Aquatic surface respiration,an adaptive response to hypoxia in the guppy,Poecilia reticulata (Pisces,Poeciliidae)

Synopsis Aquatic respiration at the air-water interface, herein termed aquatic surface respiration (ASR), is used by the guppy,Poecilia reticulata (Poeciliidae) to meet oxygen demand in hypoxid water. A specific position in which the head contacts the surface and the jaws open just beneath the surface is adopted. ASR is initiated at a P0₂ of about 50 torr and the percent time spent rises in a steep, linear fashion as PO₂ decreases. Below 4 torr more than 90% of the animal's time is spent in ASR. Males spend less time in ASR than do females. The percent time in ASR increases with increasing size of female guppies, but decreases with increasing size of male guppies. At low oxygen (18 torr) laboratory-born guppies derived from stocks likely to experience deoxygenation spend less time in ASR than do guppies derived from stocks less likely to experience deoxygenation. The percent time in ASR increases with temperature when PO₂ is held constant. Acclimation to low oxygen decreases the percent time in ASR.Guppies not permitted ASR die in 6–41 h at 14–17 torr and 10–15 min at 1–4 torr. Guppies performing ASR survive the duration of experiments at 13–35 torr (13 days), 14–17 torr (96 h), and 1–4 torr (9.5 h). Activity, courtship, and dive duration in response to a shadow stimulus are all reduced by low oxygen. ASR is an effective alternative to aerial respiration as an adaptation to hypoxic waters, but is probably energetically and temporally more costly.