Meeting Abstract
Although the The Mauthner cell (M-cell) and its neural circuit are best known for driving the startle response of fishes, some amphibians also have M-cells. One such species is Xenopus laevis, the African clawed frog, which has M-cells at the tadpole and adult life stages. These animals have drastically different startle behaviors before and after metamorphosis: an axial startle response as a tadpole and a limb-based response as an adult. Furthermore, the animal must also perform the startle response during metamorphosis, but how this change in behavior occurs through these intermediate developmental stages has not been characterized in depth. By investigating whether the transition in startle is a discrete or gradual behavioral shift, we can begin to form hypotheses about how the Mauthner circuit itself might be changing to accommodate a tetrapod body plan. We performed high-speed video recordings of X. laevis startle at stages ranging from pre-metamorphic tadpole to adult frog. Our preliminary observations are based on 20 animals ranging between stages 47 and adult. During early hindlimb development (stage 57), limbs tend to remain close to the body axis throughout the duration of the C-bend behavior. Around stage 60, the hindlimbs appear to be involved in the first stage of startle; however, unilateral axial movement still appears to be the primary mode of propulsion. Between stages 61-63, the limbs become the primary propulsors of the startle response. These observations indicate a gradual shift in startle behavior and that M-cells may be transitioning from primarily unilateral activity to bilateral activity to initiate limb movement.
