北海道噴火湾におけるマナマコの夏眠

抄録

The Japanese sea cucumber Apostichopus japonicus exhibits aestivation during high temperature season. During aestivation, they migrate to sheltered and dark microhabitats(under the surface of rocks and underneath rocks)and stop feeding and moving. Aestivation is commonly observed in the sea cucumbers inhabiting warm areas. However, in Hokkaido, which is a cool temperate zone, there is a controversy whether the sea cucumbers aestivate because of the relatively low temperature even during summer. A field experiment was conducted to verify the occurrence of aestivating individuals in Funka Bay, Hokkaido. A total of 50 cages were set up at 7 m depth, in which an individual sea cucumber and a concrete block with chambers were placed in each cage. We observed the attaching point of each individual within a cage by SCUBA diving every alternative day from late July to the mid-November 2012 for 4 months. The observations revealed that aestivation often occurred intermittently during the experiment period. The longest successive aestivation period was 12 d; however, several individuals did not aestivate. The mean bout of successive aestivating period was estimated to be 3 d. A maximum of 25% and 75% of the individuals exhibited aestivation and attached onto the shaded sites, respectively. These values correlated positively with the seawater temperature. The body weight of the sea cucumbers significantly decreased during the summer months. These results confirmed that the sea cucumbers in Funka Bay aestivate during summer even in the cool temperate zone. The aestivation behavior, such as moving into sheltered areas under rocks and decrease in body weight, will significantly affect the efficiency and quantity of sea cucumber fishing.