Locomotion in Amphibian Larvae (or "Why Aren't Tadpoles Built Like Fishes?")

SYNOPSIS. A variety of morphological features that affect locomotiondistinguish larvae of the three living amphibian orders fromfishes and their larvae. The oddest amphibian larvae are anurantadpoles. With their globose bodies, concealed forelimbs, abruptlycompressed and terminally tapered tails, tadpoles not only differradically from fishes but they—unlike caecilians or salamanders—alsodiffer radically from their adults. Tadpoles typically haveless axial musculature and much simpler myotomes than fishes.Surprisingly, in terms of mechanical (propeller) efficiencyand maximum sprint speeds, tadpoles still perform as well asmany teleosts of comparable sizes. From a consideration of hydromechanics,no amphibian larvae appear to be designed for sustained swimmingat high speeds. High maneuverability, rather than sustainablespeed, are important for amphibian larval survival.Two key featuresof tadpoles are the absence of caudal vertebrae and unexposedpectoral appendages. With only a notochord to serve as a skeleton,the tadpole tail is extremely flexible. Because of this exceptionalflexibility, tadpoles can fold their tails up against the bodyand turn rapidly with virtually no displacement of their centerof mass. Caudal flexibility can be regulated by muscle activityin the tadpole to effect turning. Lateral appendages are notneeded for this movement and are free to develop directly intotheir adult morphology; the anterior ones develop under coverof an opercular fold where they do not contribute to drag. Acase is presented, based on the ecology of metamorphosis, thatanuran transformation should be as brief as possible. With nobone to resorb, metamorphosis of the anuran caudal appendagecan, indeed, be very rapid.The basic kinematics of constantvelocity straightforward swimming for tadpoles and salamanderlarvae is reviewed, as well as the kinematics and electromyographyof starting, stopping, and turning in tadpoles. An attempt ismade to relate swimming kinematics to the characteristic morphologiesof amphibian larvae. Swimming speed in Rana, Bufo and Aynbystomalarvae, which swim only intermittently, is modulated by changingtail beat frequency. However, Xenopus, which swims constantlyby sculling with its tail, regulates swimming speed (at lowto intermediate velocities) by varying the length of the propulsivewave in its tail. Xenopus and Rana differ in the morphologyof their notochord, spinal cord, spinal nerves, and spinal motorpool distribution within the spinal cord. These differencesmay underlie the different way these larvae regulate swimming.They may also reflect their phylogenetic history.