However, the behaviors of sea cucumber may have originated from a biological clock reflecting a phylogenetic carry-out from ancestral holothurians. The reason for developing an internal clock is complex, but a major pressure for the evolution of nocturnal feeding is predation [33]. In addition, the small-sized sea cucumber had a relatively lower proportion of feeding and no second peak in feeding during the day. This difference in feeding rhythms between size classes suggested that small individuals become less sensitive to light as they grew. Body wall thickness increases with growth; thus, a large-sized sea cucumber would probably be less sensitive to light [11]. Regardless of size, A. japonicus had higher IR and FPR at night than those during the day, which was consistent with the results in feeding proportions