Gyoban is a wooden fish-shaped drum, which serves as a signal to start and end rituals, meditation sessions and meals. The fish-shaped drums are common in Zen temples in Japan. Gyoban is also called Gyoko, Mokugyoku, or Ho. In Buddhism, the fish, which never sleeps, symbolizes wakefulness and devotion to training.
Mokugyo, a wooden percussion instrument used during the recitation of sutras, mantras, or other Buddhist texts, also takes the shape of fish. Mokugo is said to have been derived from Gyoban and developed into the present form in China during the Ming Dynasty. That's why Gyoban looks more like a fish than Mokugyo.