Shelter material and shape preferences of the sea cucumber, Apostichopus japonicus

Abstract The purpose of this study was to determine the physical properties of shelter design that could enhance sea cucumber cultivation. The structural material (PVC, cement mortar, and porous cement mortar), hole shape (triangle, square, and circle), and hole size (0.0014, 0.0025, and 0.0079 m 2 ) of shelters were analyzed to determine sea cucumber preferences. Ninety sea cucumbers were used (30 individuals per body size group) and the initial weights were 20.56 ± 4.97 g, 60.80 ± 6.05 g, and 115.80 ± 9.13 g for small, medium, and large sea cucumbers, respectively. Changes in behavioral characteristics in relation to body weight at 10, 15, and 20 °C were also analyzed. Additionally, movement characteristics on surfaces of varying degrees of roughness (a. PVC [0.68–0.9 mm], b. cement mortar [1.5–1.8 mm], and c. porous cement mortar [2.0–2.2 mm]) were determined using particle image velocimetry (PIV) and 2D velocity fields. By day, we observed that sea cucumbers were attached to the inside of circular holes ( P  porous cement mortar (average 8) > PVC (average 6), regardless of water temperature or body weight. The preference for sheltering was higher in the small- and medium-size sea cucumber groups than in the group of large-size sea cucumbers. Most sea cucumbers attached to the shelters were in the weight range 20–60 g. The average distances moved by the small, medium, and large sea cucumbers were 1.501, 1.475, and 1.310 m, respectively, and their movement speeds (×10 −3  m/s) were 0.181, 0.289, and 0.269, respectively, regardless of water temperature. The movement speed of the sea cucumbers was the fastest (0.373 × 10 −3  m/s) at 20 °C. The PIV measurements showed that different velocities and forces were generated when the sea cucumbers moved to shelter materials of varying degrees of surface roughness (force: PVC > porous cement mortar > cement mortar). The sea cucumbers produced the greatest amount of force when moving over smooth surfaces, such as PVC. The smaller the sea cucumber, the less they were affected by temperature change.