ON THE NOCTURNAL BEHAVIOR OF TYLOS GRANULATUS MIERS (CRUSTACEA : ISOPODA)

Abstract

The physiological and ecological aspects of the nocturnal activity of Tylos granulatus was investigated by natural observations and laboratory experiments that brought about the following results: 1. Tylos granulatus inhabits the pebbly beach, in the part higher than the level of respective high water, especially richly under stranded matters. It is nocturnal, gathering on the stranded materials at night but hiding itself in spaces among pebbles in the daytime. 2. On the studied beach, the temperature is higher in the daytime than at night, sometimes over 35°C in the daytime but below 30°C at night in the summer season. The humidity is higher at night. As the temperature of the hiding space is rather mild even in the daytime as compared with that of the exterior, Tylos can keep itself in the mild circumstances throughout the day by its nocturnal behavior. 3. Tylos is photopositive to lower light and can survive more than 2 days under the exposure to a bright light of ca. 10000 lux in the moist state. Its normal activity is displayed in the temperature range of 20 to 30°C and the critical temperature is found at about 45°C in the moist state. It survives longer in the highly humid condition and will be killed in 2 hours when it is exposed to the natural daytime condition on the beach in summer, though it can survive more than 4.5 hours if watered. 4. The activity rhythm of Tylos is controlled by the cycle of light, temperature and humidity and follows either of any two cycle according to the applied temperatures, when they are given coordinately with each other. These results seem to explain the nocturnal activity of Tylos as an adaptation to the terrestrial life in relation to water conservation. Lastly, some speculation is made as to the development of the nocturnal activity of this animal on the above-mentioned explanation and the results of observations in the field and the laboratory.