TILDEN, Andrea R.; DANZIGER, Andrew; OWEN, Jeffrey; STERIO, Thomas; THURSTON, Kristy: Effects of Melatonin on Locomotor and Cardiac Activity in the Fiddler Crab Uca pugilator

We previously measured melatonin in fiddler crabs (Uca pugilator) and determined that melatonin cycles over a 24 hr period, with a mid-photophase peak that persists under constant conditions. Under some laboratory circumstances, fiddler crabs are diurnal, displaying enhanced locomotor activity around mid-to-late photophase. In vertebrates, melatonin expresses a circadian rhythm with a peak in production and release during scotophase, and it is involved in entraining locomotor activity potentially via its influence on neural and metabolic activity. We studied the influence of melatonin on both locomotor and cardiac activity in fiddler crabs. Initially, we established that the crabs were active during mid-to-late photophase, and then we injected melatonin in early scotophase. Melatonin-treated crabs showed a significant increase in locomotor activity that persisted into photophase. Next we used extracellular recording to determine cardiac activity in response to melatonin. Melatonin significantly increased both the rate and the amplitude of cardiac contractions, inducing significant increases after 10 min of treatment, and an even greater response after 20 min. In a previous study, we determined that melatonin influences hemolymph glucose and lactate levels in a manner consistent with the both the daytime melatonin peak and the daytime locomotor activity. These studies suggest that melatonin may be involved in entraining locomotor activity patterns in crustaceans as well as vertebrates.