Studies on the Shipworms III : Pattern of Vertical Settlement of Shipworms

Abstract

Test wood blocks (Pinus sylvestris LINNAEUS, 4 × 4 cm in section and 30 cm in length) were suspended vertically in the sea at the sea water log storage area, Takahama, Fukui Pref. The test blocks were connected by a rope and submerged at 0.3, 0.8 and 1.3 m below the water surface, and at regular intervals of 1 m from 2 m to the bottom level (about 23 m). The blocks were removed and replaced every month for examining monthly settlement of shipworms on the surfaces of the blocks. The heavier settlement of shipworms was generally observed in deeper regions, particularly at the bottom. This tendency was prominent in the months when the furious shipworm settlement was noticed. The patterns of monthly vertical settlement of shipworms on wood surfaces were divided into the following 4 types: (1) The intensity of settlement of shipworms increases proportionally with water depth so that the maximum settlement is recorded at the deepest level (September, 1975 and August, 1976). (2) The heaviest settlement is observed at the deepest level with the second peak at about 10 m depth (July and October, 1976). (3) The heaviest settlement is recorded at approximately 10 m depth; the extent of settlement increases in proportion to water depth down to 10 m level and then decreases adversely, but rises again at depths between 20 m and 23 m (August and October, 1975; September, 1976). (4) The settlement occurs very slightly and relatively evenly along the water column (June, July and December, 1975; June, November and December, 1976). The effect of light intensity and water temperature gradient with water depths would partly account for the different patterns of monthly vertical settlement of shipworms. And the active vertical locomotion of shipworms might help shipworms concentrate at deeper levels. The results suggest that the sunken logs at the bottom are the important reproductive sources of shipworms. It is therefore helpful to remove not only riddled floating wood and logs but also sunken logs for reducing shipworm attack in the sea water log storage areas.