Dietary effects of intensively reared zoeae of Portunus trituberculatus on survival and growth of paralarvae of the East Asian common octopus Octopus sinensis

Abstract

Zoea larvae of decapod crustaceans are considered to be the best prey organisms for culturing paralarvae of merobenthic octopuses. Because paralarvae consume many zoeae until settling as juveniles, a reliable method is required to rear a large number of zoeae at a high stocking density while maintaining their nutrition at a level to meet the demands of octopus paralarvae. However, when rearing zoeae, “sinking-death syndrome” (where many zoeae are deposited on the bottom of the rearing tanks) results in high mortality. In Experiment 1, to reduce larval deposition and lower the mortality, an upwelling system was installed and tested for its effectiveness in rearing a high stocking density of Portunus trituberculatus zoeae at low mortality. The effect of different supply densities of Artemia on growth and morphology of zoeae was also investigated to evaluate the nutrient sufficiency of zoeae under intensive rearing conditions. In Experiment 2, Octopus sinensis paralarvae were reared for 15 d, supplied with either early-stage zoeae or late-stage (long-term reared) zoeae, to examine the dietary effect of intensively reared zoeae on paralarval survival and growth. In Experiment 1, the proportion of zoeae sinking to the bottom was significantly reduced by using the upwelling system, and the survival rate of zoeae was higher in tanks with an upwelling system at a high stocking density of zoeae (500 individuals L−1) compared to those without. Larger body size and faster morphogenesis of larvae were obtained at the final (fourth) zoeal stage when zoeae were supplied with Artemia at high density compared with those supplied at low density. Composition of docosahexaenoic acid (DHA) in intensively-reared final-stage zoeae (10 d old) was maintained at a similar level to that in first-stage zoeae (1 d old) by supplying a high density of Artemia enriched with DHA. In Experiment 2, a higher survival rate and heavier body weight of paralarvae were obtained in the group supplied mainly with late-stage zoeae compared to that with early-stage zoeae. Because the supply of zoeae was adjusted to the same dry weight for both groups, it is concluded that supplying large-sized zoeae increases the efficiency of prey capture and nutrient absorption by the paralarvae. These results suggest that a combination of the use of an upwelling system and a high-density supply of enriched Artemia is effective for intensive rearing of zoeae, achieving reduced facility costs and more efficient production of high-quality prey for rearing octopus paralarvae.