Developing elastic mechanisms: Ultrafast motion and cavitation emerge at the millimeter scale in juvenile snapping shrimp

Organisms such as jumping froghopper insects and punching mantis shrimp
use spring-based propulsion to achieve fast motion. Studies of elastic
mechanisms primarily focus on fully developed and functional mechanisms in
adult organisms. However, the ontogeny and development of these mechanisms
can provide important insights into lower size limits of spring-based
propulsion, the ecological or behavioral relevance of ultrafast movement,
and the scaling of ultrafast movement. Here we examine the development of
the spring-latch mechanism in the big claw snapping shrimp, Alpheus
heterochaelis (Alpheidae). Adult snapping shrimp use an enlarged claw to
produce high-speed strikes that generate cavitation bubbles. However,
until now, it was unclear when the elastic mechanism emerges during
development and whether juvenile snapping shrimp can generate cavitation
at this size. We reared A. heterochaelis from eggs, through their larval
and postlarval stages. Starting one month after hatching, the snapping
shrimp snapping claw gradually developed a spring-actuated mechanism and
began snapping. We used high-speed videography (300,000 frames s-1) to
measure juvenile snaps. We discovered that juvenile snapping shrimp
generate the highest recorded accelerations (5.8x105 ± 3.3x105 m s-2) for
repeated use and underwater motion and are capable of producing cavitation
at the millimeter scale. The angular velocity of snaps did not change as
juveniles grew; however, juvenile snapping shrimp with larger claws
produced faster linear speeds and generated larger, longer-lasting
cavitation bubbles. These findings establish the development of the
elastic mechanism and cavitation in snapping shrimp and provide insights
into early life-history transitions in spring-actuated mechanisms.