A series of experiments was conducted to elucidate the events immediately preceding and following moulting (ecdysis) in the blue crab, Callinectes sapldus Rathbun. The crabs gain weight (mostly water) to a level more than twice the premoult value excluding the shed carapace. The gain begins about 4 h before ecdysis, accelerates rapidly to a maximum rate at about the time of ecdysis, and is essentially complete by 2h after ecdysis. Both calcification and net H+ excretion remain at control (intermoult) levels until 1–2 h post-moult, whereupon a very rapid increase in both begins, with the same time course for both processes. The ratio of internal to external calcium concentration drops from 1.4:1 during intermoult to 0.85:1 by 2 days after the moult, reversing the electrochemical gradient during the post-moult period. Calcification is strongly inhibited by the protein synthesis inhibitors actinomycin D and cycloheximide. Isolated whole gills and gill slices do not show significant changes in rates of calcium uptake related to moult stage, indicating that the uptake of calcium across the gills may be largely passive. A low-affinity Ca2+-activated ATPase is present in both gills and epithelium, but only the epithelial activity shows a substantial (fivefold) increase in the hours after ecdysis. The control mechanisms for stimulation of the transport processes remain elusive. Eyestalkless crabs progress normally through moult and post-moult calcification. Peptide profiles from blood and from suspected neurohormone- producing tissues differ markedly with moult stage, but several different approaches to developing a bioassay using peptides from crab tissues and vertebrate sources have been unsuccessful. The physiological events in the hours surrounding ecdysis suggest a complex and precisely timed series of control signals.

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