The nighttime activity of a Japanese spiny lobster is considerably suppressed when the brightness is higher than a threshold value of about 1.8 × 10-4lx. This brightness value is low enough that nothing is visible by human eyes, and we need special automatic devices to trace lobster movements and to investigate lobster behaviors in dark nighttime. We designed two types of recording systems which are appreciable for our experimental tanks. Lobster cages in our experimental tank are hung by three wires or supported by three pointed vertical bars. In a simpler devices, we used a rectangular cage of 0.4 m length, of 0.3 m width and of 0.3 m depth. The tension of one of three wires, the lower end of which is connected to the center of one shorter rim, is measured. The strain averaged for 9 sec are recorded every 15 sec. We define a measure of the lobster activity by counting the number of occurrence of strain change larger than 2 gw between successive data pair per unit time. The value of 2 gw is selected empirically for convenience sake, and corresponds to a movement of the 250 gw lobster over 3.2 cm distance in the longitudinal direction. This measure appears to be useful to describe gross natures of the diurnal variation pattern of lobster activities. In the other devices, we use a cylindrical cage of 0.8 m diameter, and of 0.4 m depth. The upper part of the cage is supported by a triangle frame made of aluminum angles. A pointed supporting bar is set downward from each corner of the triangle frame, and is placed on an electronic balance. The weight measured by each balance is automatically fed to a personal computer, and the position in two dimensional plane is calculated and the results are recorded and kept in a floppy disk. The position of the lobster is determined in the interval of 2.7 sec, and the accuracy of the position determination is less than 0.9 cm. This recording system appears good enough to detect the detailed movement of the lobster in two dimensional plane. The usefulness of these recording systems will be introduced with several typical experimental results.
Current observations were carried out in the coral reef lying between two islands, Izena Island and Yanaha Island, in Okinawa Prefecture. The characteristics of the currents in the reef are summerized as follows; 1) Semi-diurnal tidal currents are predominant which are caused by the gradient of the sea surface between the east end and the west end of the reef. The gradient is induced by the phase difference of tide between both ends. 2) The strength of currents responds to the incidental wave height. 3) They also respond to the sea surface elevation. As the sea level rises, the velocity tends to increase.
Deep-sea equipment using an optical fiber sensor for measuring temperature distribution over long distances is being developed, under a research program called Ridge-Flux which was started in 1993. The optical fiber sensor can produce simultaneous temperature data at up to 4000 points with an interval of 1 m along it. Brief discussions on specifications of the optical fiber sensor and the controlling/recording system are given. In an attempt to upgrade the basic technologies for the equipment, we first investigated its suitability for high precision measurement in the laboratory, with a satisfactory result. Secondly, we made several field tests at a near-shore seawater, as well as at a lake bottom. Vertical distribution measurement of seawater temperature to 500 m depth in Suruga Bay has shown that the optical fiber code is very suitable as the distributed temperature sensor, even under hydrostatic pressure. Experiments in lake Nakaumi for a period of 55 days have resulted in two interesting discoveries of its limnological phenomena: one is the temporal change of warm spring activity on the lake bottom, which might be an analog of hydrothermal activity in the mid-ocean ridges under deep sea. The other is the movement of a mass of saline water into the fresh-water lake bottom, driven by meteorological forcing. The advantage of the optical fiber distributed temperature sensor is its ability to easily obtain the temperature data as function of time and space over a long distance. Our next step is to modify the equipment, so that we may record the temperature distribution on the deep-sea floor for a long time, up to one year, for the study of energy flux estimation at mid-ocean ridges. Down-sizing the equipment and lowering its power-consumption are regarded to be the key factor. Also, there still remain some technical problems to be solved, such as how to deploy the sensor cable on the sea floor, and so on.