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 three
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 behavior 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.