Circumventing surface tension: tadpoles suck bubbles to breathe air

Surface tension in biological systems - a common problem with a variety of solutions

Water is of fundamental importance to living organisms, not only as a universal solvent to maintain metabolic activity but also due to the effects the physical properties of water have on different organismal structures. In this review, we explore some examples of how living organisms deal with surfaces covered with or in contact with water. While we do not intend to describe all possible forms of interactions in every minute detail, we would like to draw attention to this intriguing interdisciplinary subject and discuss the positive and negative effects of the interaction forces between water molecules and organisms. Topics explored include locomotion on water, wettability of surfaces, benefits of retaining a film of air while submerged (Salvinia effect), surface tension of water inhibiting air-breathing, accumulation of water in small tubes, surface tension in non-mammalian and mammalian respiratory systems. In each topic, we address the importance of interactions with water and the adaptations seen in an organism to solve the surface-related challenges, trying to explore the different selective pressures acting onto different organisms allowing exploring or compensating these surface-related interactions.

Air-breathing behavior underlies the cell death in limbs of Rana pirica tadpoles

Amphibians shape their limbs by differential outgrowth of digits and interdigital regions. In contrast, amniotes employ cell death, an additional developmental system, to determine the final shape of limbs. Previous work has shown that high oxygen availability is correlated with the induction of cell death in developing limbs. Given the diversity of life histories of amphibians, it is conceivable that some amphibians are exposed to a high-oxygen environment during the tadpole phase and exhibit cell death in their limbs. Here, we examined whether air-breathing behavior underlies the cell death in limbs of aquatic tadpoles of the frog species Rana pirica. Our experimental approach revealed that R. pirica tadpoles exhibit cell death in their limbs that is likely to be induced by oxidative stress associated with their frequent air-breathing behavior.

Air Breathing and Suction Feeding Kinematics in the West African Lungfish, Protopterus Annectens

Research on the water-to-land transition tends to focus on the locomotor changes necessary for terrestriality. But the evolution from water breathing to air breathing was also a necessary precursor to the invasion of land. Air is approximately 1,000 times less dense, 50 times less viscous, and contains hundreds of times more oxygen than water. However, unlike the transition to terrestrial locomotion, breathing air does not require body weight support, so the evolution of air breathing may have necessitated smaller changes to morphology and function. We used X-ray Reconstruction of Moving Morphology to compare the cranial kinematics of aquatic buccal pumping, such as seen in suction feeding, with the aerial buccal pumping required for lung ventilation in the West African lungfish (Protopterus annectens). During buccal pumping behaviors, the cranial bones and associated soft tissues act as valves and pumps, and the sequence of their motions controls the pattern of fluid flow. Both behaviors are characterized by an anterior-to-posterior wave of expansion and an anterior-to-posterior wave of compression. We found that the pectoral girdle and cranial rib rotate consistently during air breathing and suction feeding, and that the muscle between them shortens during buccal expansion. Overall, we conclude that the major cranial bones maintain the same basic functions (i.e., acting as valves or pumps, or transmitting power) across aquatic and aerial buccal pumping. The cranial morphology that enables aquatic buccal pumping is well-suited to perform air-breathing and accommodates the physical differences between air and water.

The mechanics of air-breathing in African clawed frog tadpoles, Xenopus laevis (Anura: Pipidae)

Frog larvae (tadpoles) undergo many physiological, morphological, and behavioral transformations through development before metamorphosing into their adult form. The surface tension of water prevents small tadpoles from breaching the surface to breathe air (including those of Xenopus laevis), forcing them to acquire air using a form of breathing called bubble-sucking. With growth, tadpoles typically make a behavioral/biomechanical transition). X. laevis tadpoles have also been shown to transition physiologically from conforming passively to ambient oxygen levels to actively regulating their blood oxygen. However, it is unknown whether these mechanical and physiological breathing transitions are temporally or functionally linked, or how both transitions relate to lung maturation and gas exchange competency. If these transitions are linked, it could mean that one biomechanical breathing mode (breaching) is more physiologically proficient at acquiring gaseous oxygen than the other. Here, we describe the mechanics and development of air-breathing and the ontogeny of lung morphology in X. laevis throughout the larval stage and examine our findings considering previous physiological work. We find that the transitions from bubble-sucking to breaching and from oxygen conforming to oxygen regulation co-occur in X. laevis tadpoles at the same larval stage (Nieuwkoop-Faber stages 53-56 and 54-57, respectively), but that the lungs do not increase significantly in vascularization until metamorphosis, suggesting that lung maturation, alone, is not sufficient to account for increased pulmonary capacity earlier in development. Although breach-breathing may confer a respiratory advantage, we remain unaware of a mechanistic explanation to account for this possibility. At present, the transition from bubble-sucking to breaching appears simply to be a consequence of growth. Finally, we consider our results in the context of comparative air-breathing mechanics across vertebrates.

Variation in upper thermal tolerance among 19 species from temperate wetlands

Communities usually possess a multitude of interconnected trophic interactions within food webs. Their regulation generally depends on a balance between bottom-up and top-down effects. However, if sensitivity to temperature varies among species, rising temperatures may change trophic interactions via direct and indirect effects. We examined the critical thermal maximum (CT_max) of 19 species from temperate wetlands (insect predators, amphibian larvae, zooplankton and amphipods) and determined if they vary in their sensitivity to warming temperatures. CT_max differed between the groups, with predatory insects having higher CT_max than amphibians (both herbivorous larval anurans and predatory larval salamanders), amphipods and zooplankton. In a scenario of global warming, these differences in thermal tolerance may affect top-down and bottom-up processes, particularly considering that insect predators are more likely to maintain or improve their performance at higher temperatures, which could lead to increased predation rates on the herbivores in the food web. Further studies are needed to understand how the energy flows through communities, how species' energy budgets may change and whether other physiological and behavioral responses (such as phenotypic plasticity and thermoregulation) can buffer or increase these changes in the top-down regulation of wetland food webs.

The mechanics of air breathing in gray tree frog tadpoles, Hyla versicolor (Anura: Hylidae)

We describe air-breathing mechanics in gray tree frog tadpoles (Hyla versicolor). We found that H. versicolor tadpoles breathe by 'bubble-sucking', a breathing mode typically employed by tadpoles too small to break the water's surface tension, in which a bubble is drawn into the buccal cavity and compressed into the lungs. In most tadpoles, bubble-sucking is replaced by breach breathing (breaking the surface to access air) at larger body sizes. In contrast, H. versicolor tadpoles bubble-suck throughout the larval period, despite reaching body sizes at which breaching is possible. Hyla versicolor tadpoles exhibit two bubble-sucking behaviors: 'single bubble-sucking', previously described in other tadpole species, is characterized by a single suction event followed by a compression phase to fill the lungs; 'double bubble-sucking' is a novel, apparently derived form of bubble-sucking that adds a second suction event. Hyla versicolor tadpoles transition from single bubble-sucking to double bubble-sucking at approximately 5.7 mm snout-vent length (SVL), which corresponds to a period of rapid lung maturation when they transition from low to high vascularization (6.0 mm SVL). Functional, behavioral and morphological evidence suggests that double bubble-sucking increases the efficiency of pulmonary gas exchange by separating expired, deoxygenated air from freshly inspired air to prevent mixing. Hyla versicolor, and possibly other hylid tadpoles, may have specialized for bubble-sucking in order to take advantage of this increased efficiency. Single and double bubble-sucking represent two- and four-stroke ventilation systems, which we discuss in the context of other anamniote air-breathing mechanisms.

Circumventing surface tension: tadpoles suck bubbles to breathe air

The surface tension of water provides a thin, elastic membrane upon which many tiny animals are adapted to live and move. We show that it may be equally important to the minute animals living beneath it by examining air-breathing mechanics in five species (three families) of anuran (frog) tadpoles. Air-breathing is essential for survival and development in most tadpoles, yet we found that all tadpoles at small body sizes were unable to break through the water's surface to access air. Nevertheless, by 3 days post-hatch and only 3 mm body length, all began to breathe air and fill the lungs. High-speed macrovideography revealed that surface tension was circumvented by a novel behaviour we call 'bubble-sucking': mouth attachment to the water's undersurface, the surface drawn into the mouth by suction, a bubble 'pinched off' within the mouth, then compressed and forced into the lungs. Growing tadpoles transitioned to air-breathing via typical surface breaching. Salamander larvae and pulmonate snails were also discovered to 'bubble-suck', and two insects used other means of circumvention, suggesting that surface tension may have a broader impact on animal phenotypes than hitherto appreciated.