The physiological role of haemoglobin in backswimmers (Notonectidae, Anisops)

Unlike all other diving insects, backswimmers of the genus Anisops can exploit the pelagic zone by temporarily achieving near-neutral buoyancy during the course of a dive. They begin a dive positively buoyant due to the large volume of air carried in their ventral air-stores, but rapidly enter a protracted period of near neutral buoyancy before becoming negatively buoyant. This dive profile is due to haemoglobin found in large tracheated cells in the abdomen. Fibre optic oxygen probes placed in the airstores of submerged bugs revealed that oxygen partial pressure (PO2) dropped in a sigmoid curve, where a linear decline preceded a plateau between 5.1 and 2.0 kPa, before a final drop. Buoyancy measurements made by attaching backswimmers to a sensitive electronic balance showed the same three phases. Inactivating the haemoglobin by fumigating backswimmers with 15 % CO eliminated both buoyancy and PO2 plateaus. Oxygen unloaded from the haemoglobin stabilises the air-store during the neutrally buoyant phase after a decrease in volume between 16 to 19 %. Using measurements of air-store PO2 and volume it was calculated that during a dive the haemoglobin and air-store contribute 0.25 and 0.26 μL of oxygen, respectively.