Impact of substrate particle size on the digging behavior of the juveniles of the smooth fan lobster <i>Ibacus novemdentatus</i> Gibbes, 1850 (Decapoda: Pleocyemata: Scyllaridae)

Abstract Aquaculture techniques for fan lobsters are still in an early stage of development due to the limited knowledge on their early-benthic stages. As the lobsters are diggers and thus the substrate influences their survival and growth, the physical characteristics of such substrates are a crucial environmental factor in an aquaculture setting. The appropriate size ranges of the substrate particles are nevertheless unknown. We studied the digging behavior of the juvenile of the smooth fan lobster, Ibacus novemdentatusGibbes, 1850, and examined its ability to dig into sand of different particle sizes to determine the optimal substrate conditions. Digging was completed by 100% of the juveniles in fine sand, 85.7% in medium-size sand, 25.7% in medium-coarse sand, and none in coarse sand. Juvenile digging involves three stages: searching for a location to dig, inserting the tail fan, and submerging into the sediment. The median length of time from the beginning of the experiment to the commencement of tail insertion in juveniles that completed digging did not differ significantly (P = 0.97) with the particle size. Juveniles placed on the substrate with medium-size and medium-coarse sand took significantly longer (P &amp;lt; 0.01) from the start of tail insertion to the complete submersion than those on fine sand. Considering the success rates of digging and the time required to complete digging, fine sand was the most appropriate substrate among the tested particle-size ranges. The juveniles that failed to complete the digging repeated a digging position and attempted to insert their tail fan into the sand without submerging. These juveniles took a significantly longer time (P &amp;lt; 0.05) to attempt to insert their tail fan than the juveniles that successfully completed digging, suggesting that juveniles placed on inappropriate substrates may spend more energy on digging and become more stressed.