Morphological observation of marine organisms by underwater ultrasonography

A high resolution underwater ultrasonic camera enclosed in a pressure-resistant case was developed to observe underwater animals. It enables to measure size, shape, and movement of living marine animals as well as tissue, shape, and movement of internal organs. Ultrasonic camera is originally used for medical diagnosis. A convex/linear type transducer array is driven by 3.5 to 5 MHz ultrasonic signals and the echo from an object is displayed as an acoustic tomogram. It covers a range from 24 to 100 cm in waters. The acoustical B mode image and an optical image by an underwater camera were transferred by underwater cable. Special features of the underwater ultrasonic camera are, (1) To observe marine animals in dark or opaque water, (2) To observe the tissue and shape of internal organ, (3) To observe the shape and movement of marine animals in natural conditions. A trial of an underwater ultrasonic camera was conducted in a water tank. Live animals such as fish, squid, jellyfish, and krill were observed by the ultrasonic camera. Also, a three dimensional image of internal structure of animals was constructed by using a series of acoustical tomograms in order to measure size and volume of internal organs. However several acoustical discontinuities in a body were found. The most important contributor for acoustic scattering was a vertebra in nonbladdered fish and a swimbladder in bladdered fish. As for invertebrate animals the major contributors for acoustic scattering were a liver in squid, an ovary in jelly fish, and a shell in crustacean respectively. Finally a real use of ultrasonic camera in coastal waters was conducted. The underwater ultrasonic camera and optical camera were set on a sea bottom at a depth of 12 m. Several interesting optical and acoustical images from wild animals were observed.