Ecdysteroid Treatment Delays Ecdysis in the Lobster, Homarus americanus

Premolt stage D3 of juvenile lobsters, Homarus americanus, was further divided into five substages according to the degree of cuticle digestion along the dorsal midline of the carapace and on the dorsal surface of the merus of the chelipeds. The mean times to ecdysis for intact lobsters (5.5 +/- 1.1 g) at each substage were 71.4, 57.7, 30.0, 16.1, and 6.6 h, respectively. The level of ecdysteroids dropped continuously during stage D3, from 0.5 μg/ml at substage 1, to less than 0.1 μg/ml at substage 5. Injections of 20-hydroxyecdysone (20-HE) (1.0 or 5.0 μg/g) delayed ecdysis in animals receiving an injection at substages 3 or 4, but not 1, 2, or 5. The staging method can be applied to eyestalk-ablated (ESX) lobsters as well; but those animals complete stage D3 and molt much more rapidly. In addition to the time of ecdysis, the rate of development (based on the degree of cuticle digestion) in both intact and ESX lobsters was decreased by injections of 20-HE. We conclude that decreased ecdysteroid titers in the hemolymph of lobsters is a prerequisite to the initiation of ecdysis, and that rates of development during stage D3 are regulated negatively by ecdysteroids. We suggest that the time of ecdysis is controlled in lobsters through the regulation of the rate of decline of ecdysteroid titers.