Ventilation of the branchial chambers in the amphibious West African freshwater crab,Sudanonautes (Convexonautes) aubryi monodi (Balss, 1929) (Brachyura,Potamonautidae)

The anatomy of the respiratory system of the savanna-zone African freshwater crab, Sudanonautes (Convexonautes) aubryi monodi [Balss, 1929], has been examined and has been found to be adapted for both aerial and aquatic gas exchange. The activities of the scaphognathites and the directions of flow of the ventilatory stream have been recorded in stressed, active and resting specimens during their exposure to a wide range of conditions from deep water to dry land.Ventilation of the branchial chambers during aquatic gas exchange in Sudanonautes kept in deep water is shown to consist of a rapid, predominantly forward water flow similar to that of fully-aquatic species. Ventilation of the branchial chambers during aerial gas exchange in Sudanonautes on land is shown to consist of a relatively slow forward air flow. This flow is continuous in post-operative crabs, pulsatile in active crabs and completely immobile in resting crabs.A second method of ventilation of the branchial chambers during aerial gas exchange is shown to consist of a pulsatile reversed air flow. This occurs (1) when Sudanonautes is kept in very shallow water and active or stressed; (2) when it has recently moved on to land; and (3) when it is completely immersed and exhibiting aerial gas exchange under water. The unusual phenomenon of aerial gas exchange under water is reported here for the first time in any species of crab.Bimodal ventilation of the branchial chambers occurs in stressed or active crabs partly immersed in shallow water. This consists of an alternation between forward water flow and reversed air flow.The morphology of the branchial chambers in Sudanonautes, and observational data on the patterns of ventilation of the branchial chambers, are discussed in relation to those described for other air-breathing decapod crustaceans.     

A functional analysis of the shift from gill- to lung-breathing during the evolution of land crabs (Crustacea, Decapoda)

The evolution of air-breathing in land crabs is associated with a progressive shift in the primary site of respiratory gas exchange from the diffusion-limited gills used for water-breathing, via a simple 'cutaneous' lung surface to the perfusion-limited, invaginated lung described in the mountain crab, Pseudothelphusa garmani. The reduced diffusion limitation over the lungs facilitates oxygen transfer from air to the tissues at lower ventilation rates but is associated with accumulation of carbon dioxide. A potential respiratory acidosis is buffered by the respiratory pigment haemocyanin and by elevation of haemolymph bicarbonate levels. These changes parallel those described in vertebrates but air-breathing crustaceans maintain relatively low carbon dioxide levels in the haemolymph, either by retaining an aquatic route for its elimination over the reduced gills or by blowing it off across the lung. Maintenance of low carbon dioxide levels may be associated with a limited capacity to buffer against an acidosis due to low levels of circulating haemocyanin (i.e. crustaceans lack red blood cells). This may ultimately limit their survival in air as an acidosis will reduce oxygen transport due to a marked Bohr effect on haemocyanin. The primary role of an invaginated lung may be to reduce rates of water loss in air.     

Clearance of Vibrio campbellii injected into the hemolymph of Callinectes sapidus, the Atlantic blue crab: the effects of prior exposure to bacteria and environmental hypoxia

The Atlantic blue crab, Callinectes sapidus (Rathbun), lives in a bacteria-rich environment that experiences daily fluctuations in water quality. In the present study, we tested the hypothesis that crustaceans with prior or ongoing exposure to bacteria in their hemolymph have an increased susceptibility to subsequent infections, and that acute exposure to low dissolved oxygen (hypoxia) and elevated carbon dioxide levels (hypercapnia) may further confound the ability of blue crabs to counter a subsequent infection. Adult male blue crabs held in well-aerated (normoxic; P O2=20.7 kPA; CO(2)<0.06 kPa; pH 7.8-8.0) or hypercapnic hypoxic (HH; P O2=4 kPa; CO(2)=1.8 kPa; pH 6.9-7.2) seawater received an injection (pre-challenge dose) of 1 x 10(5)Vibrio campbellii g(-1) crab. Control animals were injected with an equivalent dose of HEPES-buffered saline (1 microl g(-1) crab). At 2h or 24h after the pre-challenge injection, both Vibrio and saline-pre-challenged animals were injected with a dose of live V. campbellii (1 x 10(5)g(-1) crab). This second injection will be referred to as a second injection or challenge injection. Degradation in or physical removal of intact bacteria from hemolymph was quantified using real-time PCR; bacteriostasis was quantified as the percentage of intact bacteria that could not be recovered by selective plating. We demonstrated that bacteriostasis occurs in the hemolymph of blue crabs. Furthermore, blue crabs that received a challenge injection 2h after a pre-challenge dose of V. campbellii cleared culturable bacteria from their hemolymph more rapidly when compared to animals that received a pre-challenge dose of saline. This enhanced clearance of culturable bacteria was associated with an increase in antibacterial activity in the cell-free hemolymph. However, the enhanced clearance of culturable bacteria disappeared when the time interval between the pre-challenge and challenge dose was extended to 24h and when crabs were held in HH seawater throughout the experiment. Neither the time interval between the pre-challenge and the challenge dose nor exposure to HH altered the pattern of intact bacterial clearance in blue crabs. These results demonstrate that prior exposure to bacteria does not increase the susceptibility of C. sapidus to a second, sublethal dose of V. campbellii. In fact, a recent exposure to V. campbellii enhances the ability of blue crabs to render bacteria non-culturable and the immune mechanisms/effectors responsible for this are short lived and appear to be sensitive to low dissolved oxygen and high carbon dioxide concentrations in the environment.     

Oxygen and carbon dioxide sensitivity of ventilation in amphibious crabs, Cardisoma guanhumi, breathing air and water

Amphibious crabs, Cardisoma guanhumi, were acclimated to breathing either air or water and exposed to altered levels of oxygen and/or carbon dioxide in the medium. Hypercapnia (22, 36 and 73 torr CO(2)) stimulated a significant hypercapnic ventilatory response (HCVR) in both groups of crabs, with a much greater effect on scaphognathite frequency (Deltaf(SC)=+700%) in air-breathing crabs than water-breathing crabs (Deltaf(SC)=+100%). In contrast, hyperoxia induced significant hypoventilation in both sets of crabs. However, simultaneous hyperoxia and hypercapnia triggered a greater than 10-fold increase in f(SC) in air-breathing crabs but no change in water-breathing crabs. For water-breathing crabs hypoxia simultaneous with hypercapnia triggered the same response as hypoxia alone-bradycardia (-50%), and a significant increase in f(SC) at moderate exposures but not at the more extreme levels. The response of air-breathing crabs to hypoxia concurrent with hypercapnia was proportionally closer to the response to hypercapnia alone than to hypoxia. Thus, C. guanhumi were more sensitive to ambient CO(2) than O(2) when breathing air, characteristic of fully terrestrial species, and more sensitive to ambient O(2) when breathing water, characteristic of fully aquatic species. C. guanhumi possesses both an O(2)- and a CO(2)-based ventilatory drive whether breathing air or water, but the relative importance switches when the respiratory medium is altered.     

Aerial and Aquatic Respiration in the River Crab Potamonautes Warreni Calman with Notes on Gill Structure

Summary

The oxygen consumption rate (?O₂) for Potamonauteus warreni Calman (= Potamon warreni (Calman) kept in 25 °C water was 34,4 μmol 1^?1 O₂ kg^?1 and after 72 hours in 98% R.H. air the rate was 31,9 μmol 1^?1 O₂ kg^?1 min^?1. The ?O₂ values for each of the two groups are not significantly different (P > 0,05). The partial oxygen tension of pre-branchial (v = venous) haemolymph (P_vCO₂) is 15,3 mm Hg in water and 13,0 mm Hg in air); partial carbon dioxide tension of pre-branchial (v) haemolymph (P_vCO₂) is 13,2 mm Hg in water and 13,0 mm Hg in air); the total carbon dioxide concentration in pre-branchial (v) haemolymph (C_vCO₂) tot. is 12,3 mmol 1^?1 in air and 13,9 mmol 1^?1 in water) are not significantly different for the two groups (P > 0,05). The haemolymph pH and the lactate concentration for crabs in water was found to be 7,51 and 0,38 mmol 1^?1 respectively. No significant differences were found in pre-branchial haemolymph oxygen tension, carbon dioxide tension, total carbon dioxide content, haemolymph pH, lactate level, chloride concentration, P₅₀ and haemocyanin-oxygen cooperativity in control crabs kept in water, and experimental crabs held in air for 72 hours. The chloride concentration, (327,0 mmol 1^?1) for crabs kept in water does not differ from that of crabs held in air for 72 hours but is at least 15% higher than the sodium concentration (255 mmol 1^?1) for crabs kept in water. The gill surface area is 520 mm² g^?1 wet body mass; on average 9,2 gill platelets (lamellae) can be found on a gill length of one millimetre. Each lamella is spaced 60–70 μm apart, each with a thickness of 30–40 μm. It is concluded that P. warreni may be described as a truly amphibious fresh-water crab.     

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.     

Aerial respiration in the semaphore crab,Heloecius cordiformis,with or without branchial water

• 1.1. Semaphore crabs (Heloecius cordiformis) are active in air at low tide. Their branchial chambers are lined with a vascular epithelium and are expanded above the gills (five pairs) to form air cavities which could function as lungs. Water is continuously circulated over the gills.
• 2.2. The relative contribution made by the gills and lungs to gas exchange in semaphore crabs active in air and circulating branchial water, was determined by measuring oxygen consumption (at 25°C) in crabs with and without branchial water, and in crabs with their lungs subsequently occluded.
• 3.3. Activity levels and VO₂ were unaffected by the absence of branchial water.
• 4.4. With their lungs occluded, VO₂ dropped (on average) by 61% in crabs with branchial water (i.e. gills still functional) and by 81% in crabs without branchial water (gill function impaired).
• 5.5. It is concluded that semaphore crabs are obligate air breathers while active on land, despite carrying water within their branchial chambers. Lung development and gill reduction in land crabs is discussed briefly in relation to “terrestriality”.

     

Respiratory,cardiovascular, and hemolymph acid‐base changes in the amphibious crab,Cardisoma guanhumi,during immersion and emersion

The cardiorespiratory and hemolymph acid base status of bimodal breathing crabs, Cardisoma guanhumi, was monitored during the transition from breathing air to breathing water. Upon immersion, oxygen uptake (MO₂) decreased by half. Ventilatory frequency (fsc) increased more than 5 fold, causing a decrease in hemolymph carbon dioxide partial pressure (PCO₂). This was nearly fully compensated for by a gradual decrease in hemolymph bicarbonate concentration ([HCO₃ ^‐]) over 96 hours post‐immersion. After one to two weeks of immersion, when crabs were removed from the water, oxygen uptake initially increased, but eventually returned to the initial immersed value. Heart rate was unchanged but fsc slowed dramatically. The decreased ventilation resulted in a buildup of hemolymph PCO₂, causing a respiratory acidosis that was slowly compensated for by increased hemolymph [HCO₃ ^‐]. C. guanhumi appears to be a truly amphibious crab with respiratory and acid‐base adaptations found in both fully aquatic and fully terrestrial species.     

Ternary effects on the gas exchange of isotopologues of carbon dioxide

The ternary effects of transpiration rate on the rate of assimilation of carbon dioxide through stomata, and on the calculation of the intercellular concentration of carbon dioxide, are now included in standard gas exchange studies. However, the equations for carbon isotope discrimination and for the exchange of oxygen isotopologues of carbon dioxide ignore ternary effects. Here we introduce equations to take them into account. The ternary effect is greatest when the leaf-to-air vapour mole fraction difference is greatest, and its impact is greatest on parameters derived by difference, such as the mesophyll resistance to CO(2) assimilation, r(m) . We show that the mesophyll resistance to CO(2) assimilation has been underestimated in the past. The impact is also large when there is a large difference in isotopic composition between the CO(2) inside the leaf and that in the air. We show that this partially reconciles estimates of the oxygen isotopic composition of CO(2) in the chloroplast and mitochondria in the light and in the dark, with values close to equilibrium with the estimated oxygen isotopic composition of water at the sites of evaporation within the leaf.     

Morphological, Behavioral, and Physiological Characterization of Bimodal Breathing Crustaceans

SYNOPSIS Bimodal breathing crustaceans, while representing astage in the transition from the aquatic to terrestrial habitat,also constitute a distinct group that can be characterized bymorphological, behavioral, and physiological traits. Morphologically,this group displays reduced gill surface area and enlarged branchialchambers. The lining of the branchial chambers, the branchiostegites,also has increased surface area and is highly vascularized.The branchiostegites can be smooth, cutaneous epithelia, orthey can have more complex evaginations or invaginations tofurther increase surface area. In addition, the thickness ofthe branchiostegal epithelium is greatly reduced, compared tothat in the gills, thus minimizing the diffusion distance betweenair and hemolymph. These animals maintain a store of water inthe branchial chamber that covers the gills and allows for simultaneousgas exchange with two media (air and water). There is also apartitioning of gas exchange, with oxygen uptake occurring preferentiallyfrom air across the branchiostegites, and carbon dioxide excretionoccurring across the gills into the branchial water. One criticalfactor that appears to separate bimodal breathing crustaceansfrom fully terrestrial, exclusively air-breathers is the abilityof the latter group to excrete CO₂directly into air across thegills and branchiostegites. It is suggested that the incorporationof the enzyme carbonic anhydrase into the membrane fractionof the branchiostegites may have been one of the key molecularevents which allowed for pulmonary CO₂ excretion into air.     

Systemic Respiratory Adaptations to Air Exposure in Intertidal Decapod Crustaceans

SYNOPSIS: The responses of intertidal decapods to emersion areclosely related to the particular conditions of emersion, yetall members of this group of animals face the problems of watershortage and internal hypoxia during air exposure. Several speciesexhibit modification of normal ventilatory activity and thisresponse seems to enable these crabs to take up seawater fromthe substrate. Other crabs have specific morphological adaptationspermitting recirculation of water from the exhalent aperturesback into the gill chamber. The hemocyanin of some species hasa higher affinity for oxygen, and this difference may be moreprevalent in tropical animals. The higher oxygen affinity undoubtedlycompensates in part for the lower internal oxygen tensions duringair exposure. Structural specialization of the branchial apparatusmay prevent the gill lamellae from adhering together, a processwhich reduces the surface available for gas exchange. Thereis a wide range of responses to emersion and yet relativelyfew specific adaptations. Some species are able to merely tolerateair exposure, while others are able to more fully exploit thehabitat.     

Stereological determination of tracheal volume and diffusing capacity of the tracheal walls in the stick insect Carausius morosus (Phasmatodea, Lonchodidae)

First instars of Carausius morosus provide a good model for morphometric evaluation of the diffusing capacity between the tracheal system and hemolymph: air sacs are lacking, tracheoles do not penetrate the organs and muscles, and entire animals can be evaluated electron microscopically without subsampling. The tracheal volume makes up 1.3% of the volume of the whole insect excluding appendages. We calculated the lateral diffusing capacity for oxygen and carbon dioxide for five classes of tracheae according to their diameters, from 0.2 microm to 35 microm. The harmonic mean thickness of the tracheal epithelium is lowest in smallest tracheae and increases with increasing tracheal diameter. Although the smallest tracheae make up 70% (O2) and 60% (CO2) of the total diffusing capacity, the proximal four classes may also be significant in diffusion of oxygen and particularly of carbon dioxide. The suppression of the development of respiratory pigments in the evolution of terrestrial insects may have increased the relative importance of small tracheal elements for local oxygen consumption.     

Acute pulmonary edema induced by CO2 in rats

High concentrations (FiCO2 0.3) of carbon dioxide in the inspired air provoke acute pulmonary oedema in the vagotomized rats only, the animals dying of acute asphyxia in a few minutes. Vagotomy accelerates the production of pulmonary transudates mainly by increasing capillary-alveolar transmural pressure: tidal volume and pleural depression are largely augmented after stimulation of the respiratory centers by CO2.     

Ecosystem-atmosphere CO(2) exchange: interpreting signals of change using stable isotope ratios

Changes in the concentration and stable isotope ratio of atmospheric CO(2) can be used to study variations in the net exchange of carbon dioxide in terrestrial ecosystems (net difference between total photosynthesis and respiration). Changes in the timing of seasonal fluctuations in atmospheric CO(2) concentration have suggested that net uptake of carbon dioxide has been increasing in northern latitude ecosystems in association with warmer temperatures and a lengthening of the growing season. Stable isotope techniques allow a more detailed separation of differences between ecosystem photosynthesis and respiration because these two processes have contrasting effects on both the carbon and oxygen isotope ratio of atmospheric CO(2). Future applications of stable isotope analyses include documenting and monitoring the influence of global environmental change on ecosystem CO(2) exchange at regional scales (10-1000km(2)).     

Respiration and Circulation in Land Crabs: Novel Variations on the Marine Design

Both the "true" crabs (Brachyura) and hermit crabs (Anomura)include species that show numerous behavioral, morphological,and physiological specializations permitting terrestrial life.This paper examines respiratory and circulatory adaptationsfor air breathing in these land crabs. Respiratory specializationsinclude modification of gas exchange structures for air breathing(gills and elaborated branchial chamber linings), ventilatorymechanisms permitting effective air pumping, an elevated hemolymphoxygen capacity, and a primarily CO₂- rather than O₂- sensitiveventilatory control system. The qualitative aspects of hemolymphoxygen transport and metabolic rate are apparently unchangedfrom that of marine crabs. While the basic cardiovascular morphologyof land crabs appears similar to that of marine forms, thereis considerable elaboration of the vasculature of the branchialchamber lining, which in some species includes a unique doubleportal system. Cardiac output is lower in land crabs (probablyrelated to their higher hemolymph O₂ capacity), but insufficientdata on hemolymph pressures prevent comparisons with marineforms. In general, land crabs have modified (sometimes extensively)existing structures and processes found in their marine relativesrather than evolving structures for terrestrial life de novo.Accordingly, land crabs present a useful model for the evolutionof terrestriality, showing that even subtle anatomical changescan result in the large changes in physiological function necessaryfor the terrestrial invasion.     

Functional genomics and sexual differentiation in amphibians

To succeed on land rather than in water, crabs require a suite of physiological and morphological changes, and ultimately the ability to reproduce without access open water. Some species have modified gills to assist in gas exchange but accessory gas exchange organs, usually lungs, occur in many species. In accomplished air-breathers the lung becomes larger and more vascularised with pulmonary vessels directing oxygenated haemolymph to the heart. The relative abundance of O₂ in air promotes relative hypoventilation and thus an internal hypercapnia to drive CO₂ excretion. Land crabs have a dual circulation via either lungs or gills and shunting between the two may depend on respiratory media or exercise state. During their breeding migration on Christmas Island Gecarcoidea natalis maintained arterial Po₂ by branchial O₂ uptake, while pulmonary O₂ pressure was reduced; partly because exercise doubled relative haemolymph flow through the gills. Related species rely on elevated haemocyanin concentration and affinity for O₂ to assist uptake but this compromises unloading at the tissues and thus the aerobic scope of tissues. Aquatic crabs exchange salt and ammonia with water via the gills but in land crabs this is not possible. Birgus latro has adopted uricotelism but other species excrete ammonia in either the urine or as gas. Land crabs minimise urinary salt loss using a filtration-reabsorption system analogous to the kidney. Urine is redirected across the gills where salt reabsorption occurs in systems under hormonal control, although in G. natalis this is stimulatory and in B. latro inhibitory. While crabs occupy a range of habitats from aquatic to terrestrial, these species do not comprise a physiological continuum but across the crab taxa individual species possess appropriate and specific physiological features to survive in their individual habitat.     

Effects of exercise and time elapsed after exercise on VO2, VCO2 and R responses to norepinephrine in rats

The effects of norepinephrine (NE) injection (300 microgram . kg-1 of body weight) on oxygen consumption (VO2), carbon dioxide production (VCO2) and respiratory exchange ratio (R) were investigated in female rats after 1 h of running on a treadmill (21.5 m . min-1) at 10% inclination. Six groups of animals were injected respectively at various times after the exercise (1, 3, 6, 9, 21, and 47 h), and were compared to six non-exercised groups injected at corresponding times. VO2 and VCO2 were monitored continuously during the 20 min preceding injection and for the 60 min following it. The increases in VO2 and VCO2, and the decrease in R were of similar magnitude in both exercised and non-exercised rats (about 30% and 20% for VO2 and VCO2, respectively, and -12% for R). Peak VO2 and R values attained after NE injection varied however with time of injection, specially in exercised animals 1 and 9 h after the run. Exercise significantly delayed time of response to NE for VO2 and VCO2 particularly 1 and 9 h after the running bout. It is concluded that time of day, exercise, and time elapsed after exercise are important factors to consider when studying metabolic responses to catecholamines. Furthermore, it is suggested that such experimental controls might be meaningful in human studies as well.     

Behavioral influences on the physiological responses of Cancer gracilis, the graceful crab, during hyposaline exposure

The relationship between the behavioral and physiological responses to hyposaline exposure was investigated in Cancer gracilis, the graceful crab. The status of C. gracilis as an osmoconformer was confirmed. Survival decreased with salinity: the LT(50) in 50% seawater (a practical salinity of 16, or 16 per thousand) was 31.5 +/- 22.7 h and in 25% seawater (a salinity of 8) was 8.0 +/- 0.7 h. When exposed to a salinity gradient, most crabs moved towards the highest salinity. However, in the salinity range of 55% to 65% seawater, they became quiescent. This "closure response" was also evident at low salinities: the mouthparts were tightly closed and animals remained motionless for 2 to 2.5 h. During closure, crabs were able to maintain the salinity of water within the branchial chambers at a level that was about 30% higher than that of the surrounding medium. The closure response was closely linked to a short-term decrease in oxygen uptake. During closure, oxygen within the branchial chamber was rapidly depleted, with oxygen uptake returning to pretreatment levels upon the resumption of activity. In addition to the short-term decrease in oxygen uptake, there was a longer-term bradycardia, which may serve to further reduce diffusive ion loss across the gills. By exhibiting a closure response during acute hyposaline exposure and an avoidance reaction during prolonged or severe hyposaline exposure, C. gracilis is able to use behavior to exploit areas prone to frequent episodes of low salinity.     

CUTANEOUS GAS EXCHANGE IN VERTEBRATES: DESIGN, PATTERNS, CONTROL AND IMPLICATIONS

1. The exchange of oxygen and carbon dioxide between skin and environment is commonplace in the vertebrates. In many lower vertebrates, the skin is the major or even sole avenue for respiration.
2. As implied by the physical laws governing diffusion of gases, the skin diffusion coefficient, surface area, gas diffusion distance and transcutaneous gas partial pressures may independently or jointly affect cutaneous respiration. In vertebrates, each of these variables has undergone modification that may be related to dependence upon cutaneous gas exchange.
3. Both theoretical models and experimental data suggest that cutaneous gas exchange is limited by the rate of diffusion. However, changes in convection of the respiratory medium and of blood may partially compensate for diffusion limitation, and potentially function in the regulation of cutaneous gas exchange.
4. Typically, the skin is one of several gas exchangers, although many salamanders and some species in other vertebrate groups breathe solely through the skin. The cutaneous contribution to overall gas exchange is often most important in small animals, at cool temperatures, at low levels of activity and in normoxic and normocapnic conditions. Branchial and pulmonary respiration increasingly predominate in other circumstances.
5. Often, the skin figures more prominently in CO₂, excretion than in O₂, uptake.
6. Cutaneous gas exchange emerges in vertebrates as a process perhaps less effective and more constrained than branchial or pulmonary exchange but also less energetically costly. Its utility is indicated by its wide and successful exploitation in vertebrates occupying a diverse array of habitats.     

Molecular keys unlock the mysteries of variable survival responses of blue crabs to hypoxia

Hypoxia is a major stressor in coastal ecosystems, yet generalizing its impacts on fish and shellfish populations across hypoxic events is difficult due to variability among individuals in their history of exposure to hypoxia and related abiotic variables, and subsequent behavioral and survival responses. Although aquatic animals have diverse physiological responses to cope with hypoxia, we know little about how inter-individual variation in physiological state affects survival and behavioral decisions under hypoxic conditions. Laboratory experiments coupled with molecular techniques determined how extrinsic factors (e.g., water body and temperature) and respiratory physiology (hemocyanin concentration and structure) affected survival and behavior of adult blue crabs (Callinectes sapidus) exposed to different levels of hypoxia over a 30-h time period. Nearly 100% of crabs survived the 1.3 mg dissolved oxygen (DO) l^?1 treatment (18.4% air saturation), suggesting that adult blue crabs are tolerant of severe hypoxia. Probability of survival decreased with increasing hypoxic exposure time, lower DO, and increasing temperature. Individual-level differences in survival correlated with water body and crab size. Crabs collected from the oligo/mesohaline and hypoxic Neuse River Estuary (NRE), North Carolina, USA survived hypoxic exposures longer than crabs from the euhaline and normoxic Bogue and Back Sounds, North Carolina. Furthermore, small NRE crabs survived longer than large NRE crabs. Hemocyanin (Hcy) concentration did not explain these individual-level differences, however, hypoxia-tolerant crabs had Hcy structures indicative of a high-O₂-affinity form of Hcy, suggesting Hcy “quality” (i.e., structure) may be more important for hypoxia survival than Hcy “quantity” (i.e., concentration). The geographic differences in survival we observed also highlight the importance of carefully selecting experimental animals when planning to extrapolate results to the population level.