Avoidance reaction of yolk-sac larvae of the inland silverside Menidia beryllina (Atherinidae) to hypoxia

Larvae of the inland silverside Menidia beryllina (Atherinidae) were observ ed in a chamber with two horizontal water layers to study their aversive re sponse to hypoxia. When larvae descended from an upper normoxic layer into a lower hypoxic layer, they displayed a characteristic avoidance reaction c onsisting of a burst of fast swimming that always ended in an upward direct ion leading the larva away from the hypoxic region. Each swimming burst las ted for approximately 2-3 s, with a maximum speed of approximately 25 mm s( -1), which is approximately four times the maximum swimming speed observed in the normoxic layer. The avoidance reaction was present in larvae from 6 h until 64 h post hatching. At 72 h we observed 50 % mortality as a result of starvation. The avoidance reaction was correlated with the O-2 concentra tion but not with the Na concentration nor with salinity or with salinity g radients. The avoidance response was observed at O-2 levels ranging from 4. 7 to 0.8 mg O-2 l(-1) For all concentrations below 3.8 mg O-2 l(-1), the di stribution was significantly different from that observed in normoxic contr ol layers of the same salinity. No avoidance responses were observed at 6.6 mg O-2 l(-1). We propose that the avoidance reaction of larvae of M. beryl lina to hypoxic water is an innate behavioural adaptation to the hypoxic co nditions present in the estuaries that are the spawning, hatching and nurse ry grounds of this species.