Subsurface behaviours facilitate respiration by a physical gill in an adult giant water bug, Abedus herberti

Abedus herberti is an aquatic bug that carries a subalar store of atmospheric air for use during submersion. When submersed, adult bugs routinely show two patterns of behaviour we call gaping and dynamic gaping. We studied these behaviours to determine when they are expressed and tested the hypothesis that the behaviours serve a respiratory function. Gaping and dynamic gaping occurred when bugs were positioned sufficiently deep in water that they could not refresh air stores without releasing their hold on the substrate and floating to the surface. Gaping and dynamic gaping occurred more frequently as water depth increased, and bugs performing these behaviours remained submersed longer than bugs that did not express these behaviours. Bugs permitted to express gaping and dynamic gaping remain submersed longer than bugs that were experimentally prevented from performing the behaviours. Bugs also remained submersed longer in high oxygen water than in low oxygen water. We conclude that gaping allows the subalar air store to function as a physical gill and that dynamic gaping is a form of behavioural ventilation. Adult A. herberti are buoyant and must swim or hold onto a substrate to remain submersed. Extending submersion time increases predatory efficiency, reduces the frequency of surfacing, saves energy and reduces predation risk.