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 <i>Apostichopus japonicus</i>. However, food sources for juvenile <i>A. japonicus</i> 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 <i>A. japonicus</i> 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 (<i>Tabularia</i>) 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, <i>Tabularia</i> 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 <i>Tabularia</i> sp. after excretion, supporting its digestibility. These findings have broadened our understanding of optimal food sources for <i>A. japonicus</i> juveniles as well as their feeding ecology in natural environments, informing the selection of more suitable diets and potentially enhancing stock enhancement practices.