Seasonal Variation in Diatom Availability and Utilization by Juvenile Japanese Sea Cucumber Apostichopus japonicus

Diatoms are considered an important food source for the commercially valuable sea cucumber Apostichopus japonicus. However, food sources for juvenile A. japonicus in the wild remain understudied, despite their importance for effective stock enhancement. In this study, seasonal diatom assemblages and viability in the feces of juvenile A. japonicus and in the feeding environments (biofilm, water column) were investigated using direct microscopy to evaluate diatom availability and utilization by the juveniles. Additionally, a laboratory feeding experiment was conducted to examine the digestibility of the predominant diatom (Tabularia) in the feces during the field survey. Field surveys have shown that diatoms are a major food item for juveniles, although their contribution varies seasonally with shifts in dominant food sources. Notably, Tabularia spp. occurred at a higher relative proportion in fecal assemblages than in the feeding environments and exhibited high mortality in the feces (96.3 ± 2.4%). Consistently, laboratory experiments showed marked degradation of intracellular contents in Tabularia sp. after excretion, supporting its digestibility. These findings have broadened our understanding of optimal food sources for A. japonicus juveniles as well as their feeding ecology in natural environments, informing the selection of more suitable diets and potentially enhancing stock enhancement practices.