BURTON, M.M.*; HOPKINS, P.M.; Univ. of Oklahoma: Ecdysteroid Control of the Molt Cycle in the Fiddler Crab, Uca pugilator

Arthropods have a non-living exoskeleton that is composed of a lipoprotein epicuticle, underlying exocuticle and deeper thick endocuticle. The deepest component, the membranous layer, is an uncalcified zone immediately above a sheet of epithelial cells called the hypodermis. Molting, or ecdysis, is achieved when the animal manufactures a new exoskeleton, separates from and then crawls out of the old exoskeleton. Under the influence of a group of invertebrate steroids named ecdysteroids, hypodermal cells secrete a hydrolytic molting fluid that begins digesting the membranous layer. As hydrolysis completes, the endocuticle separates from the hypodermis by a process termed apolysis. We report here preliminary results investigating the role of ecdysteroids in regulation of apolysis. Hemolymph ecdysteroid titers from fiddler crabs forced to molt by multiple autotomy (reflexive loss of limbs) were quantified by radioimmunoassay. Two distinct titer trends have emerged. Certain crabs follow the short cycle. A rise in hemolymph ecdysteroid levels occurs at R3 (relative growth indicator determined by dividing the limb bud length by the carapace width) of about 16, limb buds continue to grow and the group molts within 3 – 5 weeks after autotomy. This premolt hormone peak evidently initiates apolysis. Some animals did not progress through the molt cycle with their cohorts. Their ecdysteroid titers remained low, limb buds did not grow and hormone levels did not peak during premolt. These crabs, following a long cycle, did not molt with their cohorts. Instead, all specimens not sacrificed died without molting. It appears that this premolt peak of ecdysteroids is necessary for completion of the molt cycle. We are currently completing studies that will correlate the hormone titers from experimental crabs with the completion of apolysis.