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

Abstract

ABSTRACTLarvae of the inland silverside Menidia beryllina (Atherinidae) were observed in a chamber with two horizontal water layers to study their aversive response to hypoxia. When larvae descended from an upper normoxic layer into a lower hypoxic layer, they displayed a characteristic avoidance reaction consisting of a burst of fast swimming that always ended in an upward direction leading the larva away from the hypoxic region. Each swimming burst lasted 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 O2concentration but not with the N2 concentration nor with salinity or with salinity gradients. The avoidance response was observed at O2 levels ranging from 4.7 to 0.8 mg O2 l−1. For all concentrations below 3.8 mg O2 l−1, the distribution was significantly different from that observed in normoxic control layers of the same salinity. No avoidance responses were observed at 6.6 mg O2 l−1. We propose that the avoidance reaction of larvae of M. beryllina to hypoxic water is an innate behavioural adaptation to the hypoxic conditions present in the estuaries that are the spawning, hatching and nursery grounds of this species.