Evidence that ecdysteroids and methyl farnesoate control allometric growth and differentiation in a crustacean

Jan 5, 2002Insect biochemistry and molecular biology

Ecdysteroids and methyl farnesoate may control growth and development in a crustacean

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Abstract

The propodus of male spider crabs can grow disproportionately, exceeding carapace length by as much as 35%.

Male spider crabs exhibit different morphotypes, with large-clawed males becoming primary reproductives.
Eyestalk ablation, which removes growth-inhibiting hormones, accelerated molting and increased ecdysteroid levels to a peak of 150 ng/ml before the molt.
In control males, ecdysteroids peaked at 90 ng/ml three days prior to molting, while methyl farnesoate (MF) remained below 0.5 ng/ml.
Ablated males showed increased MF levels (1 to 1.5 ng/ml) and larger carapace size, but retained shorter, non-allometric claws.
These findings suggest that ecdysteroids and lower MF concentrations promote allometric growth in crustaceans.

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Allometric, or disproportionate, growth of body parts is a basic problem in morphogenesis. Male spider crabs, Libinia emarginata, have several forms or morphotypes. During the terminal molt, the propodus enlarges disproportionately, exceeding the carapace length by as much as 35%. Even though shorter clawed males are reproductive, the large-clawed males become primary reproductives. We stimulated penultimate stage males to molt by eyestalk ablation, which removes molt inhibiting hormone (MIH) and mandibular organ inhibiting hormone (MIOH), and measured ecdysones by radioimmunoassay and methyl farnesoate (MF) in hemolymph by high-performance liquid chromatography (HPLC) using an internal standard. Eyestalk ablation accelerated molting and increased ecdysteroids to peak at 150 ng/ml before the molt. In control animals the ecdysteroids peaked at 90 ng/ml 3 days before the molt, with MF remaining less than 0.5 ng/ml. These became large males with large allometric claws. In contrast, the ablated ones, with increased MF (1 to 1.5 ng/ml), increased carapace size, but retained shorter non-allometric claws, with length shorter than the carapace. The results are consistent with experiments that we have performed with MF administration (Abdu et al., Biol. Bull., Woods Hole, MA 195 (1998) 112; Laufer et al., Gen. Comp. Endocrinol. 111 (1998) 113; Laufer et al., in: IV Symp. Aquaculture in Central America: Focusing on Shrimp and Tilapia, (1997a), p. 161; Laufer et al., Invert. Reprod. Dev. 31 (1997b) 63) which led to the interpretation that ecdysteroids and low MF concentrations promote allometric growth, while ecdysones with relatively higher concentrations of MF inhibited allometric growth. These results indicate and support the conclusion that MF and ecdysteroids determine the control of morphogenesis in allometric growth of Crustacea.

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