Environmental modulation of the onset of air breathing and survival of Betta splendens and Trichopodus trichopterus

The effect of hypoxia on air‐breathing onset and survival was determined in larvae of the air‐breathing fishes, the three spot gourami Trichopodus trichopterus and the Siamese fighting fish Betta splendens . Larvae were exposed continuously or intermittently (12 h nightly) to an oxygen partial pressure ( P O 2 ) of 20, 17 and 14 kPa from 1 to 40 days post‐fertilization ( dpf ). Survival and onset of air breathing were measured daily. Continuous normoxic conditions produced a larval survival rate of 65–75% for B. splendens and 15–30% for T. trichopterus , but all larvae of both species died at 9 dpf in continuous hypoxia conditions. Larvae under intermittent (nocturnal) hypoxia showed a 15% elevated survival rate in both species. The same conditions altered the onset of air breathing, advancing onset by 4 days in B. splendens and delaying onset by 9 days in T. trichopterus . These interspecific differences were attributed to air‐breathing characteristics: B. splendens was a non‐obligatory air breather after 36 dpf , whereas T. trichopterus was an obligatory air breather after 32 dpf .