Effects of four fresh microalgae in diet on growth and energy budget of juvenile sea cucumber Apostichopus japonicus (Selenka)

• Nutrient contents and their availability of microalgae are critical for the animal.
• Animal fed Cylindrotheca fusiformis grew well due to its easily broken cell walls.
• Animal fed high protein Spirulina platensis undergrew due to cellulose cell wall.

A 60 day feeding experiment was conducted to evaluate the growth performance and energy budget of the sea cucumber Apostichopus japonicus fed on four fresh microalgae (Cylindrotheca fusiformis, Dicrateria inornata, Nitzschia closterium, Spirulina platensis) and one granulated macroalga (Sargassum thunbergii) mixed with sea mud or yellow soil as diets. The results showed that there was no significant difference between the specific growth rates (SGRs) of the sea cucumbers fed diets containing sea mud and yellow soil. Though the nutrient contents of C. fusiformis were low, it is large in size with weakly silicified cell wall, which can be easily broken by A. japonicus. The sea cucumbers fed C. fusiformis diet had significantly higher SGR and energy proportion deposited to growth than those fed other four diets. S. platensis possesses a complete cellulose cell wall which can hardly be broken by the sea cucumbers. Therefore, the test animals fed S. platensis had the lowest growth rate and the highest energy proportion lost in feces among all treatments, even though S. platensis had the highest protein contents among the test algae. The SGRs of the animals fed N. closterium and D. inornata were in between of C. fusiformis and S. platensis, partly because N. closterium and D. inornata were small in cell sizes. The results indicate that the effects of fresh microalgae in diet on the growth of the sea cucumber A. japonicus are influenced not only by nutrient contents of the algal species but also by the availability of these nutrients, which is partially determined by the structure and size of their cell wall.