Bimodal Respiration in Aquatic and Terrestrial Apodan Amphibians

SYNOPSIS. New data on resting metabolism, aerobic capacity,respiratory morphology and bimodal breathing capacities offourcaecilian species were combined with previously known data oncaecilian and amphibian biology to discern differing or unifyingthemes of gas exchange within the Gymnophiona and among theAmphibia. Discoveries include 1) a uniform resting metabolismamong caecilians that is distinctly lower than that of anuransand urodeles, 2) an aerobic capacity following activity thatrivals or exceeds that of other amphibians, and 3) despite previoussuggestions to the contrary, a substantial contribution of theskin to respiratory gas exchange, apparently sufficient to sustainthe low resting metabolism. In contrast, pulmonary gas transportappears to dominate during metabolic recovery from exercise,which is comparatively fast. The contributing roles of the skinand lungs to this extreme scope of respiratory needs in caeciliansare further discussed in light of the structural nature of andcirculatory supply to each respiratory bed, and the respiratoryproperties of caecilian blood.     

Absence of Intracellular Potential Gradients in Amphibian Embryos

THE developmental biology literature contains many references to the existence of bioelectric potential gradients in embryonic and regenerating systems^1–5 and, because the polarity of regenerating hydroids and planarians can be altered by application of electric fields^6–8, a role in the control of spatial patterns of cellular organization has been assigned to potential differences which have been measured in vivo. With few exceptions, the experimental evidence in support of the existence of potential gradients is based on measurements made with extracellular electrodes applied to the surface of the particular organism or tissue under consideration. The reliability of some of these measurements is difficult to assess because the potential values quoted tend to be rather small, in the range 10^?6 to 10^?2 V, while information concerning, the magnitude of errors arising from electrode junction potential asymmetries is often not available. If extracellular measurements reflect a true intracellular potential gradient in the organism, electrodes introduced directly into the interior of those cells situated at opposite ends of its major axis should record a larger gradient provided that the internal resistance is greater than that of the external fluid. This is because potential values recorded extracellularly are shunted by the conducting medium in which the tissue is usually bathed.     

Subsidies of Aquatic Resources in Terrestrial Ecosystems

Ecosystems - Floods, spatially complex water flows, and organism movements all generate important fluxes of aquatic-derived materials into terrestrial habitats, counteracting the gravity-driven...     

Comparative Physiology of Respiration in Aquatic Fungi

A comparative study of terminal respiration was undertaken in five genera of aquatic fungi in the Leptomitales. The cytochrome system in this group of fungi contained cytochrome a-a₃ (605 nm), cytochrome c (551 nm), cytochrome b (557 nm), and cytochomo b (564 nm). A representative of each of three aerobic genera, Leptomitus, Apodachlya, and Sapromyces, had a total cytochrome content of about 2×10^?10 mol/mg dry weight. An endogenous respiration rate of 21 μl O₂ uptake/ (h × mg dry weight) at 21.7°C was found in Leptomitus and Apodmhlya and 14 in Sapromyces. The strain belonging to the fermenlative genus Mindeniella had approximately one-third of the total cytochrome content and one-third of the endogenous respiration rate observed in Leptomitus and Apodachlya. Mindeniella and Sapromyces contained less total cytochrome when grown under reduced oxygen tension than when grown in air. Only about one-half of the b-type cytochrome was redueible by endogenous substrates. Both cytochrome a₃ and an unidentified pigment bound CO. The endogenous respiration of Leptomitus, Apodachlyo, and Sapromyces was strongly Inhibited by sodium cyanide, sodium azide, antimycin A, and sodium fluoroacetate.     

Herbivores Enforce Sharp Boundaries Between Terrestrial and Aquatic Ecosystems

The transitions between ecosystems (ecotones) are often biodiversity hotspots, but we know little about the forces that shape them. Today, often sharp boundaries with low diversity are found between terrestrial and aquatic ecosystems. This has been attributed to environmental factors that hamper succession. However, ecosystem properties are often controlled by both bottom-up and top-down forces, but their relative importance in shaping riparian boundaries is not known. We hypothesize that (1) herbivores may enforce sharp transitions between terrestrial and aquatic ecosystems by inhibiting emergent vegetation expansion and reducing the width of the transition zone and (2) the vegetation expansion, diversity, and species turnover are related to abiotic factors in the absence of herbivores, but not in their presence. We tested these hypotheses in 50 paired grazed and ungrazed plots spread over ten wetlands, during two years. Excluding grazers increased vegetation expansion, cover, biomass, and species richness. In ungrazed plots, vegetation cover was negatively related to water depth, whereas plant species richness was negatively related to the vegetation N:P ratio. The presence of (mainly aquatic) herbivores overruled the effect of water depth on vegetation cover increase but did not interact with vegetation N:P ratio. Increased local extinction in the presence of herbivores explained the negative effect of herbivores on species richness, as local colonization rates were unaffected by grazing. We conclude that (aquatic) herbivores can strongly inhibit expansion of the riparian vegetation and reduce vegetation diversity over a range of environmental conditions. Consequently, herbivores enforce sharp boundaries between terrestrial and aquatic ecosystems.     

Aquatic Terrestrial Linkages Along a Braided-River: Riparian Arthropods Feeding on Aquatic Insects

Rivers can provide important sources of energy for riparian biota. Stable isotope analysis (δ¹³C, δ¹⁵N) together with linear mixing models, were used to quantify the importance of aquatic insects as a food source for a riparian arthropod assemblage inhabiting the shore of the braided Tagliamento River (NE Italy). Proportional aquatic prey contributions to riparian arthropod diets differed considerable among taxa. Carabid beetles of the genus Bembidion and Nebria picicornis fed entirely on aquatic insects. Aquatic insects made up 80% of the diet of the dominant staphylinid beetle Paederidus rubrothoracicus. The diets of the dominant lycosid spiders Arctosa cinerea and Pardosa wagleri consisted of 56 and 48% aquatic insects, respectively. In contrast, the ant Manica rubida fed mainly on terrestrial sources. The proportion of aquatic insects in the diet of lycosid spiders changed seasonally, being related to the seasonal abundance of lycosid spiders along the stream edge. The degree of spatial and seasonal aggregation of riparian arthropods at the river edge coincided with their proportional use of aquatic subsidies. The results suggest that predation by riparian arthropods is a quantitatively important process in the transfer of aquatic secondary production to the riparian food web.     

Terrestrial Versus Aquatic Biology: The Medium and Its Message

Many basic biological functions are constrained by the physicalproperties of the fluids in which organisms live. Here I explorefour selected examples in which physical differences betweenair and water have contributed to the functional divergenceof terrestrial and aquatic organisms. 1. Water is about 800times as dense as air. As a result, while the cost of locomotionis generally less for aquatic organisms, the hydrodynamic forcesthey encounter are larger. 2. The combined effects of densityand viscosity insure that the capture of suspended particlesis mechanically more effective on land than in water. 3. Thespeed of sound is four times greater in water than in air, requiringaquatic organisms to use higher frequency sounds in echolocationsystems. 4. The resistivity of air is 16 orders of magnitudelarger than that of seawater, which might explain why aquaticanimals use electrical sensing organs to detect prey but terrestrialanimals do not.     

Cardiovascular and Other Physiological Correlates of Hibernation in Aquatic and Terrestrial Turtles

Recent research has provided new insight into the physiologyof hibernation in freshwater, marine, and terrestrial turtles.In this paper I review what is known about the mechanisms thatpermit the hearts of these turtles to withstand several monthsof hypoxia or anoxia. I also report new research that indicatesthat a terrestrial turtle, unlike freshwater and marine species,does not experience hypoxia in its winter burrow and thus doesnot rely on glycolysis to supply ATP, at least under moderatewinter conditions.     

Terrestrial Dispersal and Potential Environmental Transmission of the Amphibian Chytrid Fungus (Batrachochytrium dendrobatidis)

Dispersal and exposure to amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) is not confined to the aquatic habitat, but little is known about pathways that facilitate exposure to wild terrestrial amphibians that do not typically enter bodies of water. We explored the possible spread of Bd from an aquatic reservoir to terrestrial substrates by the emergence of recently metamorphosed infected amphibians and potential deposition of Bd-positive residue on riparian vegetation in Cusuco National Park, Honduras (CNP). Amphibians and their respective leaf perches were both sampled for Bd presence and the pathogen was detected on 76.1% (35/46) of leaves where a Bd-positive frog had rested. Although the viability of Bd detected on these leaves cannot be discerned from our quantitative PCR results, the cool air temperature, closed canopy, and high humidity of this cloud forest environment in CNP is expected to encourage pathogen persistence. High prevalence of infection (88.5%) detected in the recently metamorphosed amphibians and frequent shedding of Bd-positive residue on foliage demonstrates a pathway of Bd dispersal between aquatic and terrestrial habitats. This pathway provides the opportunity for environmental transmission of Bd among and between amphibian species without direct physical contact or exposure to an aquatic habitat.     

The American Alligator Detects Food Chemicals in Aquatic and Terrestrial Environments

American alligators (Alligator mississippiensis) were offered raw meat or animal carcasses under water or on land in experiments where only chemoreception could be used to discriminate between these and control materials. Alligators also were presented with aqueous extracts and airborne chemicals from meat. Juvenile alligators, tested in indoor tanks, opened more cheesecloth packets containing meat than control packets when these materials were placed under water and on platforms above the water surface. Adult alligators, tested in outdoor semi-natural enclosures, removed more cheesecloth-wrapped meat presented under water, and more of both meat and raccoon (Procyon lotor) carcasses placed under perforated baskets on land, than control materials. Juvenile alligators, tested in tanks partially filled with water, exhibited more lateral head movements and mouth-openings to an aqueous beef extract than to water alone. Juvenile alligators, tested in an olfactometer, exhibited more gular pumps to airborne chemicals from beef than to distilled water. These experiments indicate that alligators may use chemical cues to locate food both on land and under water, and that they detect both water- and airborne chemicals from meat.     

Ecological, Evolutionary, and Physical Factors Influencing Aquatic Animal Respiration

Compared to air-breathers, animals that respire aquaticallyhave limited access to O₂ and their habitats are more subjectto hypoxia. Because O₂ diffuses more slowly through water thanair, animals in water experience greater diffusion boundarylayer effects on respiratory gas diffusion. While ventilationand specialized exchange surfaces mitigate O₂ diffusion limitationson respiration, most animal phyla, particularly those confinedto aquatic habitats, lack these. Diffusion limitation influencesthe ontogeny of aquatic animals and may have also shaped Precambrianmetazoans. In spite of a more limited O₂ access, aquatic animalsdisplay a much greater spectrum of respiratory adaptation, rangingfrom the loss of Hb in icefishes to the independent evolution,invention, and acquisition of Hb in many invertebrates confinedto hypoxic habitats. Three features of aquatic respiratory systemsdistinguishing them from aerial systems are the widespread occurrenceof integumental respiration, the frequent presence of combinedrespiratory and feeding surfaces, and the profound effect ofhypoxia on shaping respiratory adaptation, both in shallow waterand in the deep sea.     

Bacterial Growth on Photochemically Transformed Leachates from Aquatic and Terrestrial Primary Producers

Abstract We measured bacterial growth on phototransformed dissolved organic matter (DOM) leached from eight different primary producers. Leachates (10 mg C liter⁻¹) were exposed to artificial UVA + UVB radiation, or kept in darkness, for 20 h. DOM solutions were subsequently inoculated with lake water bacteria. Photoproduction of dissolved inorganic carbon (DIC), ranging from 3 to 16 μg C liter⁻¹ h⁻¹, and changes in the absorptive characteristics of the DOM were observed for all leachates upon UV irradiation. The effects of irradiation exposure on DOM bioavailability varied greatly, depending on leachate and type of bacterial growth criterion. Bacterial carbon utilization (biomass production plus respiration) over the entire incubation period (120 h) was enhanced by UV radiation of leachate from the terrestrial leaves, relative to carbon utilization in non-irradiated leachates. Conversely, carbon utilization was reduced by radiation of the leachates from aquatic macrophytes. In a separate experiment, the stable C and N isotope composition of bacteria grown on irradiated and non-irradiated DOM was estimated. Bacterial growth on UV-irradiated DOM was enriched in ¹³C relative to the bacteria in the non-irradiated treatments; this result may be explained by selective assimilation of photochemically produced, isotopically enriched labile compounds. Received: 17 February 2000; Accepted: 1 May 2000; Online Publication: 28 August 2000     

Trophic Transfer of Arsenic from an Aquatic Insect to Terrestrial Insect Predators

The movement of energy and nutrients from aquatic to terrestrial ecosystems can be substantial, and emergent aquatic insects can serve as biovectors not only for nutrients, but also for contaminants present in the aquatic environment. The terrestrial predators Tenodera aridifolia sinensis (Mantodea: Mantidae) and Tidarren haemorrhoidale (Araneae: Theridiidae) and the aquatic predator Buenoa scimitra (Hemiptera: Notonectidae) were chosen to evaluate the efficacy of arsenic transfer between aquatic and terrestrial environments. Culex tarsalis larvae were reared in either control water or water containing 1000 µg l⁻¹ arsenic. Adults that emerged from the control and arsenic treatments were fed to the terrestrial predators, and fourth instar larvae were fed to the aquatic predator reared in control or arsenic contaminated water. Tenodera a. sinensis fed arsenic-treated Cx. tarsalis accumulated 658±130 ng g⁻¹ of arsenic. There was no significant difference between control and arsenic-fed T. haemorrhoidale (range 142–290 ng g⁻¹). Buenoa scimitra accumulated 5120±406 ng g⁻¹ of arsenic when exposed to arsenic-fed Cx. tarsalis and reared in water containing 1000 µg l⁻¹ arsenic. There was no significant difference between controls or arsenic-fed B. scimitra that were not exposed to water-borne arsenic, indicating that for this species environmental exposure was more important in accumulation than strictly dietary arsenic. These results indicate that transfer to terrestrial predators may play an important role in arsenic cycling, which would be particularly true during periods of mass emergence of potential insect biovectors. Trophic transfer within the aquatic environment may still occur with secondary predation, or in predators with different feeding strategies.     

A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.     

Sequestration,Phytoreduction, and Phytooxidation of Halogenated Organic Chemicals by Aquatic and Terrestrial Plants

Laboratory data from plant-mediated transformation of chlorinated and brominated alkanes, alkenes, and chlorinated pesticides, including phytotransformation data from field plants currently used in phytoremediation of trichloroethylene (TCE), were reviewed for the purpose of identifying important phytoprocesses and their respective roles in phytoremediation of halogenated organic compounds (HOCs). The results of the laboratory experiments indicated that the initial very rapid removal of hydrophobic HOCs from water or the gas phase by aquatic and terrestrial plants is primarily due to sequestration. The amount of HOC sequestered is controlled by the plant species and the physicochemical properties (e.g., K_ow, aqueous solubility, volatility) of the contaminant. Phytodegradation studies conducted in both the gas and aqueous phases indicated that hexachloroethane (HCA) is dechlorinated to the same metabolites by sterilized and axenically cultivated aquatic plants and an isolated plant dehalogenase factor. Similar results were obtained in experiments conducted with o,p'-DDT and p,p'-DDT in aqueous solution. The sterilized and axenically cultivated aquatic plants also oxidized HCA to similar chloroacetic acids. The metabolism of HOCs to the corresponding oxidative and reductive transformation products identified in the plant rhizosphere, stems, and leaves suggested that more than one pathway, requiring different enzymes, may be involved in phytotransformation reactions. Four phytoprocesses (mechanisms) were found to be important in the removal of the probe HOCs from water by aquatic plants, namely, (1) rapid sequestration by partitioning to the lipophilic plant cuticles; (2) phytoreduction to less halogenated metabolites; (3) phytooxidation to haloethanols, haloacetic acids, and unidentified metabolites; and (4) assimilation into the plant tissues as nonphytotoxic products, presumably produced by covalent binding with the plant tissues. Laboratory and field data indicate that the distribution of metabolites of perchloroethylene (PCE) and TCE in cottonwood and willow trees is determined by the growth stage or age of these vascular plants, the plant species, and the duration of exposure to the compound. For terrestrial plants, the predominant phytoprocesses by which HOCs are attenuated in the environment include sequestration, rhizodegradation, uptake, phytodegradation, and phytovolatilization. Using PCE as a model chlorinated organic solvent, possible phytotransformation pathways are proposed to account for the different metabolites identified in the rhizosphere and tissues of laboratory and field plants. The proposed pathways also combine phytoreduction reactions that occur in plant tissues and are likely catalyzed by plant dehalogenase(s) for example, enzyme(s) such as glutathione-S-transferase and Fe-S clusters in chloroplast ferredoxin, with phytooxidation and covalent binding (phytoassimilation) reactions mediated by oxidative-enzymes (possibly cytochrome P-450 with monooxygenase activity, glutathione or laccase). Depending on the characteristics of the field site, the phytoprocesses identified in this study are vital in the design and implementation of phytoremediation of halogenated organic contaminants.     

Terrestrial Contributions to the Aquatic Food Web in the Middle Yangtze River

Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ¹³C and δ¹⁵N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C₄ plants was more important than that of C₃ plants at the upstream site while C₃ plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.