Snapping shrimps, which make peculiar pulse sounds, are found everywhere in the world. This research examined the validity of pulse counts of snapping shrimps as an indication of sea environmental conditions. Pulse counts can be gained without special biological knowledge merely by recording their sounds for a few minutes using a hydrophone. The pulses can be heard not only in Japan but also in other parts of the world. However, the pulse count varies by region and seems to decrease in places where the water quality is not very good. Especially, few pulse counts were obtained at areas where anoxia occurs in the summertime. We found from the Ministry of the Environment that the pulse count correlates with the water quality of sea areas. From field surveys and laboratory experiments, this method can be a useful index of the effect of water pollution on benthic animals. A fixed-point observation is very important because the pulse count shows regional variations.
Passive acoustical monitoring systems were used to monitor local migrations of Yangtze finless porpoises in China. We stationed 3 acoustic data loggers (W20-ASII) at 3 different places in the confluence area of Poyang Lake and the Yangtze River on 27-29 April 2006. Acoustic data loggers recorded ultrasonic signals from porpoises swimming in a 300m radius approximate range. Visual observations were conducted simultaneously at two stations. Acoustical and visual observation could detect the presence of porpoises at 73.9 ± 20.9 % and 7.0 ± 6.5 % at all observation times, respectively. Visually-counted group sizes were underestimated compared with acoustical observations for groups of up to 5 individuals, whereas the opposite was true when the group size was more than 6. In summary, a passive acoustical method was proved to be effective for the long term monitoring of finless porpoises swimming underwater. The limitation of group size estimations is thought to be caused by a less accurate measurement of sound source direction due to the short baseline of W20-ASII. It can be improved by employing longer baseline stereo hydrophones.
To explore behavioral responses to sound and the detectable frequency range of Octopus ocellatus, responses of the octopus to 120 dB rms sound stimuli of various frequencies were observed. The octopus did not respond at 200-1000 Hz, but clearly responded at 50-150 Hz by showing lengthened respiratory activities. Their respiratory activity was frequently suppressed for more than 5 sec after the sound stimulation, and the longest respiratory suppression reached 55.6 sec, while the mean respiratory activity length without sound stimulation was 1.23 sec. In the case of long-lasting respiratory suppression, they retracted the basal parts of their eyes simultaneously. This suggests that underwater sound may play an important role in the life of the octopus, possibly to detect predators.