Tilden, A.R.*; Schwartz, J.; McGann, L.: Influence of Melatonin on Hemolymph Metabolites in the Fiddler Crab Uca Pugilator

Melatonin levels were measured in the eyestalks of fiddler crabs (Uca pugilator) acclimated to a LD 12:12 photoperiod, constant dark, and constant light. Melatonin cycled with a photophase peak during the LD 12:12 photoperiod. The cycle persisted in constant dark and constant light, with significantly greater melatonin production in constant light. These results are contrary to the scotophase melatonin peaks observed in most other organisms but may reflect a general crustacean phenomenon. We also studied the influence of melatonin on hemolymph glucose and lactate levels in fiddler crabs. Crabs were acclimated to a LD 12:12 photoperiod and were injected with melatonin at the onset of light. Hemolymph samples were taken every three hours over 24 hr, and glucose and lactate were measured simultaneously with enzymatic assays. In intact crabs, glucose and lactate levels cycled, with peaks in both occurring during early and late photophase. Melatonin shifted both glucose and lactate cycles, with one peak occurring closer to mid-photophase. Glucose did not cycle in eyestalk-ablated crabs, and overall lactate levels were lower than in intact crabs. Melatonin injection caused a delayed hyperglycemic response in eyestalk-ablated crabs with a less pronounced but similar effect on lactate levels. Melatonin may be involved in the synchronization of hemolymph metabolite levels with activity patterns, an influence that may not involve eyestalk factors.