Circumventing surface tension: Tadpoles suck bubbles to breathe air

Abstract

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.