McHenry M J
Department of Integrative Biology, University of California, Berkeley, 94720, USA.
J Exp Biol. 2001 Sep;204(Pt 17):2959-73. doi: 10.1242/jeb.204.17.2959.
A great diversity of unicellular and invertebrate organisms swim along a helical path, but it is not well understood how asymmetries in the body shape or the movement of propulsive structures affect a swimmer's ability to perform the body rotation necessary to move helically. The present study found no significant asymmetries in the body shape of ascidian larvae (Distaplia occidentalis) that could operate to rotate the body during swimming. By recording the three-dimensional movement of free-swimming larvae, it was found that the tail possessed two bends, each with constant curvature along their length. As these bends traveled posteriorly, the amplitude of curvature changes was significantly greater in the concave-left direction than in the concave-right direction. In addition to this asymmetry, the tail oscillated at an oblique angle to the midline of the trunk. These asymmetries generated a yawing moment that rotated the body in the counterclockwise direction from a dorsal view, according to calculations from hydrodynamic theory. The tails of resting larvae were bent in the concave-left direction with a curvature statistically indistinguishable from the median value for tail curvature during swimming. The flexural stiffness of the tails of larvae, measured in three-point bending, may be great enough to allow the resting curvature of the tail to have an effect on the symmetry of kinematics. This work suggests that asymmetrical tail motion is an important mechanism for generating a yawing moment during swimming in ascidian larvae and that these asymmetries may be caused by the tail's bent shape. Since helical motion requires that moments also be generated in the pitching or rolling directions, other mechanisms are required to explain fully how ascidian larvae generate and control helical swimming.
多种多样的单细胞生物和无脊椎动物沿着螺旋路径游动，但身体形状的不对称或推进结构的运动如何影响游泳者进行螺旋运动所需的身体旋转能力，目前还不太清楚。本研究发现，海鞘幼虫（西方住囊虫）的身体形状不存在明显的不对称性，而这种不对称性在游泳时可使身体发生旋转。通过记录自由游动幼虫的三维运动，发现其尾巴有两个弯曲，每个弯曲沿其长度方向曲率恒定。当这些弯曲向后移动时，曲率变化的幅度在凹向左的方向上明显大于凹向右的方向。除了这种不对称性外，尾巴相对于躯干中线以倾斜角度摆动。根据流体动力学理论计算，这些不对称性产生了一个偏航力矩，从背面看，该力矩使身体沿逆时针方向旋转。静止幼虫的尾巴向凹向左的方向弯曲，其曲率在统计学上与游泳时尾巴曲率的中值没有区别。在三点弯曲试验中测量的幼虫尾巴的弯曲刚度可能足够大，使得尾巴的静止曲率能够影响运动学的对称性。这项工作表明，不对称的尾巴运动是海鞘幼虫游泳时产生偏航力矩的重要机制，并且这些不对称性可能是由尾巴的弯曲形状引起的。由于螺旋运动还需要在俯仰或滚动方向上产生力矩，因此需要其他机制来全面解释海鞘幼虫如何产生和控制螺旋游动。
