Metamorphic shortening of the alimentary tract in anuran larvae (Rana catesbeiana)

Abstract

The premetamorphic alimentary tract in anurans can be more than 10 times a tadpole's body length but then dramatically shortens to a third or less of that length by the end of metamorphosis. Although there have been many studies on histological changes in the anuran gut with metamorphosis, the broader question of where the major shortening occurs has not been previously addressed. This topic is investigated here. We began our study by labeling intestinal coils in situ in preserved Rana catesbeiana tadpoles and then uncoiling their intestines, locating, and measuring the labeled points. This allowed us to map the coiled gut of the tadpole, such that the distance along the oral-anal axis could be determined by simply counting coils. We next implanted markers into the intestinal coils of live R. catesbeiana tadpoles at five known locations along the oral-anal axis, established from the prior mapping. The tadpoles were then induced to metamorphose by immersion in thyroid hormone. After the gut had shortened to a third of its premetamorphic length, the positions of the implanted markers were determined through dissection. Relative distances between the marked points did not change when the gut shortened. The results indicate that during metamorphosis the intestine shortens uniformly along its length and not preferentially from one region or another. Although metamorphosis was artificially induced, the shortening that we observed matches that occurring during natural metamorphosis.