Kaji, Anker, Wirkner & Palmer, 2018.
• The evolutionary history of remarkable snapping claws in shrimp is reconstructed
• Two novel claw-joint types—slip joints and torque-reversal joints—preceded snapping
• The transition “slip joint → torque-reversal joint → snapping” occurred in two families
• Subtle changes in joint form yielded dramatic changes in claw function (e.g., speed)
How do stunning functional innovations evolve from unspecialized progenitors? This puzzle is particularly acute for ultrafast movements of appendages in arthropods as diverse as shrimps, stomatopods, insects, and spiders. For example, the spectacular snapping claws of alpheid shrimps close so fast (∼0.5 ms) that jetted water creates a cavitation bubble and an immensely powerful snap upon bubble collapse. Such extreme movements depend on (1) an energy-storage mechanism (e.g., some kind of spring) and (2) a latching mechanism to release stored energy quickly. Clearly, rapid claw closure must have evolved before the ability to snap, but its evolutionary origins are unknown. Unearthing the functional mechanics of transitional stages is therefore essential to understand how such radical novel abilities arise. We reconstructed the evolutionary history of shrimp claw form and function by sampling 114 species from 19 families, including two unrelated families within which snapping evolved independently (Alpheidae and Palaemonidae). Our comparative analyses, using micro-computed tomography (microCT) and confocal imaging, high-speed video, and kinematic experiments with select 3D-printed scale models, revealed a previously unrecognized “slip joint” in non-snapping shrimp claws. This slip joint facilitated the parallel evolution of a novel energy-storage and cocking mechanism—a torque-reversal joint—an apparent precondition for snapping. Remarkably, these key functional transitions between ancestral (simple pinching) and derived (snapping) claws were achieved by minute differences in joint structure. Therefore, subtle changes in form appear to have facilitated wholly novel functional change in a saltational manner.
Keywords: Alpheidae, Palaemonidae, innovation, functional morphology, biomechanics, evolutionary morphology, evo-devo, comparative morphology, saltational evolution, torque-reversal joint
Tomonari Kaji, Arthur Anker, Christian S. Wirkner and A. Richard Palmer. 2018. Parallel Saltational Evolution of Ultrafast Movements in Snapping Shrimp Claws. Current Biology. 28(1); 106-113. DOI: 10.1016/j.cub.2017.11.044
An adaptation 150 million years in the making phy.so/434192683 via @physorg_com