Rana tadpoles are known to have a left-handed turning bias whereas Xenopus larvae lack such a preference in turning direction. Since Rana tadpoles have a single, external, left-handed spiracle, we previously suggested that a turning bias in tadpoles may be obligatorily linked to that external asymmetry. We have tested this idea by examining turning bias in the startle response of Microhyla ornata tadpoles. Microhyla tadpoles are, like Xenopus, externally symmetrical, but phylogenetically they are more closely related to Rana.

Individual Microhyla tadpoles were startled by a solenoid-driven plunger that sent a shock wave up through the bottom of a container holding each tadpole. High speed videography (250 frames per second) was used to witness the tadpole's response.

Microhyla tadpoles show no turning bias during the first few days post-hatching, when they are very small. However, they develop a left-handed turning bias while still in the earliest free-swimming stage (Gosner stage 25) and that bias persists through stage 41. At stage 42, after forelimbs emerge, the laterality in startle responses fades away.

Since Microhyla larvae are externally symmetrical, yet preferentially turn to the left, we can reject the hypothesis that a turning bias in tadpoles is obligated by external morphological asymmetry. An alternative working hypothesis, given the limited taxa that have been examined to date, is that handedness in tadpoles is phylogenetically conserved and independent of spiracle position in tadpoles.