The Kinematics of Swimming in Anuran Larvae

Abstract

ABSTRACT The kinematics of swimming in tadpoles from four species of anurans (Rana catesbeiana Shaw, Rana septentrionalis Baird, Rana clamitans Latreille and Bufo americanus Holbrook) was studied using computer-assisted analysis of high speed (⩾200 frames s− 1) ciné records. Tadpoles exhibit the same positive, linear relationship between tail beat frequency and specific swimming speed commonly reported for sub-carangiform fishes. Tadpoles show an increase in the maximum amplitude of the tail beat with increasing swimming speed up to approximately 4 lengths s− 1. Above 4 lengths s− 1, amplitude approaches an asymptote at approximately 25% of length. Tadpoles with relatively longer tails have lower specific amplitudes. Froude efficiencies for tadpoles are similar to those reported for most subcarangiform fishes. Bufo larvae tend to have higher specific maximum amplitude, higher tail beat frequencies, lower propeller efficiencies (at least at intermediate speeds) and substantially less axial musculature than do comparable-sized Rana larvae. These differences may relate to the fact that Bufo larvae are noxious to many potential predators and consequently need not rely solely on locomotion for defence. Tadpoles exhibit larger amounts of lateral movement at the snout than do most adult fishes. The point of least lateral movement during swimming in tadpoles is at the level of the semi-circular canals, as assumed in models on the evolution of the vertebrate inner ear. Passive oscillation of anaesthetized and curarized tadpoles at the base of their tail produces normal kinematics in the rest of the tail. This supports the idea that muscular activity in the posterior, tapered portion of the tadpole tail does not serve a major role in thrust production during normal, straightforward swimming at constant velocity. The angle of incidence and lateral velocity of the tail tip as it crosses the path of motion are not consistent with theoretical predictions of how thrust should be generated. The same parameters evaluated at the high point of the tail fin (approximately midtail) suggest that that portion of the tail generates thrust most effectively. Ablation of the end of the tail in passively oscillated tadpoles confirms that the terminal portion of the tadpole tail serves to reduce excessive amplitude in the more anterior portion of the tail, where most thrust is generated. The posterior portion of the tail is important in reducing turbulence around a tadpole. It may also function to produce thrust during irregular, intricate movements, such as swimming backwards. Tadpoles are comparable to subcarangiform fishes of similar size in their maximum swimming speed and mechanical efficiency, despite the fact that they have much less axial musculature and lack the elaborate skeletal elements that stiffen the fins in fishes. The simple shape of the tadpole tail appears to allow these animals efficient locomotion over short distances and high manoeuvrability, while maintaining the potential for rapid morphological change at metamorphosis.