抱卵型カイアシ類Oithona oculata のバイオリアクターを用いた試験的培養
- Experimental cultivation of the egg-carrying copepod Oithona oculata in a pilot bioreactor
In aquaculture and ornamental industries, copepods are recognized as preferred live feeds for marine fish larvae over commonly used organisms such as Artemia and rotifers. Marine fish larvae fed with copepods show better survival and growth. Despite obvious advantages of copepods as live feed, their use is still limited owing to low productivity and cost-efficiency when mass cultured. Copepods can be divided into free-spawner and sac-spawner (egg-carrying) according to their spawning style. In cultivation of egg-carrying copepod, a simple nauplii collection/harvesting method with low labor has not been established, because females carry the egg-sac until hatching. Manual collection of nauplii using a siphon hose and mesh-filters is a common method in copepod cultivation, but automation is necessary to reduce labor costs if copepods are to be viably cultured at a commercial scale. Recently, we devised a zooplankton bioreactor for rapid separation of free-spawner copepod eggs from adults in a tank. The automation collects eggs on a mesh filter in a water flow system which can also function as water exchange device. In the present study, we apply this bioreactor to egg-carrying copepod, and report the results from cultivation at laboratory-scale for 45 days. Species of Oithona are good candidates for live feed because their body size and biochemical composition are suitable for many marine fish larvae which have small gapes. Oithona oculata is widely distributed and a typical dominant species in coastal waters, and was chosen as target species of egg-carrying copepod in present study. 150 adult individuals of O. oculata were placed into a culture chamber which were immersed in a reproduction tank containing 3 L seawater. The culture chamber to retain the copepods has a 100-μm nylon mesh placed 5 mm above the bottom, while allowing passage of nauplii. The copepods were fed daily with a sufficient amount of Thalassiosira weissflogii and Isochrysis galbana and incubated at 28°C for 45 days. Produced nauplii in the reproduction tank were collected daily using water flow of 1.5 L min-1. The species abundance in each development, ovigerous rate and collected number of nauplii were measured daily. In the semi-continuous culture, the collected nauplii were cultured in maturation vessels and returned to the production tank after their maturation, and 45 days cultivation (four generations) was achieved. Total abundance was gradually increased from day 4 to day 20 and reached at 500 inds. L-1. Ovigerous rate varied from 0 to 88% during the incubation period. The maximum number of collected nauplii was 380 nauplii L-1 at day 5. The results obtained in present study suggest that the bioreactor can also be applied to egg-carrying copepod by optimization of the mesh size and the water flow velocity.
プランクトン工学研究 1 22-31, 2021-06-28