Gas gill respiration enhances underwater copulation in a water scorpion.

To overcome physiological constraints during submergence, many air-breathing aquatic animals utilize an air store. If this store functions as a gas gill for dissolved oxygen uptake, this will extend underwater copulation when atmospheric oxygen is inaccessible. We tested this hypothesis using water scorpion Nepa hoffmanni (Hemiptera: Nepidae). Our field surveys showed that, although N. hoffmanni typically inhabit shallow water, males cannot access atmospheric oxygen via their respiratory siphon during underwater copulation. Laboratory experiments revealed active oxygen use from the air store beneath the wings during submergence. Furthermore, under higher dissolved oxygen treatments, males copulated longer and more frequently, while mate-guarding behavior remained unaffected. These findings provide the first direct evidence that dissolved oxygen uptake via a gas gill enhances copulation efficiency in air-breathing aquatic insects, highlighting the importance of environmental oxygen availability for aquatic reproductive strategies, especially in the context of ongoing climate-driven changes to freshwater ecosystems.