Mantis Shrimp Locomotion: Coordination and Variation of Hybrid Metachronal Swimming.

Across countless marine invertebrates, coordination of closely spaced swimming appendages is key to producing diverse locomotory behaviors. Using a widespread mechanism termed hybrid metachronal propulsion, mantis shrimp swim by moving five paddle-like pleopods along their abdomen in a posterior to anterior sequence during the power stroke and a near-synchronous motion during the recovery stroke. Despite the ubiquity of this mechanism, it is not clear how hybrid metachronal swimmers coordinate and modify individual appendage movements to achieve a range of swimming capabilities. Using high-speed imaging, we measured pleopod kinematics of mantis shrimp (Neogonodactylus bredini), while they performed two swimming behaviors: burst swimming and taking off from the substrate. By tracking each of the five pleopods, we tested how stroke kinematics vary across swimming speeds and the two swimming behaviors. We found that mantis shrimp achieve faster swimming speeds through a combination of higher beat frequencies, smaller stroke durations, and partially via larger stroke angles. The five pleopods exhibit non-uniform kinematics that contribute to the coordination and forward propulsion of the whole system. Micro-hook structures (retinacula) connect each of the five pleopod pairs and differ in their attachment across pleopods-possibly contributing to passive kinematic control. We compare our findings in N. bredini to previous studies to identify commonalities across hybrid metachronal swimmers at high Reynolds numbers and centimeter scales. Through our large experimental dataset and by tracking each pleopod's movements, our study reveals key parameters by which mantis shrimp adjust and control their swimming, yielding diverse locomotor abilities.