Endocrine control of spine epidermis resorption during metamorphosis in crab larvae.

During premolt of the fourth zoeal instar ofRhithropanopeus harrisii, the epidermal tissue of the dorsal spine is resorbed (=spine epidermis resorption, SER). 20-Hydroxyecdysone (20-HE) accelerated SER in fourth, but not in third instar zoeae in vivo. Dorsal spines explanted from day-1 third or fourth instar larvae underwent SER in response to 20-HE (0.001-5 μg/ml) in vitro, while spines from day-2 third instar larvae showed no 20-HE-stimulated SER in vitro. Extracts of third, but not fourth instar zoeae inhibited 20-HE-elicited SER in explanted spines from third and fourth instar larvae in vitro. The results suggest that a factor is present in the larvae that inhibits metamorphic activity in the larval epidermis.