Effect of melatonin on hemolymph glucose and lactate levels in the fiddlercrab Uca pugilator

Melatonin was injected into intact and eyestalk-ablated fiddler crabs (Uca pugilator), and its effects on hemolymph glucose and lactate levels were st udied. In intact crabs, glucose and lactate levels cycled simultaneously, w ith peaks occurring during early and late photophase. Melatonin caused a sh ift in the glucose and lactate cycles, with only one peak occurring closer to mid-photophase. In eyestalk-ablated animals, the glucose rhythmicity was lost; lactate cycled, but levels were significantly lower than in intact a nimals. Melatonin caused a delayed hyperglycemia in eyestalk-ablated animal s, with concurrent but much lower increases in lactate. Overall, melatonin demonstrated delayed hyperglycemic effects that do not appear to be mediate d solely via eyestalk factors such as crustacean hyperglycemic hormone (CHH ), though involvement of the eyestalks cannot be ruled out. An influence on extra-eyestalk CHH secretion is a potential mechanism of melatonin activit y. (C) 2001 Wiley-Liss, Inc.