Life in the slow lane: the effect of reduced mobility on tadpole limb development.

Movement is thought to be a primary agent eliciting basic responses in the vertebrate body, such as the proper development of the musculoskeletal system. Embryos do not passively await hatching or birth but rather begin active movement very early on in their development. Most studies dealing with embryonic responses to changes in mobility have been performed in chickens or mammals. Herein, we investigate for the first time whether the embryos of organisms that are free-living during development demonstrate the same morphological responses to reduced mobility as embryos that undergo development in controlled environments such as in utero or in a shelled egg. We changed the viscosity of the environment in which free-living anuran tadpoles grow by rearing them in an agar medium. We thus increased the viscosity of the growth medium resulting in a decrease in larval movement. We predicted that a substantial increase in viscosity of the medium in which the larvae were reared would have at least two consequences: (1) a reduction of tadpole mobility and (2) a delayed onset of skeletogenesis thus producing shorter long bones. Our predictions were upheld and tadpoles reared in an agar medium remain immobile longer and showed a delayed onset of skeletogenesis compared with controls. We propose that the developmental responses to the same stimulus are similar throughout tetrapods, regardless of their developmental context (i.e., intrauterine, within an egg, or free-living).