Influence of temperature and substrates on burrowing 

behavior of Scapharca kagoshimensis with different body sizes

Abstract: Behavioral plasticity plays an important role in the adaptation of animals to environmental changes. The tidal flat substrate is the place for burrowing shellfish to forage, inhabit and avoid predators. The tidal flat substrate environment directly affects the species distribution, composition, growth and reproduction of shellfish, as well as their burrowing behavior. The ark shell Scapharca kagoshimensis is an important economic clam. In recent years, due to overfishing and environmental changes, the natural resources of Scapharca kagoshimensis have greatly declined. In order to restore the declining natural resources, stock enhancement of S. kagoshimensis has been carried out in many regions in China. Scapharca kagoshimensis is a typical burrowing shellfish. The time and depth of sand burrowing affect its ability to feed, survive and avoid predators. These parameters are important indicators for assessing the health of Scapharca kagoshimensis and selecting a suitable area for bottom farming. Therefore, it is of great significance to study the burrowing behavior of S. kagoshimensis for its stock enhancement and farming. This study can provide basic data for future study on the environmental adaptation and habitat selection of S. kagoshimensis, and provide guidance for the selection of bottom farming areas of S. kagoshimensis. We collected S. kagoshimensis in Xiangshan Bay, Zhejiang Province, China, and investigated the effects of water temperature and substrate particle size on the burrowing behavior of S. kagoshimensis with different body sizes in a nearby laboratory. Two body sizes were set in the experiment: the average shell length (21.15 ± 1.64) mm and (9.78 ± 2.79) mm, which corresponded to the sizes of firstyearold seedlings and secondyearold spat, respectively. According to the average seawater temperature in different seasons in Xiangshan Bay, 4 temperature gradients were set: 12 ℃, 20 ℃, 25 ℃, 32 ℃. According to the bottom sediment composition, four kinds of sediment particle sizes were set: coarse sand (>1 700 μm), medium sand (8301 700 μm), fine sand (380830 μm), and sea mud. The experiment was carried out in plexiglass experimental devices with a length of 25 cm, a width of 4 cm, and a height of 15 cm. A digital camera with a resolution of 2 160×1 080 pixels was placed in parallel above the plexiglass experimental device to observe and record the burrowing process and behavior. The first inmud time, the maximum burrowing depth and the burrowing rate of the clams were measured and calculated respectively. The first inmud time: the time when the first S. kagoshimensis began to burrow into the sand, and the judgement criterion was that more than half of the shells burrowed into the sand. Burrowing rate = (1-Et/20)×100%, where Et was the number of clams in the experimental device who did not burrow into the sand in time, and 20 was the total number of clams. The sand burrowing process of S. kagoshimensis was intermittent, and the burrowing depth in the sand was limited to submerge of most of the shell. The first inmud time of smallsize S.kagoshimensis was significantly lower than those of largesize ones (P<0.05), indicating that smallsize S.kagoshimensis adapted to the environment faster, probably because the shell of smallsize S.kagoshimensis was thinner. In order to avoid predators, it was necessary to burrow into the substrate as soon as possible. The burrowing rate of smallsize S.kagoshimensis was significantly higher than that of largesize ones with different particle sizes and time(P<0.05). Considering the first inmud time and the burrowing rate, the sand burrowing capability of smallsize S.kagoshimensis was stronger than that of largesize S.kagoshimensis. The burrowing depth in the fine sand, medium sand and coarse sand of largesize S.kagoshimensis was significantly higher than that of smallsize ones (P<0.05). This might be due to the difference in locomotion and sand burrowing abilities of different sizes. Largesize S.kagoshimensis had larger foot, which generated greater friction with the substrate, allowing them to burrow deeper. When the temperature was lowered to 12 ℃, the first inmud time of two sizes of S.kagoshimensis was both prolonged, indicating that the low temperature had an inhibitory effect on the burrowing speed of S.kagoshimensis. The maximum burrowing depths of S.kagoshimensis at 12 ℃ and 32 ℃ were significantly lower than those at 20 ℃ and 25 ℃ (P<0.05), indicating that too low or too high temperature would both influence the sand burrowing ability. Under low temperature conditions, as a survival strategy, S.kagoshimensis might reduce the burrowing speed to cope with the energy loss, thereby improving their survival rate. Sea mud had the most significant effect on the burrowing behavior of S.kagoshimensis. The two body sizes of clams had the longest burrowing time in sea mud, and the burrowing depth in sea mud was also very small. The first inmud time of smallsize S.kagoshimensis in fine sand was the shortest, while the largesize clams were most likely to burrow in coarse sand, indicating that the particle size of the substrate had a significant effect on the behavior of S. kagoshimensis, and sandy substrates with large particle size were more suitable for the survival of S.kagoshimensis. The burrowing rate increased with the sand particle size. This might be due to the fact that the algae attached to the sand substrate were more than that in the mud substrate, which was more suitable for the feeding activities of shellfish. At the same time, the sand substrate had a larger gap between particles than the mud sediment and was easy to disperse, so the resistance to shellfish in the process of burrowing was relatively small. The present study showed that water temperature, substrate particle size and body size all have significant effects on the burrowing behavior of S. kagoshimensis. The smallsize S.kagoshimensis had shorter first inmud time, higher burrowing rate, and stronger burrowing capacity than the largesize ones. In the sandy substrate, S.kagoshimensis had shorter first inmud time, higher burrowing rate and burrowing depth than those in sea mud. The burrowing performance of S.kagoshimensis at 20 ℃ and 32 ℃ was better than that at 12 ℃ and 32 ℃. It is suggested to choose sandy substrates with larger particle size in spring and autumn for bottom farming and stock enhancement of S. kagoshimensis.

Keywords: Scapharca kagoshimensis; water temperature; substrate particle size; body sizes; burrowing behavior