From the point of view that the desolation of seaweed-beds was largely due to increased quantities of micro-particles suspended in water and/or deposited on the seabed, the influence exerted on sedimentation velocity and zoospore insertions of brown algae by suspended matter was examined in our previous report. In this report, the influence exerted on the insertion of zoospores, germination, servival, and maturation of gametophytes of brown algae in accordance with quantities of sediments has been studied. Both Undaria pinnatifida and Ecklonia cava are employed also for the experiments of this time as samples. If the number of zoosporei nsertions under conditions of no sediments adhering to the substrate is taken to be 100%, it decreases to 50% when a sediment quantity of 0.5mg/cm2 (i.e. approx. 0.008mm in thickness) is deposited on the substrate, and no zoospore insertions take place when the sediment deposition becomes as much as 3mg/cmz. Following a similar way of thinking in respect to the germination of their gametophytes, it (i.e. 100%) also decreases to approx. 40% under conditions of 1mg/cm2, and no germination takes place under conditions of 10mg/cm2. Their survival rate also decreases to approx. 50% under conditions of 1mg/cm2, and there are no survivals at all under conditions of 10mg/cm2. Their maturation rate also reduces to approx. 50% under conditions of 0.25mg/cm2, and no maturation develops under conditions of 1mg/cm2.

No abstract

The visual contrast threshold of Japanese common squid Todarodes pacificus Steenstrup was sought through a series of experiments by observing their responses to contrast test boards. The con trast test boards consisted of three vertical stripes of which the outer two were painted in black and the middle band was in 11 different hues from white to black, exhibiting the apparent contrast values of 31.3 to zero (0). The experiments were carried on under the light intensity of 1.0 lx at the water surface, and the total number of snnid used was 68.When an experimental squid was confronted with the test board of which apparent contrast was less than 0.63, he made a jet-hit response to it. This jet-hit response was particularly prominent when the ap parent contrast was less than 0.008 and decreased in frequency with the increase of contrast value. At contrast values above 1.45, he avoided the middle band. By means of the 50-percent point of jet-hit response probability, the visual contrast threshold of the squid was estimated to be 0.006.

Variations in degrees of fish's aversion to intermittent light in response to different wavelengths of lights and different background irradiances under projected light were studied from an ecological point of view in this report. A school of one-year-old Japanese horse mackerel Trachurus japonicus soon after being caught, were used for the experiments.As a result, it is clarified that the fish school shows higher degree of aversion to blue intermittent light than that to red intermittent light; that, notwithstanding that the underwater energy of the blue intermittent light is only 1/6 that of the white intermittent light, both lights have almost no difference in degrees of fish's aversion. Furthermore, it is revealed that, when the intermittent light of 100lx is projected, the most prominent degree of aversion to light is seen when background irradiance is less than 0.003lx, and it decreases with increasing background irradiance but a certain degree of aversion still remains up to irradiance of 0.02lx and then, it disappears completely when irradiance reaches to 0.2lx.