Abstract
Copepods are agile microcrustaceans that are capable of maneuvering freely in water. However, the physical mechanisms driving their rotational motion are not entirely clear in small larvae (nauplii). Here we report high-speed video observations of copepod nauplii performing acrobatic feats with three pairs of appendages. Our results show rotations about three principal axes of the body: yaw, roll, and pitch. The yaw rotation turns the body to one side and results in a circular swimming path. The roll rotation consists of the body spiraling around a nearly linear path, similar to an aileron roll of an airplane. We interpret the yaw and roll rotations to be facilitated by appendage pronation or supination. The pitch rotation consists of flipping on the spot in a maneuver that resembles a backflip somersault. The pitch rotation involved tail bending and was not observed in the earliest stages of nauplii. The maneuvering strategies adopted by plankton may inspire the design of microscopic robots, equipped with suitable controls for reorienting autonomously in three dimensions.
Highlights
Animals change the orientation of their body by coordinating the movements of various body parts
The axes were defined such that the roll axis is along the length of the body; the pitch axis extends across the body, toward the right-hand side of the body, as perceived from the body; and the yaw axis is perpendicular to the plane formed by the pitch and roll axes, pointing in the direction from the ventral to the dorsal side of the body
The appendages generate fluid flow at Reynolds number Re = L(L/T)/ν~0.3, where ν~1 mm2 s−1 is the kinematic viscosity of water at room temperature, L~0.05 mm is the length scale set by the average length of all appendages, and L/T is the velocity scale
Summary
Animals change the orientation of their body by coordinating the movements of various body parts. For microscopic organisms with small inertia, they must actively and repeatedly move their body parts in order to rotate adequately in fluids dominated by viscosity. In this physical regime of low Reynolds number (Re), it is well known that bacteria can tumble [3], and phototactic algae can reorient [4], using flexible flagella. Microcrustaceans such as larval copepods have relatively stiff bodies and appendages. Further research is needed to unravel the physical mechanisms underlying the rotational motion of larval copepods
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