (Pcrit), and calculated the oxygen supply capacity (α) of both the red (Grimothea monodon) and yellow (Grimothea johni) squat lobsters, under two pCO2 scenarios (~414 and 1400 μatm). We also measured haemolymph pH, haemocyanin oxygen binding affinity (p50), and haemolymph lactate content in both species under normoxia, low pCO2 hypoxia and high pCO2 hypoxia. Our results revealed that both species show extremely low Pcrit and P50 values. The MRa increased under high pCO2 condition in both species but hypoxia tolerance was not negatively impacted by pCO2. Furthermore, hypoxia tolerance is enhanced at high pCO2 in the yellow squat lobster, and although not statically significant, α value follows the same trend. The red squat lobster has a better pHe regulation and lower reliance on anaerobic metabolism. While the yellow squat lobster had a poorer pHe regulation during high pCO2 hypoxia, relying more on anaerobic metabolism. Our research suggests that elevated pCO2 is crucial on respiratory processes in hypoxia tolerant organisms, ameliorating the effects of hypoxia alone. Learning from OMZ adapted species contribute to better predicting climate change consequences on these important ecosystems.

The role of the antennal glands and gills in acid-base regulation and ammonia excretion of a marine osmoconforming brachyuran

The excretory mechanisms of stenohaline marine osmoconforming crabs are often compared to those of the more extensively characterized euryhaline osmoregulating crabs. These comparisons may have limitations, given that unlike euryhaline brachyurans the gills of stenohaline marine osmoconformers possess ion-leaky paracellular pathways and lack the capacity to undergo ultrastructural changes that can promote ion-transport processes in dilute media. Furthermore, the antennal glands of stenohaline marine osmoconformers are poorly characterized making it difficult to determine what role urinary processes play in excretion. In the presented study, ammonia excretory processes as well as related acid-base equivalent transport rates and mechanisms were investigated in the Dungeness crab, Metacarcinus magister - an economically valuable stenohaline marine osmoconforming crab. Isolated and perfused gills were found to predominantly eliminate ammonia through a microtubule network-dependent active NH4+ transport mechanism that is likely performed by cells lining the arterial pockets of the gill lamella where critical Na+/K+-ATPase detection was observed. The V-type H+-ATPase - a vital component to transbranchial ammonia excretion mechanisms of euryhaline crabs - was not found to contribute significantly to ammonia excretion; however, this may be due to the transporter's unexpected apical localization. Although unconnected to ammonia excretion rates, a membrane-bound isoform of carbonic anhydrase was localized to the apical and basolateral membranes of lamella suited for respiration. Urine was found to contain significantly less ammonia as well as carbonate species than the hemolymph, indicating that unlike those of some euryhaline crabs the antennal glands of the Dungeness crab reabsorb these molecules rather than eliminate them for excretion.

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.

Transcriptome analysis of the gills of Eriocheir sinensis provide novel insights into the molecular mechanisms of the pH stress response

Eriocheir sinensis is an important economic species in China, which is easily affected by pH changes. However, the molecular mechanism of the pH stress response in E. sinensis is still unclear. Therefore, this study aimed to examine the molecular response mechanism of E. sinensis based on pH variation surveillance, histopathological evaluation and transcriptomic analyses. Firstly, pH variation surveillance showed that E. sinensis could actively regulate the pH of its environment. Meanwhile, the histopathological evaluation suggested that pH stress seriously damaged the gills, especially at high pH. Finally, transcriptome analysis showed that the expression of genes related to ion transport, immune stress, and energy metabolism significantly changed. Many genes played an important role in the pH response of E. sinensis, such as carbonic anhydrase (CA), mitochondrial proton/calcium exchanger protein (LETM1), recombinant sodium/hydrogen exchanger 3 (SLC9A3/NHE3), heat shock protein 90 alpha family class a member (HSP90A), alkylglycerone phosphate synthase (AGPS), succinate-CoA ligase ADP-forming subunit beta (LSC2), and superoxide dismutase (SOD). Our study revealed the molecular response mechanism of E. sinensis in response to pH stress, thus providing a basis for further research on the molecular mechanism of response to pH stress in aquatic animals.

A Multi-Species Comparison and Evolutionary Perspectives on Ion Regulation in the Antennal Gland of Brachyurans

Brachyurans inhabit a variety of habitats and have evolved diverse osmoregulatory patterns. Gills, antennal glands and a lung-like structure are important organs of crabs that maintain their homeostasis in different habitats. Species use different processes to regulate ions in the antennal gland, especially those with high terrestriality such as Grapsoidea and Ocypodoidea. Our phylogenetic generalized least square (PGLS) result also suggested that there is a correlation between antennal gland NKA activity and urine-hemolymph ratio for Na⁺ concentration in hypo-osmotic environments among crabs. Species with higher antennal gland NKA activity showed a lower urine-hemolymph ratio for Na⁺ concentration under hypo-osmotic stress. These phenomenon may correlate to the structural and functional differences in gills and lung-like structure among crabs. However, a limited number of studies have focused on the structural and functional differences in the antennal gland among brachyurans. Integrative and systemic methods like next generation sequencing and proteomics method can be useful for investigating the differences in multi-gene expression and sequences among species. These perspectives can be combined to further elucidate the phylogenetic history of crab antennal glands.

Postprandial nitrogen and acid-base regulation in the seawater acclimated green crab, Carcinus maenas

The effects of feeding (meal of 3% of body mass) on acid-base and nitrogen homeostasis were investigated in the seawater acclimated green shore crab, Carcinus maenas. Feeding did not change gastric fluid pH (~pH 6); however, feeding was associated with a respiratory acidosis. Hemolymph HCO₃^- did not increase during this acidosis, although titratable and net acid efflux changed from an uptake to an excretion. Feeding affected the crabs' nitrogen homeostasis causing a substantial increase in hemolymph ammonia and urea concentrations after six hours. At this point, hemolymph urea accounted for ~1/3 of nitrogenous waste accumulated within the hemolymph, suggesting a potential role in ammonia detoxification. The postprandial increase in hemolymph ammonia coincided with an 18-fold increase in ammonia excretion rates that accounted for the majority of net acid excreted by the crabs. Urea excretion rates did not increase after feeding; however, branchial urease activity increased, implying that the gills may possess a mechanism to form excretable ammonia through the catabolism of urea. Our results demonstrate that despite an acidic gastric compartment, C. maenas does not experience a postprandial alkaline tide and that any feeding related acid-base challenges are primarily derived from metabolic acid production. Our findings also indicate that unlike the bicarbonate buffering acid-base compensatory response induced by hypercapnia and emersion, acid-base challenges upon feeding are compensated through changes in the excretion of acid equivalents, mainly in the form of ammonia.

Effects of single and dual-stressor elevation of environmental temperature and PCO2 on metabolism and acid-base regulation in the Louisiana red swamp crayfish, Procambarus clarkii

Elevation of temperature and CO₂ levels within the world's aquatic environments is expected to cause numerous physiological challenges to their inhabitants. While effects on marine ecosystems have been well studied, freshwater ecosystems have rarely been examined using a dual-stressor approach leaving our understanding of its inhabitants upon these challenges unclear. We aimed to identify the affects of elevated temperature and hypercapnia in isolation and in combination on the metabolic and acid-base regulatory processes of a freshwater crayfish, Procambarus clarkii. Crayfish were exposed to freshwater conditions that may be prevalent by the year 2100 and metabolic responses were determined after 14-days of exposure. In addition, changes in branchial mRNA expression of acid-base linked transporters were investigated. Interactions between exposure conditions influenced extracellular pH as well as the nitrogen physiology and routine metabolic rate of the crayfish. Crayfish exposed to individual and combined elevations in temperature and/or hypercapnia maintained an extracellular pH similar to that of control crayfish. Dual-stressor exposed crayfish seem to elevate the importance of ammonium as an excretable acid-equivalent based on an overall increase in the branchial mRNA expression of transporters related to ammonia excretion including the Na⁺/K⁺-ATPase, Rhesus-protein, and the V-type H⁺-ATPase. Overall, hypercapnia and dual-stressor conditions caused a metabolic depression that may have long-lasting consequences such as limited locomotion, growth, and reproduction. Future generations of crayfish given the chance to adapt over several generations may ameliorate these consequences.

Microbial interaction promotes desulfurization efficiency under high pH condition

The existence of H₂S in biogas may cause equipment corrosion and considerable SO₂ emission. Commonly used biotrickling filters may cause biogas dilution or generation of explosive mixtures. Compared with biotrickling filters, two-step process such as bioscrubber filters can overcome these shortages. However, its removal efficiency was still limited due to low microbial activity under high pH condition. Here, a bioreactor filter was carried out under pH 9.0. Removal efficiency higher than 99% was achieved under sulfide loading rate reaching 4.24 kg S m^-3d^-1. Results of network and high throughput sequencing showed that Thiobacillus acted as both dominant species (accounting for 75%) and unique kinless hub in this bioreactor. Other bacteria (accounting for 25%) contributed 75% to the network, which implied the intensive interaction between Thiobacillus and others. Sulfide removal ability and pH tolerance of pure bacteria and mixed culture were considered to verify how microbial interaction influenced them. Compared with pure bacteria, mixed culture had better performance under high pH condition, which confirmed that microbial interaction promoted desulfurization efficiency under high pH condition. These results showed that intensive microbial interaction might be the key to enhance sulfide removal efficiency under high pH condition.

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.