Startle response and turning bias in Microhyla tadpoles

Rana tadpoles are known to have a left-handed turning bias whereas Xenopus larvae lack such a preference in turning direction. Since Rana tadpoles hav e a single, external, left-handed spiracle, we previously suggested that a turning bias in tadpoles may be obligatorily linked to that external asymme try. We have tested this idea by examining turning bias in the startle resp onse of Microhyla ornata tadpoles. Microhyla tadpoles are, like Xenopus, ex ternally symmetrical, but phylogenetically they are more closely related to Rana. Individual Microhyla tadpoles were startled by a solenoid-driven plunger th at sent a shock wave up through the bottom of a container holding each tadp ole. 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-hatc hing, 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 i s obligated by external morphological asymmetry. An alternative working hyp othesis, given the limited taxa that have been examined to date, is that ha ndedness in tadpoles is phylogenetically conserved and independent of spira cle position in tadpoles.