Snaps of a tiny amphipod push the boundary of ultrafast, repeatable movement.

ABSTRACT

Surprisingly, the fastest motions are not produced by large animals or robots. Rather, small organisms or structures, including cnidarian stinging cells, fungal shooting spores, and mandible strikes of ants, termites, and spiders, hold the world acceleration records.1-5 These diverse systems share common features they rapidly convert potential energy - stored in deformed material or fluid - into kinetic energy when a latch is released.4-6 However, the fastest of these are not repeatable, because mechanical components are broken or ejected.5,6 Furthermore, some of these systems must overcome the added challenge of moving in water, where high density and viscosity constrain acceleration at small sizes. Here we report the kinematics of repeatable, ultrafast snaps by tiny marine amphipods (Dulichiella cf. appendiculata). Males use their enlarged major claw, which can exceed 30% of body mass, to snap a 1 mm-long dactyl with a diameter equivalent to a human hair (184 µm). The claw snaps closed extremely rapidly, averaging 93 µs, 17 m s-1, and 2.4 x 105 m s-2. These snaps are among the smallest and fastest of any documented repeatable movement, and are sufficiently fast to operate in the inertial hydrodynamic regime (Reynolds number (Re) >10,000). They generate audible pops and rapid water jets, which occasionally yield cavitation, and may be used for defense. These amphipod snaps push the boundaries of acceleration and size for repeatable movements, particularly in water, and exemplify how new biomechanical insights can arise from unassuming animals. VIDEO ABSTRACT.