Swimming kinematics and hydrodynamics of barnacle larvae throughout development

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.

Population Genetic Differentiation on the Hydrothermal Vent Crabs Xenograpsus testudinatus along Depth and Geographical Gradients in the Western Pacific

Connectivity in deep-sea organisms must be considered across both depth gradient and horizontal geographical scales. The depth-differentiation hypothesis suggests that strong environmental gradients (e.g., light, temperature, pressure) and habitat heterogeneity in the deep-sea can create selection pressure, and this can result in genetic population divergence. The hydrothermal vent crab Xenograpsus testudinatus (Xenograpsidae) is common in vents at Kueishan Island, Taiwan, ranging from 10 to about 300 m depths. Xenograpsus testudinatus has also been found in shallow water vents (3–20 m) at Kagoshima and the Izu archipelago of Japan. We examine the sequence divergences in the mitochondrial COI, 16S rRNA and D-loop genes, to test the hypothesis that there is significant genetic differentiation among populations of X. testudinatus along the depth gradient at Kueishan Island (30, 200, 209–224 and 250–275 m), and among different geographical regions (Kueishan, Kagoshima and the Izu archipelago) in the West Pacific. There is neither significant population differentiation among shallow or deep-sea vents, nor between geographical locations. Vertical migration of zoea, upwelling on the eastern coast of Taiwan and the strong effect of the Kuroshio Currents has probably resulted in a high level of planktonic larval dispersal of X. testudinatus along the depth and geographical gradients in the Western Pacific.

A tail's tale: Biomechanical roles of dorsal thoracic spine of barnacle nauplii

Many marine invertebrates have complex life histories that begin with a planktonic larval stage. Similar to other plankton, these larval invertebrates often possess protruding body extensions, but their function beyond predator deterrence is not well-documented. For example, the planktonic nauplii of crustaceans have spines. Using the epibiotic pedunculate barnacle Octolasmis spp., we investigated how the dorsal thoracic spine affects swimming and fluid disturbance by comparing nauplii with their spines partially removed against those with intact spines. Our motion analysis showed that amputated Octolasmis spp. swam slower, in jerkier trajectories, and were less efficient per stroke cycle than those with intact spines. Amputees showed alterations in limb beat pattern: larger beat amplitude, increased phase lag, and reduced contralateral symmetry. These changes might partially help increase propulsive force generation and streamline the flow, but were insufficient to restore full function. Particle image velocimetry further showed that amputees had a larger relative area of influence, implying elevated risk by rheotactic predator. Body extensions and their interactions with limb motion play important biomechanical roles in shaping larval performance, which likely influences the evolution of form.

Swimming kinematics of cyprids of the barnacle Balanus glandula

Larvae of barnacles typically pass through naupliar and cyprid planktonic stages before settlement and metamorphosis. As the final larval stage, cyprids swim much faster than nauplii and in turbulent fluid environments with high shears as they seek habitat. Cyprids swim with six pairs of reciprocating thoracic appendages and use two anterior antennules during settlement. Our understanding of how thoracic appendages generate movement is limited due to short stroke intervals (∼5 ms) that impede observations of the shape and trajectory of appendages. Here, we used high speed videography to observe both free-swimming and tethered cyprids of the intertidal acorn barnacle Balanus glandula to produce a comprehensive description of thoracic appendage swimming kinematics. Cyprids used a drag-based method of swimming: their six pairs of thoracic appendages moved through metachronal power strokes and synchronous recovery strokes similar to the thoracopod motions in calanoid copepods during escape swimming. During the power stroke, plumose setae on each appendage pair spread laterally into a high surface area and high drag paddle composed of a meshwork of fused setules. This interconnected setal array collapsed into a low surface area and low drag shape during the recovery stroke. These effective swimming appendages allowed cyprids to move upwards at an average speed of 1.4 cm s-1 (about 25 body lengths s-1) with an average beat frequency of 16 beats s-1, and reach an instantaneous velocity of up to 6 cm s-1. Beat frequency of the thoracic appendages was significantly associated with speed, with higher beat frequencies indicating faster swimming speed. At their average speed, cyprids moved at the intermediate Reynolds number of ∼10, in which both viscous and inertial forces affected movement. Cyprids could alter swimming direction by sweeping the posterior-most appendage pair to one side and beating the remaining thoracic appendages synchronously through the power stroke with greater motion on the outside of their turn. These results greatly enhance our understanding both of cyprid motility and how small planktonic organisms can use swimming appendages with fused setule arrays to reach high swimming speeds and affect directional changes.