Evaluation of physiological stress and nutritional deficiency related to cannibalism in early paralarvae of Patagonian red octopus Enteroctopus megalocyathus

Patagonian red octopus Enteroctopus megalocyathus is one of the two octopuses' species of great commercial importance in Chile. At the present time, the life cycle of this species in captivity has been closed at experimental level; however, high mortalities at the early paralarvae stage are a limiting factor for attaining sustainable production. Cannibalism has been identified as one of the main mortality causes at the early paralarvae rearing. Stress has been proposed as an important cause of cannibalism in cephalopods. Likewise, nutritional deficiency has been suggested as a potential cause to onset cannibalism. However, neither of both factors has been directly assessed in cephalopod paralarvae. In the present study, we aimed to elucidate if physiological stress due to a food shortage and/or to a high rearing density of paralarvae trigger cannibalism in E. megalocyathus. An experiment was carried out with newly hatched paralarvae, under conditions that trigger cannibalism with densities of 10 and 15 paralarvae L−1 combined with two measures of prey availability: 0 and 2 prey paralarva−1 day−1. Paralarvae alone in the aquarium fed ad libitum was used as a control to avoid cannibalism. Nutritional status was assessed as the total protein and energy content per paralarvae; and the 70 kD heat shock protein (HSP70) transcript level was used as a bioindicator of physiological stress, documented as a suitable stress biomarker for cephalopods. No differences were observed between the treatments and control in the absolute protein levels and energy content. There was a strong induction in HSP70 transcriptional level when high density and prey availability where combined, proving that there are only a few paralarvae rearing combinations that cause critical stress and trigger HSP70 transcription. In experimental conditions, nutritional status indicators showed that early paralarvae, with similar reserves of protein and energy, exhibited cannibalism. Results suggest that mortality as well as cannibalism is associated with competition for prey and with nutritional and energy imbalance due to food shortage that determines the expression of the related stress gene.