To investigate the nocturnal habits of the Japanese spiny lobster, (Panulirus japonicus), we produced diurnal brightness changes of a 12-h light period (daytime) and a 12-h dark period (nighttime) in our tanks. Under a condition with 3.3 × 102 lux daytime brightness
and 0 lux nighttime brightness (the standard condition), movements of a spiny lobster exhibited a clear diurnal pattern, almost in phase with the brightness change. However, the pattern collapses under several conditions: when the nighttime brightness was higher than a threshold value of about
1.8 × 10−4 lux (the nighttime activity was considerably suppressed); when the daytime brightness was changed from the standard value to 0 lux, the diurnal pattern was skewed and its variation period shortened and became 23 h. (By using this scheme, we determined the
minimum daytime brightness recognizable by spiny lobsters to be about 2.3 × 10−5 lux); when a spiny lobster was placed in new circumstances, the activity of the lobster was considerably increased for the first few days. This increase occurs in parallel both in nighttime
and in daytime activities, and a collapse of the pattern occurred not only due to external conditions but also due to internal conditions.
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