The daily colour rhythm of the fiddler crab Uca rapax on Curaçao

1. Uca rapax of Curaçao showed a daily colour rhythm. The animal is dark during the day and light during the night. This rhythm persisted in animals which were placed in constant darkness for 110 days, and in those which were placed in constant illumination for 40 days. The latter condition caused a gradual phase shift of the maximum darkening from 12 (noon) to 3 AM within a period of 18 days. The maxima and minima of animals kept in constant darkness were higher than in control animals. No correlation between tide levels and colour change was found. 2. Eyestalk extracts were found to contain a pigment dispersing activity. There was no difference in the amount of dispersing material between the extracts of eyestalks removed either in daytime or at night. Males reacted more strongly upon injection of eyestalk extracts than females. 3. Blood taken from eyestalkless or normal animals had no dispersing activity, although a “non-specific” dispersing activity can be demonstrated in blood kept outside the body for 45 minutes.