The effects of viscosity on the axial motor pattern and kinematics of the African lungfish (Protopterus annectens) during lateral undulatory swimming

Separate studies of terrestrial and aquatic locomotion are abundant, but research addressing locomotion in transitional environments (e. g. mud) is scant. The African lungfish (Protopterus annectens) moves in a gradation of water to mud conditions during seasonal droughts, and breathes air. Thus, the lungfish was an ideal organism for our study to determine the effects of a wide range of viscosities on lateral undulatory swimming and to simulate some of the muddy conditions early tetrapods may have encountered. Regardless of viscosity, several aspects of lungfish swimming were similar to those of other swimming vertebrates including: posteriorly propagated muscle activity that was unilateral and alternated between the left and right sides at each longitudinal location, and posterior increases in the amount of bending, the amplitude of muscle activity and the timing differences between muscle activity and bending. With increased viscosity (1-1000 cSt), significant increases occurred in the amount of lateral bending of the vertebral column and the amplitude of muscle activity, particularly in the most anterior sites, but the distance the fish traveled per tail beat decreased. The magnitude of the phase shift between EMG onset relative to bending increased by as much as 13% of a cycle with increased viscosity, so that the muscles were increasingly active during lengthening rather than shortening. Therefore, with increased viscosity the relationship between axial muscle activity and bending in the lungfish became more dissimilar rather than converging on the motor pattern used by other ectothermic vertebrates when undulating in fully terrestrial environments.