The mayfly Neocloeon triangulifer senses decreasing oxygen availability (PO2) and responds by reducing ion uptake and altering gene expression.

Oxygen availability is central to the energetic budget of aquatic animals and may vary naturally and/or in response to anthropogenic activities. Yet, we know little about how oxygen availability is linked to fundamental processes such as ion transport in aquatic insects. We hypothesized and observed that ion (22Na and 35SO4) uptake would be significantly decreased at O2 partial pressures below the mean Pcrit (5.4 kPa) where metabolic rates (MO2) are compromised, and ATP production is limited. However, we were surprised to observe marked reductions in ion uptake at oxygen partial pressures well above the Pcrit, where MO2 was stable. For example, SO4 uptake decreased by 51% at 11.7kPa, and 82% at the Pcrit (5.4kPa) while Na uptake decreased by 19% at 11.7kPa, and 60% at the Pcrit. Nymphs held for longer time periods at reduced PO2 exhibited stronger reductions in ion uptake rates. Fluids from whole body homogenates exhibited a 29% decrease in osmolality in the most hypoxic condition. The differential expression of atypical guanylyl cyclase (gcy-88e) in response to changing PO2 conditions provides evidence for its potential role as an oxygen sensor. Several ion transport genes (e.g., chloride channel and sodium-potassium ATPase) and hypoxia-associated genes (e.g., ldh and egl-9) were also impacted by decreased oxygen availability. Together, our work suggests that N. triangulifer can sense decreased oxygen availability and perhaps conserves energy accordingly, even when MO2 is not impacted.