Two Designs of Marine Egg Masses and their Divergent Consequences for Oxygen Supply and Desiccation in Air

Two common types of egg masses rely on differing routes of supplyof oxygen in water. When embryos are embedded in a gelatinousmatrix, oxygen is supplied by diffusion through the gel, andthicker masses require more gel per embryo. When an adherentmass of eggs lacks a gel matrix, oxygen can be provided fromwater flowing through the open interstices between eggs, andlarger eggs provide larger channels and thus less resistanceto flow. Both types occur intertidally, where they are periodicallyexposed to air. Exposure to air can have a greater effect onoxygen supply via interstices than on supply via gel. Oxygendiffusing in interstices drained of water provides increasedrates of supply to masses of adherent eggs. In contrast, diffusionthrough gel is similar for masses in air and water. Effectsof emersion on desiccation also differ for the two types ofegg masses. Additional gel matrix can reduce salinity changefrom desiccation while enhancing oxygen supply, whereas drainingof interstices, though necessary for oxygen supply, may increaserisk of desiccation.     

The effects of nest temperature,nest substrate,and clutch size on the oxygenation of embryos and larvae of the Australian moss frog,Bryobatrachus nimbus

The jelly around amphibian eggs presents a formidable barrier to oxygen diffusion. Therefore, egg capsules must be thin enough, and the dimensions of globular egg masses small enough, to avoid oxygen limitation leading to developmental retardation or death. The eggs of the Australian moss frog, Bryobatrachus nimbus, have the thickest jelly capsule known for any anuran amphibian. Laboratory measurements of respirometric variables predict that single prehatching embryos should be normoxic between 5 degrees and 20 degrees C, with Po(2 in) maintained above critical levels (10.2-17.0 kPa). However, numerical models of embryos amid larger egg masses (13-20 eggs) predict hypoxia at temperatures above 5 degrees C. Contrary to model predictions, however, B. nimbus embryos rarely experience hypoxia in natural nests, because embryos occur in one or two layers and the moss substrate permits aeration of the lower surface while photosynthesis probably supplies oxygen directly. After hatching, larvae move to oxygen-rich regions of the jelly mass and disperse more widely within the mass as temperatures increase. Although nest characteristics relieve diffusive constraints, small clutch sizes, low rates of embryonic and larval respiration, and the cool climate occupied by B. nimbus are the main characteristics that prevent hypoxia.     

Respiration of Aquatic and Terrestrial Amphibian Embryos

Respiratory constraints on the structure of single eggs andegg masses have affected the mode of amphibian reproductionin water and in air. Aquatic eggs generally require less oxygen,develop faster, and hatch earlier, but these characteristicsare related to small ovum size. A comparison of two speciesof aquatic and terrestrial breeding frogs with similarly sizedova shows no differences in hatching stage, maximum rate ofoxygen uptake, oxygen conductance of the egg capsule, or Po₂difference across the capsule. However, the aquatic speciesdevelops about 2.4 times faster and tolerates lower environmentalPo₂, suggesting adaptation for development in ephemeral water.Modelling of diffusive oxygen transport into a single aquaticegg shows that a large amount of jelly (or a boundary layer)around the capsule may not greatly restrict gas exchange, ifthe inner radius of the capsule is large. However, gelatinousegg masses that contain other embryos that compete for oxygenare therefore limited in size, unless the eggs are ventilatedby convection of water among them. Aquatic egg are often suspendedin masses above the substrate, promoting oxygen movement intothe mass from all directions. Terrestrial egg masses are morediffusion limited, because gravity and surface tension collapsethem, preventing convection between the eggs, and restrictingthe source for oxygen diffusion. Terrestrial embryos are oftenlarger than their aquatic counterparts and have higher demandsfor oxygen. Terrestrial conditions have selected for adaptationsthat reduce respiratory competition between embryos, for example,separating of embryos by large volumes of jelly or reducingthe number of eggs in a clutch. The size of foam nests is unlimited,because oxygen for each embryo is supplied directly from thefoam.     

Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity

BACKGROUND: Flooding causes substantial stress for terrestrial plants, particularly if the floodwater completely submerges the shoot. The main problems during submergence are shortage of oxygen due to the slow diffusion rates of gases in water, and depletion of carbohydrates, which is the substrate for respiration. These two factors together lead to loss of biomass and eventually death of the submerged plants. Although conditions under water are unfavourable with respect to light and carbon dioxide supply, photosynthesis may provide both oxygen and carbohydrates, resulting in continuation of aerobic respiration. SCOPE: This review focuses on evidence in the literature that photosynthesis contributes to survival of terrestrial plants during complete submergence. Furthermore, we discuss relevant morphological and physiological responses of the shoot of terrestrial plant species that enable the positive effects of light on underwater plant performance. CONCLUSIONS: Light increases the survival of terrestrial plants under water, indicating that photosynthesis commonly occurs under these submerged conditions. Such underwater photosynthesis increases both internal oxygen concentrations and carbohydrate contents, compared with plants submerged in the dark, and thereby alleviates the adverse effects of flooding. Additionally, several terrestrial species show high plasticity with respect to their leaf development. In a number of species, leaf morphology changes in response to submergence, probably to facilitate underwater gas exchange. Such increased gas exchange may result in higher assimilation rates, and lower carbon dioxide compensation points under water, which is particularly important at the low carbon dioxide concentrations observed in the field. As a result of higher internal carbon dioxide concentrations in submergence-acclimated plants, underwater photorespiration rates are expected to be lower than in non-acclimated plants. Furthermore, the regulatory mechanisms that induce the switch from terrestrial to submergence-acclimated leaves may be controlled by the same pathways as described for heterophyllous aquatic plants.     

Acclimation of a terrestrial plant to submergence facilitates gas exchange under water

Flooding imposes stress upon terrestrial plants since it severely hampers gas exchange rates between the shoot and the environment. The resulting oxygen deficiency is considered to be the major problem for submerged plants. Oxygen microelectrode studies have, however, shown that aquatic plants maintain relatively high internal oxygen pressures under water, and even may release oxygen via the roots into the sediment, also in dark. Based on these results, we challenge the dogma that oxygen pressures in submerged terrestrial plants immediately drop to levels at which aerobic respiration is impaired. The present study demonstrates that the internal oxygen pressure in the petioles of Rumex palustris plants under water is indeed well above the critical oxygen pressure for aerobic respiration, provided that the air‐saturated water is not completely stagnant. The beneficial effect of shoot acclimation of this terrestrial plant species to submergence for gas exchange capacity is also shown. Shoot acclimation to submergence involved a reduction of the diffusion resistance to gases, which was not only functional by increasing diffusion of oxygen into the plant, but also by increasing influx of CO₂, which enhances underwater photosynthesis.     

Developmental consequences of association with a photosynthetic substrate for encapsulated embryos of an intertidal gastropod

Aggregation of embryos in clutches that lack internal circulation can increase the risk of hypoxia by limiting gas exchange. As a result, limits on oxygen solubility and diffusion in water can constrain the size and embryo concentration of aquatic egg clutches. Hypoxia in egg masses can slow embryo development, increase mortality, and reduce size at hatching. The risk of hypoxia for embryos, however, can be reduced by association with photosynthetic organisms. We examined whether embryo development in egg ribbons of the cephalaspidean mollusk Haminoea vesicula is significantly influenced by oviposition on eelgrass (Zostera marina). Association with the photosynthetic substrate had marked effects on development relative to association with non-photosynthetic substrates, and the direction of these effects was mediated by light conditions. Under intermediate and high light levels, association with eelgrass accelerated embryo development, while under dim light, the presence of the macrophyte increased development rate and reduced hatchling shell size. Benefits of association with eelgrass at higher light levels likely result from oxygen production by eelgrass photosynthesis, while we attribute costs under low light to oxygen depletion by eelgrass respiration. Association with Z. marina also limited microphyte growth in egg ribbons of H. vesicula. In the field, measurements of light attenuation within an eelgrass bed showed that conditions under which benefits accrue to embryos are ecologically relevant and correspond to spatial patterns of oviposition on eelgrass in the field. The choice of a photosynthetic oviposition substrate under appropriate light conditions can improve embryo fitness by accelerating embryo development without compromising hatchling size and by reducing the potential for excessive and harmful fouling by microphytes.     

Energy status and its control on embryogenesis of legumes. Embryo photosynthesis contributes to oxygen supply and is coupled to biosynthetic fluxes

Legume seeds are heterotrophic and dependent on mitochondrial respiration. Due to the limited diffusional gas exchange, embryos grow in an environment of low oxygen. O(2) levels within embryo tissues were measured using microsensors and are lowest in early stages and during night, up to 0.4% of atmospheric O(2) concentration (1.1 micro M). Embryo respiration was more strongly inhibited by low O(2) during earlier than later stages. ATP content and adenylate energy charge were lowest in young embryos, whereas ethanol emission and alcohol dehydrogenase activity were high, indicating restricted ATP synthesis and fermentative metabolism. In vitro and in vivo experiments further revealed that embryo metabolism is O(2) limited. During maturation, ATP levels increased and fermentative metabolism disappeared. This indicates that embryos become adapted to the low O(2) and can adjust its energy state on a higher level. Embryos become green and photosynthetically active during differentiation. Photosynthetic O(2) production elevated the internal level up to approximately 50% of atmospheric O(2) concentration (135 micro M). Upon light conditions, embryos partitioned approximately 3-fold more [(14)C]sucrose into starch. The light-dependent increase of starch synthesis was developmentally regulated. However, steady-state levels of nucleotides, free amino acids, sugars, and glycolytic intermediates did not change upon light or dark conditions. Maturing embryos responded to low O(2) supply by adjusting metabolic fluxes rather than the steady-state levels of metabolites. We conclude that embryogenic photosynthesis increases biosynthetic fluxes probably by providing O(2) and energy that is readily used for biosynthesis and respiration.     

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.     

Limits to Diffusive O2 Transport: Flow,Form, and Function in Nudibranch Egg Masses from Temperate and Polar Regions

Background

Many aquatic animals enclose embryos in gelatinous masses, and these embryos rely on diffusion to supply oxygen. Mass structure plays an important role in limiting or facilitating O₂ supply, but external factors such as temperature and photosynthesis can play important roles as well. Other external factors are less well understood.

Methodology/Principal Findings

We first explored the effects of water flow on O₂ levels inside nudibranch embryo masses and compared the effects of flow on masses from temperate and polar regions. Water flow (still vs. vigorously bubbled) had a strong effect on central O₂ levels in all masses; in still water, masses were considerably more hypoxic than in bubbled water. This effect was stronger in temperate than in polar masses, likely due to the increased metabolic demand and O₂ consumption of temperate masses. Second, we made what are to our knowledge the first measurements of O₂ in invertebrate masses in the field. Consistent with laboratory experiments, O₂ in Antarctic masses was high in masses in situ, suggesting that boundary-layer effects do not substantially limit O₂ supply to polar embryos in the field.

Conclusions/Significance

All else being equal, boundary layers are more likely to depress O₂ in masses in temperate or tropical regions; thus, selection on parents to choose high-flow sites for mass deposition is likely greater in warm water. Because of the large number of variables affecting diffusive O₂ supply to embryos in their natural environment, field observations are necessary to test hypotheses generated from laboratory experiments and mathematical modeling.     

Continuous production of L-arginine using immobilized growing Serratia marcescens cells: Effectiveness of supply of oxygen gas

Summary The immobilized growing cell system using Serratia marcescens was applied to continuous L-arginine production. From the determination of oxygen uptake rate, it was shown that the cells entrapped in carrageenan gel were in an oxygen-limited state due to the diffusion barrier to oxygen transport created by the gel layer. This limited state in gel was relieved by supply of oxygen-enriched gas instead of air into the medium. The maximum population of immobilized cells increased to five times that of free cells with the supply of pure oxygen gas. The L-arginine-producing activity of the immobilized growing cells was proportional to the concentration of oxygen gas supplied and was 6 mg/h per millilitre in gel supplied with pure oxyges gas. The continuous L-arginine containing production was constantly maintained by controlling the medium penicillin G at pH 6.5 and more than 10 mg/ml of L-arginine were obtained at 10h of residence time for at least 12 days.     

主养草鱼高密度池塘溶氧收支平衡的研究

采用原位生态学的方法测定广东省中山市9口主养草鱼高密度池塘中浮游植物光合作用产氧量、水柱呼吸耗氧量、底泥呼吸耗氧量和鱼呼吸耗氧量, 并用数学模型计算增氧机增氧量及用差减法计算大气扩散作用引起的得氧或失氧, 对高密度养殖池塘中溶氧收支平衡状况进行了研究。结果显示: 在水深为1.5-2.0 m的主养草鱼高密度池塘中, 光合作用产氧量随着水深的增加而显著降低(P0.05), 底层出现负值呈现氧债现象。水呼吸耗氧量在表层、中层和底层之间没有显著差异(P0.05)。表层水光合作用产氧量显著大于水呼吸耗氧量(P0.05), 而中层和底层水光合作用产氧量却显著小于水呼吸耗氧量(P0.05)。在主养草鱼高密度池塘溶氧的收入中, 浮游植物光合作用产氧量、增氧机增氧量和大气扩散溶入氧量分别占总溶氧来源的44.7%、42.3%和13.0%, 机械增氧作用已接近光合作用, 成为溶氧来源的主要贡献者; 在池塘溶氧的支出中, 水呼吸、鱼呼吸和底泥呼吸耗氧量分别占总耗氧量的45.9%、45.0%和9.1%, 鱼呼吸耗氧与水呼吸耗氧相当, 成为水体中氧气的主要消耗者。结果表明在草鱼高密度养殖过程中, 合理使用机械增氧是池塘溶氧管理的有效措施。       

A big‐microsite framework for soil carbon modeling

Soil carbon cycling processes potentially play a large role in biotic feedbacks to climate change, but little agreement exists at present on what the core of numerical soil C cycling models should look like. In contrast, most canopy models of photosynthesis and leaf gas exchange share a common ‘Farquhaur‐model’ core structure. Here, we explore why a similar core model structure for heterotrophic soil respiration remains elusive and how a pathway to that goal might be envisioned. The spatial and temporal variation in soil microsite conditions greatly complicates modeling efforts, but we believe it is possible to develop a tractable number of parameterizable equations that are organized into a coherent, modular, numerical model structure. First, we show parallels in insights gleaned from linking Arrhenius and Michaelis–Menten kinetics for both photosynthesis and soil respiration. Additional equations and layers of complexity are then added to simulate substrate supply. For soils, model modules that simulate carbon stabilization processes will be key to estimating the fraction of soil C that is accessible to enzymes. Potential modules for dynamic photosynthate input, wetting‐event inputs, freeze–thaw impacts on substrate diffusion, aggregate turnover, soluble‐C sorption, gas transport, methane respiration, and microbial dynamics are described for conceptually and numerically linking our understanding of fast‐response processes of soil gas exchange with longer‐term dynamics of soil carbon and nitrogen stocks.     

Photosynthesis in the Pericarp of Developing Wheat Grains

Oxygen exchange in grains of wheat was measured in both lightand dark over the period of grain development. Between 10 dand 30 d after anthesis, the rate of photosynthesis exceededthe rate of respiration. Peak photosynthetic activity was observedat 20 d after anthesis, coinciding with maximum chlorophyllcontent in the pericarp green layer. Removal of the pericarptransparent layer increased rates of oxygen exchange in boththe light and the dark. Attempts to inhibit photosynthesis withDCMU were only successful with the pericarp transparent layerremoved. Key words: Wheat, pericarp, photosynthesis     

Ethylene – and oxygen signalling – drive plant survival during flooding

Flooding is a widely occurring environmental stress both for natural and cultivated plant species. The primary problems associated with flooding arise due to restricted gas diffusion underwater. This hampers gas exchange needed for the critical processes of photosynthesis and respiration. Plant acclimation to flooding includes the adaptation of a suite of traits that helps alleviate or avoid these stressful conditions and improves or restores exchange of O₂ and CO₂. The manifestation of these traits is, however, reliant on the timely perception of signals that convey the underwater status. Flooding‐associated reduced gas diffusion imposes a drastic change in the internal gas composition within submerged plant organs. One of the earliest changes is an increase in the levels of the gaseous plant hormone ethylene. Depending on the species, organ, flooding conditions and time of the day, plants will also subsequently experience a reduction in oxygen levels. This review provides a comprehensive overview on the roles of ethylene and oxygen as critical signals of flooding stress. It includes a discussion of the dynamics of these gases in plants when underwater, their interaction, current knowledge of their perception mechanisms and the resulting downstream changes that mediate important acclimative processes that allow endurance and survival under flooded conditions.     

The effect of excess moisture on the germination of Spinacia oleracea L.

Summary The reversible inhibition of the germination of spinach (Spinacia oleracea L.) seeds in conditions which are even slightly wetter than optimal has been traced to the production, in a wet environment, of a layer of mucilage around and within the fruit coat which surrounds the true seed. Such wet seeds may however germinate readily when the temperature is lowered, or the oxygen pressure of the environment is raised, or the intact seeds are placed for a short time in hydrogen peroxide before being transferred to what normally would be an excess of water. Even in the absence of an increased oxygen supply the seeds will germinate under water provided the fruit coat, or even a small part of it where it covers the radicle, is crefully removed. No evidence has been found of a water soluble inhibitor and the findings are consistent with the hypothesis that germination is dependent on a sufficiently high rate of supply of oxygen to the sites embryonic respiration. The mucilage which is formed under wet conditions forms a barrier which prevents the transfer of oxygen to the embryo by gaseous diffusion or aqueous convection currents and restricts it to the process of aqueous diffusion, and under these conditions the rate of oxygen supply may not reach the threshold level required for germination.     

Internal heterotrophy following the switch from macrophytes to algae in Lake Apopka, Florida

Measurements of photosynthesis and community respiration in Lake Apopka, Florida, U.S.A. indicate that this lake may be heterotrophic, and that the source of extra organic carbon is internal rather than external to the lake. This large and shallow lake (area 124 km², mean depth 1.7 m) was dominated by macrophytes until hurricane-associated winds disrupted the plants in 1947, and the lake switched to a turbid, algal state. A layer of flocculent, organic sediments covers the lakebed to an average depth of 45 cm and winds regularly resuspend the upper portion into the water column. We used the diel oxygen curve method to estimate production and respiration and also reanalyzed the results of five past studies of production in the lake. The production measurements did not support the hypothesis that the flocculent layer represented excess algal production since 1947. Community respiration exceeded gross production on 60 out of 76 days sampled with statistically significant negative net production found in two of the three studies using the light and dark bottle oxygen method. External supplies of organic carbon are relatively small and are balanced by losses through the outlet. If the lake is heterotrophic, the excess respiration is most likely supported by the remains of macrophytes deposited in the sediments prior to the switch to an algal state. Similar sediment oxidation probably occurs in other shallow lakes that switch from the macrophyte to the algal state.     

A model describing photosynthesis in terms of gas diffusion and enzyme kinetics

Summary A model predicting net photosynthesis of individual plant leaves for a variety of environmental conditions has been developed. It is based on an electrical analogue describing gas diffusion from the free atmosphere to the sites of CO₂ fixation and a Michaelis-Menten equation describing CO₂ fixation. The model is presented in two versions, a simplified form without respiration and a more complex form including respiration. Both versions include terms for light and temperature dependence of CO₂ fixation and light control of stomatal resistance. The second version also includes terms for temperature, light, and oxygen dependence of respiration and O₂ dependence of CO₂ fixation.The model is illustrated with curves based on representative values of the various environmental and biological parameters. These curves relate net photosynthesis to light intensity, [CO₂], [O₂], temperature, and resistances to CO₂ uptake. The shape of the [CO₂]-net photosynthesis curves depends on the total diffusion resistance to CO₂ uptake and the Michaelis constant for CO₂ uptake. The curves range from typical Michaelis-Menten to Blackman types.The model is combined with a model of leaf energy exchange permitting simultaneous estimation of net photosynthesis and transpiration. The combined model is illustrated with curves relating transpiration to photosynthesis under a wide variety of environmental conditions. Environmental regimes yielding maximum efficiency of water use are identified for the given assumptions and biological parameters.     

Photosynthesis drives oxygen levels in macrophyte-associated gastropod egg masses

Many aquatic animals deposit fertilized eggs in adherent clutches or gelatinous masses. Egg aggregation carries certain risks, including the potential for inadequate oxygen supply to embryos. Physical and biological conditions alter such risks. We examined the effects of light levels and associated photosynthetic organisms on the distribution of oxygen inside gelatinous egg masses of four temperate gastropod species. Egg masses of two species, the opisthobranchs Melanochlamys diomedea and Haminoea callidegenita, contained significant populations of diatoms but generally were not associated with macrophytes. Egg masses of the other two species, the opisthobranch Haminoea vesicula and the prosobranch Lacuna sp., occurred commonly on subtidal macrophytes and appeared not to contain significant populations of diatoms. In the laboratory, we used microelectrodes to measure oxygen levels inside masses exposed to alternating dark and light conditions; light level had an enormous influence on oxygen profiles in egg masses of all four species. Masses of H. vesicula and Lacuna sp., when experimentally separated from their macrophytes, showed only slight increases in oxygen upon light exposure, indicating that the main source of oxygen in situ was the macrophyte rather than associated microalgae. Our findings indicate that photosynthesis by macrophytes can drive large changes in internal oxygen profiles.     

Reduction of molecular gas diffusion through gaskets in leaf gas exchange cuvettes by leaf‐mediated pores

There is an ongoing debate on how to correct leaf gas exchange measurements for the unavoidable diffusion leakage that occurs when measurements are done in non‐ambient CO₂ concentrations. In this study, we present a theory on how the CO₂ diffusion gradient over the gasket is affected by leaf‐mediated pores (LMP) and how LMP reduce diffusive exchange across the gaskets. Recent discussions have so far neglected the processes in the quasi‐laminar boundary layer around the gasket. Counter intuitively, LMP reduce the leakage through gaskets, which can be explained by assuming that the boundary layer at the exterior of the cuvette is enriched with air from the inside of the cuvette. The effect can thus be reduced by reducing the boundary layer thickness. The theory clarifies conflicting results from earlier studies. We developed leaf adaptor frames that eliminate LMP during measurements on delicate plant material such as grass leaves with circular cross section, and the effectiveness is shown with respiration measurements on a harp of Deschampsia flexuosa leaves. We conclude that the best solution for measurements with portable photosynthesis systems is to avoid LMP rather than trying to correct for the effects.     

Morphometric analysis of the tracheal walls of the harvestmen Nemastoma lugubre (Arachnida, Opiliones, Nemastomatidae)

The tracheal system of harvestmen consists of two stem tracheae, which give rise to higher order tracheae that supply the extremities and internal organs. In this study, we used stereological morphometric methods to investigate diffusing capacities of the walls ('lateral diffusing capacity') of the tracheae of adult males and females of Nemastoma lugubre. Diffusing barriers of the tracheal walls tend to be thinnest (0.17-0.19 microm) for the smallest tracheae (inner diameter 0.5-2 microm). In other tracheal classes the diffusing barriers increase with increasing diameters. Calculation of the mass-specific diffusing capacity for oxygen (D(O2)) of the walls of all higher order tracheae revealed 10.57 microl min(-1)g(-1)kPa(-1) for the females (mean body mass 3.8 mg) and 25.23 microl min(-1)g(-1)kPa(-1) for the males (mean body mass 1.4 mg). In both animal groups, the main D(O2) (58-67%) lies in the tracheae with an inner diameter of 0.5-2 microm, but also tracheae up to an inner diameter of 20 microm allow gas exchange via the tracheal walls. Stem tracheae are of no importance for lateral diffusion. Our results are consistent with the hypothesis that the functional morphology of the tracheal system of harvestmen represents an 'intermediate state' between the tracheal system of insects in which gas exchange is focused on the distal portions and that of spiders, in which the walls of all tracheae serve in gas exchange.