When are resting water-breathers lacking O2? Arterial PO2 at the anaerobic threshold in crab

The Neurobiology of Ocean Change - insights from decapod crustaceans

The unprecedented rate of carbon dioxide accumulation in the atmosphere has led to increased warming, acidification and oxygen depletion in the world's oceans, with projected impacts also on ocean salinity. In this perspective article, we highlight potential impacts of these factors on neuronal responses in decapod crustaceans. Decapod crustaceans comprise more than 8,800 marine species which have colonized a wide range of habitats that are particularly affected by global ocean change, including estuarine, intertidal, and coastal areas. Many decapod species have large economic value and high ecological importance because of their global invasive potential and impact on local ecosystems. Global warming has already led to considerable changes in decapod species' behavior and habitat range. Relatively little is known about how the decapod nervous system, which is the ultimate driver of all behaviors, copes with environmental stressors. We use select examples to summarize current findings and evaluate the impact of current and expected environmental changes. While data indicate a surprising robustness against stressors like temperature and pH, we find that only a handful of species have been studied and long-term effects on neuronal activity remain mostly unknown. A further conclusion is that the combined effects of multiple stressors are understudied. We call for greater research efforts towards long-term effects on neuronal physiology and expansion of cross-species comparisons to address these issues.

Ocean deoxygenation and zooplankton: Very small oxygen differences matter

Oxygen minimum zones (OMZs), large midwater regions of very low oxygen, are expected to expand as a result of climate change. While oxygen is known to be important in structuring midwater ecosystems, a precise and mechanistic understanding of the effects of oxygen on zooplankton is lacking. Zooplankton are important components of midwater food webs and biogeochemical cycles. Here, we show that, in the eastern tropical North Pacific OMZ, previously undescribed submesoscale oxygen variability has a direct effect on the distribution of many major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton live near their physiological limits and respond to slight (≤1%) changes in oxygen. Ocean oxygen loss (deoxygenation) may, thus, elicit major unanticipated changes to midwater ecosystem structure and function.

Modulation of network pacemaker neurons by oxygen at the anaerobic threshold

Previous in vitro and in vivo studies showed that the frequency of rhythmic pyloric network activity in the lobster is modulated directly by oxygen partial pressure (PO(2)). We have extended these results by (1) increasing the period of exposure to low PO(2) and by (2) testing the sensitivity of the pyloric network to changes in PO(2) that are within the narrow range normally experienced by the lobster (1 to 6 kPa). We found that the pyloric network rhythm was indeed altered by changes in PO(2) within the range typically observed in vivo. Furthermore, a previous study showed that the lateral pyloric constrictor motor neuron (LP) contributes to the O(2) sensitivity of the pyloric network. Here, we expanded on this idea by testing the hypothesis that pyloric pacemaker neurons also contribute to pyloric O(2) sensitivity. A 2-h exposure to 1 kPa PO(2), which was twice the period used previously, decreased the frequency of an isolated group of pacemaker neurons, suggesting that changes in the rhythmogenic properties of these cells contribute to pyloric O(2) sensitivity during long-term near-anaerobic (anaerobic threshold, 0.7-1.2 kPa) conditions.

Primitive, and protective, our cellular oxygenation status?

The primitive atmosphere where aerobic life started on earth was hypoxic and hypercapnic. Remarkably, an adaptation strategy whereby O2 partial pressure, PO2, in the arterial blood is maintained within a low and narrow range of 1-3 kPa, largely independent of inspired PO2, has also been reported in modern water-breathers. In mammalian tissues, including brain, the most frequently measured PO2 is also in the same low range. Based on the postulate that basic cellular machinery has been established since the early stages of evolution, we propose that this similarity in oxygenation status is the consequence of an early adaptation strategy which, subsequently, throughout the course of evolution, maintained cellular oxygenation in the same low and primitive range independent of environmental changes. Specialized enzymes aimed at protecting cells against O2 toxicity are thought to have appeared very early in evolution but we suggest that preventing high PO2's is also the simplest and most efficient tool for limiting reactive oxygen species (ROS) production. It could be a cue mechanism to widen our understanding of the ageing process.

A modulatory role for oxygen in shaping rhythmic motor output patterns of neuronal networks

It is becoming increasingly evident that O(2)-uptake in animal tissue is not only devoted to energy production. Here we review recent findings on a novel role of tissue oxygenation, notably in controlling the operation of neuronal networks in the central nervous system. Electrophysiological recordings in vivo and in vitro from rhythmically-active motor pattern generating networks in the lobster stomatogastric ganglion (STG) have revealed that oxygen is able to act in a manner equivalent to a classical neuromodulator. Local P(O(2)) variations within the low, but physiological range of 1-6 kPa are able to shape ongoing activity of these networks and therefore the motor behaviours in which they are involved. Oxygen's contribution to two of these, feeding and moulting, have been investigated. Importantly, the P(O(2)) effects are not related to hypoxic depression but are highly specific in terms of the network, neuron and even the synapse targeted. Our results are discussed in terms of functional significance and new research directions for mammalian physiology.

From low arterial- to low tissue-oxygenation strategy. An evolutionary theory

The primitive atmosphere where aerobic life started on earth was hypoxic and hypercapnic. Remarkably, an adaptation strategy whereby O(2) partial pressure, P(O(2)), in the arterial blood is maintained within a low and narrow range of 1-3 kPa, largely independent of inspired P(O(2)), has also been reported in modern water-breathers. In mammalian tissues, including brain, the most frequently measured P(O(2)) is in the same low range. Based on the postulate that basic cellular machinery has been established since the early stages of evolution, we propose that this similarity in oxygenation status is the consequence of an early adaptation strategy which, subsequently throughout the course of evolution, maintained cellular oxygenation in the same low and primitive range independent of environmental changes. The rational for such an evolutionary theory is discussed in terms of an equilibrium between physiological and pathological reactions associated with O(2) excess vs O(2) lack and emerging concepts about the importance of cellular O(2)-dependent mechanisms in the low but physiological P(O(2)) range.

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.

Blood oxygen requirements in resting crab (Carcinus maenas) 24 h after feeding

Numerous resting unfed water-breathers have a strategy of gas-exchange regulation that consists of setting the arterial partial pressure of oxygen (Pao2) at 1-3 kPa. This raises a question concerning the extent to which physiological functions are limited in this situation. To obtain insight into this problem, we studied the steady-state adaptation of the blood-oxygen transfer system in the crab Carcinus maenas during the doubling of the oxygen consumption rate, Mo2 (i.e., during the period of specific dynamic action of food (SDA)), that occurs 24 h after feeding. We showed that this increase in the oxygen consumption rate 24 h after a meal is not limited by a blood partial pressure of oxygen (Po2) as low as 0.8-1.5 kPa in either normoxia or hypoxia (Po2 of the inspired water = 4 kPa). In normoxia, adaptation of the oxygen-transport system, if any, consisted of a combined set of adaptations of small amplitude (in absolute value), rather than major changes in blood oxygenation status, blood flow rate, or oxygen affinity (although blood pH decreases). In hypoxia, the SDA was mainly associated with an increase in blood flow rate and blood pH, with no changes in blood lactate, urate, calcium, and haemocyanin concentrations. The results are discussed, in an environmental context, in terms of minimal oxygen requirements in water-breathers.

Changes in motor network expression related to moulting behaviour in lobster: role of moult-induced deep hypoxia

The well known rhythmically active pyloric neural network in intact and freely behaving lobsters Homarus gammarus was monitored prior to and following ecdysis. Despite long-lasting hormonal and metabolic alterations associated with this process, spontaneous pyloric network activity remained largely unaltered until the last 12–48 h before exuviation. At this time, the most notable change was a progressive lengthening of pyloric cycle period, which eventually attained 500–600 % of control values. It was only in the very last minutes before ecdysis that burst patterning became irregular and the otherwise strictly alternating motor sequence broke down. After the moult, coordinated rhythmicity was re-established within 10 min. Concomitant with these final changes in motor network expression at ecdysis was a drastic reduction in blood oxygen levels which led to a temporary near-anoxia. By imposing similarly deep hypoxic conditions both on intermoult animals and on the pyloric network in vitro, we mimicked to a large extent the moult-induced changes in pyloric network performance. Our data suggest that, despite major surrounding physiological perturbations, the pyloric network in vivo retains stable pattern-generating properties throughout much of the moulting process. Moreover, some of the most significant modifications in motor expression just prior to ecdysis can be related to a substantial reduction in oxygen levels in the blood.

In vivo modulation of interacting central pattern generators in lobster stomatogastric ganglion: influence of feeding and partial pressure of oxygen

The stomatogastric ganglion (STG) of the European lobster Homarus gammarus contains two rhythm-generating networks (the gastric and pyloric circuits) that in resting, unfed animals produce two distinct, yet strongly interacting, motor patterns. By using simultaneous EMG recordings from the gastric and pyloric muscles in vivo, we found that after feeding, the gastropyloric interaction disappears as the two networks express accelerated motor rhythms. The return to control levels of network activity occurs progressively over the following 1-2 d and is associated with a gradual reappearance of the gastropyloric interaction. In parallel with this change in network activity is an alteration of oxygen levels in the blood. In resting, unfed animals, arterial partial pressure of oxygen (PO2) is most often between 1 and 2 kPa and then doubles within 1 hr after feeding, before returning to control values some 24 hr later. In vivo, experimental prevention of the arterial PO2 increase after feeding leads to a slowing of pyloric rhythmicity toward control values and a reappearance of the gastropyloric interaction, without apparent effect on gastric network operation. Using in vitro preparations of the stomatogastric nervous system and by changing oxygen levels uniquely at the level of the STG within the range observed in the intact animal, we were able to mimic most of the effects observed in vivo. Our data indicate that the gastropyloric interaction appears only during a "free run" mode of foregut activity and that the coordinated operation of multiple neural networks may be modulated by local changes in oxygenation.

Modulation of a neural network by physiological levels of oxygen in lobster stomatogastric ganglion

Although a large body of literature has been devoted to the role of O2 in the CNS, how neural networks function during long-term exposures to low but physiological O2 partial pressure (PO2) has never been studied. We addressed this issue in crustaceans, where arterial blood PO2 is set in the 1-3 kPa range, a level that is similar to the most frequently measured tissue PO2 in the vertebrate CNS. We demonstrate that over its physiological range, O2 can reversibly modify the activity of the pyloric network in the lobster Homarus gammarus. This network is composed of 12 identified neurons that spontaneously generate a triphasic rhythmic motor output in vitro as well as in vivo. When PO2 decreased from 20 to 1 kPa, the pyloric cycle period increased by 30-40%, and the neuronal pattern was modified. These effects were all dose- and state-dependent. Specifically, we found that the single lateral pyloric (LP) neuron was responsible for the O2-mediated changes. At low PO2, the LP burst duration increased without change in its intraburst firing frequency. Because LP inhibits the pyloric pacemaker neurons, the increased LP burst duration delayed the onset of each rhythmic pacemaker burst, thereby reducing significantly the cycling frequency. When we deleted LP, the network was no longer O2-sensitive. In conclusion, we propose that (1) O2 has specific neuromodulator-like actions in the CNS and that (2) the physiological role of this reduction of activity and energy expenditure could be a key adaptation for tolerating low but physiological PO2 in sensitive neural networks.

A field versus laboratory study of blood oxygen status in normoxic crabs at different temperatures

The blood oxygen status of two species of active crabs (Carcinus maenas and Necora puber) was studied in the field and compared with the results of previous laboratory experiments performed on a wide spectrum of physiologically different water-breathers. The aim was to determine whether, as in the laboratory, the functioning of the O2supply system in the field could be based on maintaining the arterial [Formula: see text] in the low range, 1–3 kPa. The O2partial pressures and concentrations in the arterial and venous blood, arterial blood pH, and blood respiratory pigment concentration were measured in normoxic water at various temperatures ranging from 10 to 20 °C and in various seasons. In the field, [Formula: see text] values in normoxic C. maenas and N. puber were in the low range, 1–3 kPa, independently of temperature, season, and blood haemocyanin concentration. It is concluded that in the field as in the laboratory, [Formula: see text] values mainly in the low range provide a head pressure sufficient to meet O2needs. The changes that appear to occur in other respiratory variables are discussed in relation to field versus laboratory conditions and temperature differences. The consequences for analysing problems of hypoxaemia in hypoxic waters or situations are discussed.