Influence of Photoperiod on N-Acetyltransferase Activity and Melatonin in the Fiddler Crab Uca pugilator

Abstract

Melatonin and N-acetyltransferase (NAT) activity were measured in the eyestalks of fiddler crabs acclimated to various photoperiods: constant light, a L:D 12:12 h photoperiod, or constant dark. Following acclimation, eyestalks were collected every 3 h over a 24-h period; they were assayed for melatonin with a radioimmunoassay and for NAT activity with a radioenzymatic assay. In constant light, melatonin levels increased at 1300 h, from 142 to 431 pg · mg−1 eyestalk; NAT activity increased concurrently, from 97 to 203 pmol · h−1 · mg−1 eyestalk, and both remained elevated until 0400 h. In the L:D 12:12 h photoperiod, melatonin levels increased at 1300 h from 28 to 230 pg · mg−1 eyestalk, and though NAT activity increased significantly, from 80 to 122 pmol · h−1 · mg−1 eyestalk, an even greater increase occurred at 0400 h, when melatonin levels were low. In constant dark, melatonin levels increased at 1600 h, from 22 to 196 pg · mg−1 eyestalk, with a concurrent increase in NAT activity from 93 to 140 pmol · mg−1 · h−1 eyestalk. However, the second peak in melatonin (111 pg · mg−1), occurring at 0400 h, was out of phase with the second peak of NAT activity (113 pmol · mg−1 · h−1 eyestalk) which occurred at 0700 h. NAT may be a rate-limiting step in melatonin synthesis in fiddler crabs under some conditions (constant light and the 1300 h peak in constant dark); however, NAT activity correlates poorly with melatonin levels in a L:D 12:12 h photoperiod and in constant dark relative to the 0400 h melatonin peak.