Morphophysiological responses of Octopus tehuelchus juveniles during the transition period between endogenous and exogenous feeding

Abstract

In cephalopod hatchlings there is a transitional period considered critical, in which the digestive system undergoes morphophysiological changes associated with the passage from the consumption of vitelline reserves to an independent feeding. During this period, the characterization of the digestive dynamics and growth is key information to define their nutritional requirements under culture. Octopus tehuelchus, which has become increasingly interesting for cultivation, is a species endemic to South America with holobenthic development and large juveniles. In this work, the enzymatic and cytological changes of the digestive gland (DG) along with the variation in body weight and size of O. tehuelchus hatchlings were evaluated when reared at 16 °C under two feeding treatments: fed with the isopod Exosphaeroma sp. and starved. In fed juveniles, the predatory activity began a few hours after hatching; and up to 6 days post hatching (DPH) there was neither somatic growth nor cellular differentiation of the DG, but there was an activation of acid enzymes. From 6 to 10 DPH, there was an increase in the size and weight of juveniles, and the maturation of the DG began with the appearance of heterolysosomes and heterophagosomes. Furthermore, lipases activity reached its maximum level and there was a considerable increase in both alkaline phosphatases and peptidases. From 15 to 25 DPH, the DG became fully mature and the lipase activity reached minimum levels, while the alkaline peptidases continued increasing. At the end of the experiment, at 25 DPH, fed juveniles doubled their weight. In the starved juveniles, neither somatic growth nor cytological differentiations of the DG were registered. Besides, in this treatment, acid phosphatases and lipase activities (i.e. enzymes related to yolk consumption) remained at basal levels or decreased over time, even with yolk platelets still present, suggesting that the consumption of yolk reserves would be delayed, regulating the survival time. In fed juveniles, the ingestion of isopods seems to act as a modulating factor of the digestive activity, triggering an increment in the activity of lipases, acid phosphatases, as well as alkaline peptidases which are related to the digestion of exogenous food. It is concluded that morphological and physiological changes during the digestive maturation are related to the ingestion of Exosphaeroma sp. In this sense, this isopod species would be a suitable food which promotes the rapid growth of O. tehuelchus during their early post-hatching stage under culture.