Direct Observation of Deep-sea Animals Using a Deep Seafloor Observatory at the Depth of 3,572 m in Nankai Trough, Japan

The Japan Marine Science and Technology Center (JAMSTEC) installed a seafloor monitoring system for monitoring submarine earthquakes, in Nankai Trough, western Japan at a depth of 3,572 meters in March 1997. It is equipped with a color video camera and six 100-watt underwater lights. 97 separate video recordings of deep-sea life were taken from May 1997 to December 1999, from two to four times a month. The observation area was 10 m². However, the area was reduced by half caused by light trouble in October 1999. Over 71 species of deep-sea animals have been filmed in the space of four years. There is no seasonal variation in the total number. The average annual number of deep-sea animals in 1999 halved as compared to that in 1997 and 1998. It seems that the change in the total number of deep-sea animals was influenced by the reduction of the observation area. We are successful in making the first observations ever of molting, feeding, locomotion, body cleaning and defending a territory of deep-sea galatheid crabs. Threatening behaviors and fights among them were also observed. For example, two Munidopsis sp. were having an intense face-off, waving their chelae at each other and then began grappling. After rolled over, they immediately caught at each other again. It seems that they are able to square up to each other by sensing the vibrations from the waving of their chelae. In the deep-sea, where vision is impossible, a pallesthesia is an important means of communication.