Suitable dissolved oxygen levels for common octopus (Octopus vulgaris cuvier, 1797) at different weights and temperatures: analysis of respiratory behaviour

The dissolved oxygen level is an essential parameter for defining water quality in aquaculture. However, the optimal values of this gas can vary greatly depending on the species, body weight, or water temperature. In this study, different oxygen level categories (optimal, suboptimal, dangerous, and lethal) were established for Octopus vulgaris (0.18-2.20 kg; 15.5-27.4 °C) according to ventilatory frequency (Vf) and oxygen consumption (MO2) in response to gradually decreasing dissolved oxygen levels. Initial and maximum ventilatory frequency (Vf0 and Vfmax, respectively) and oxygen saturation at which ventilatory frequency is altered (SVf) showed mean values of 17.7±3.29 bts/min, 33.3±5.86 bts/min, and 62.8±15.94% (4.5±0.95 mgO2/l), respectively, and all variables were positively correlated with temperature (P<0.05). The species in question maintained a constant MO2 rate regardless of the concentration of dissolved oxygen, until a critical oxygen saturation (Scrit) was reached. The Scrit showed a mean value of 31.6±8.32% (2.3±0.57 mgO2/l) and was correlated positively with temperature (P<0.05). In the ideal range of temperature for growth and food intake (17-20 °C), our findings suggest an optimal oxygen saturation of between 100% and 65% (constant Vf and MO2), a suboptimal value of 65-35% (altered Vf but not MO2), with anything below 35% being dangerous (altered Vf and MO2). Lethal oxygen saturation levels were lower than 11% in all the animals tested, although resistance was higher at low temperatures and in small animals. We conclude that suitable oxygen levels for O. vulgaris are highly variable and depend on water temperature rather than body weight.