Acute exposure to high environmental ammonia (HEA) triggers the emersion response in the green shore crab

The physiological effects of high environmental ammonia (HEA) exposure have been well documented in many aquatic species. In particular, it has recently been demonstrated that exposure to ammonia in fish leads to a similar hyperventilatory response as observed during exposure to hypoxia. In littoral crabs, such as the green crab (Carcinus maenas), exposure to severe hypoxia triggers an emersion response whereby crabs escape hypoxia to breathe air. We hypothesized that exposure to HEA in green crabs would lead to a similar behavioural response which is specific to ammonia. Using an experimental arena containing a rock bed onto which crabs could emerse, we established that exposure to HEA (4mmol/l NH₄HCO₃) for 15min triggers emersion in crabs. In experiments utilizing NaHCO₃ controls and NH₄HCO₃ injections, we further determined that emersion was triggered specifically by external ammonia and was independent of secondary acid-base or respiratory disturbances caused by HEA. We then hypothesized that emersion from HEA provides a physiological benefit, similar to emersion from hypoxia. Exposure to 15min of HEA without emersion (no rock bed present) caused significant increases in arterial haemolymph total ammonia (T_amm), pH, and [HCO₃^-]_. When emersion was allowed, arterial haemolymph T_amm and [HCO₃^-] increased, but no alkalosis developed. Moreover, emersion decreased haemolymph partial pressure of NH₃ relative to crabs which could not emerse. Overall, we demonstrate a novel behavioural response to HEA exposure in crabs which we propose may share similar mechanistic pathways with the emersion response triggered by hypoxia.