Tardigrades exhibit robust interlimb coordination across walking speeds and terrains

Comparative analysis of tardigrade locomotion across life stage, species, and disulfiram treatment

Animal locomotion requires coordination between the central and peripheral nervous systems, between sensory inputs and motor outputs, and between nerves and muscles. Analysis of locomotion thus provides a comprehensive and sensitive readout of nervous system function and dysfunction. Tardigrades, the smallest known walking animals, coordinate movement of their eight legs with a relatively simple nervous system, and are a promising model for neuronal control of limb-driven locomotion. Here, we developed open-source tools for automated tracking of tardigrade locomotion in an unconstrained two-dimensional environment, for measuring multiple parameters of individual leg movements, and for quantifying interleg coordination. We used these tools to analyze >13,000 complete strides in >100 tardigrades, and identified preferred walking speeds and distinct step coordination patterns associated with those speeds. In addition, the rear legs of tardigrades, although they have distinct anatomy and step kinematics, were nonetheless incorporated into overall patterns of interleg coordination. Finally, comparisons of tardigrade locomotion across lifespan, between species, and upon disulfiram treatment suggested that neuronal regulation of high-level aspects of walking (e.g. speed, turns, walking bout initiation) operate independently from circuits controlling individual leg movements and interleg coordination.

Cretaceous amber inclusions illuminate the evolutionary origin of tardigrades

Tardigrades are a diverse phylum of microscopic invertebrates widely known for their extreme survival capabilities. Molecular clocks suggest that tardigrades diverged from other panarthropods before the Cambrian, but their fossil record is extremely sparse. Only the fossil tardigrades Milnesium swolenskyi (Late Cretaceous) and Paradoryphoribius chronocaribbeus (Miocene) have resolved taxonomic positions, restricting the availability of calibration points for estimating for the origin of this phylum. Here, we revise two crown-group tardigrades from Canadian Cretaceous-aged amber using confocal fluorescence microscopy, revealing critical morphological characters that resolve their taxonomic positions. Formal morphological redescription of Beorn leggi reveals that it features Hypsibius-type claws. We also describe Aerobius dactylus gen. et sp. nov. based on its unique combination of claw characters. Phylogenetic analyses indicate that Beo. leggi and Aer. dactylus belong to the eutardigrade superfamily Hypsibioidea, adding a critical fossil calibration point to investigate tardigrade origins. Our molecular clock estimates suggest an early Paleozoic diversification of crown-group Tardigrada and highlight the importance of Beo. leggi as a calibration point that directly impacts estimates of shallow nodes. Our results suggest that independent terrestrialization of eutardigrades and heterotardigrades occurred around the end-Carboniferous and Lower Jurassic, respectively. These estimates also provide minimum ages for convergent acquisition of cryptobiosis.

Trackoscope: A low-cost, open, autonomous tracking microscope for long-term observations of microscale organisms

Cells and microorganisms are motile, yet the stationary nature of conventional microscopes impedes comprehensive, long-term behavioral and biomechanical analysis. The limitations are twofold: a narrow focus permits high-resolution imaging but sacrifices the broader context of organism behavior, while a wider focus compromises microscopic detail. This trade-off is especially problematic when investigating rapidly motile ciliates, which often have to be confined to small volumes between coverslips affecting their natural behavior. To address this challenge, we introduce Trackoscope, a 2-axis autonomous tracking microscope designed to follow swimming organisms ranging from 10μm to 2mm across a 325cm2 area (equivalent to an A5 sheet) for extended durations-ranging from hours to days-at high resolution. Utilizing Trackoscope, we captured a diverse array of behaviors, from the air-water swimming locomotion of Amoeba to bacterial hunting dynamics in Actinosphaerium, walking gait in Tardigrada, and binary fission in motile Blepharisma. Trackoscope is a cost-effective solution well-suited for diverse settings, from high school labs to resource-constrained research environments. Its capability to capture diverse behaviors in larger, more realistic ecosystems extends our understanding of the physics of living systems. The low-cost, open architecture democratizes scientific discovery, offering a dynamic window into the lives of previously inaccessible small aquatic organisms.

Kinematics and coordination of moth flies walking on smooth and rough surfaces

The moth fly, Clogmia albipunctata, is a common synanthropic insect with a worldwide range that lives in nearly any area with moist, decaying organic matter. These habitats comprise both smooth, slippery substrates (e.g., bathroom drains) and heterogeneous, bumpy ground (e.g., soil in plant pots). By using terrain of varying levels of roughness, we focus specifically on how substrate roughness at the approximate size scale of the organism affects kinematics and coordination in adult moth flies. Finally, we compare and contrast our characterizations of locomotion in C. albipunctata with previous work of insect walking in naturalistic environments.

Sensorimotor delays constrain robust locomotion in a 3D kinematic model of fly walking

Walking animals must maintain stability in the presence of external perturbations, despite significant temporal delays in neural signaling and muscle actuation. Here, we develop a 3D kinematic model with a layered control architecture to investigate how sensorimotor delays constrain robustness of walking behavior in the fruit fly, Drosophila. Motivated by the anatomical architecture of insect locomotor control circuits, our model consists of three component layers: a neural network that generates realistic 3D joint kinematics for each leg, an optimal controller that executes the joint kinematics while accounting for delays, and an inter-leg coordinator. The model generates realistic simulated walking that matches real fly walking kinematics and sustains walking even when subjected to unexpected perturbations, generalizing beyond its training data. However, we found that the model's robustness to perturbations deteriorates when sensorimotor delay parameters exceed the physiological range. These results suggest that fly sensorimotor control circuits operate close to the temporal limit at which they can detect and respond to external perturbations. More broadly, we show how a modular, layered model architecture can be used to investigate physiological constraints on animal behavior.

Leg-body coordination strategies for obstacle avoidance and narrow space navigation of multi-segmented, legged robots

Introduction

Millipedes can avoid obstacle while navigating complex environments with their multi-segmented body. Biological evidence indicates that when the millipede navigates around an obstacle, it first bends the anterior segments of its corresponding anterior segment of its body, and then gradually propagates this body bending mechanism from anterior to posterior segments. Simultaneously, the stride length between pairs of legs inside the bending curve decreases to coordinate the leg motions with the bending mechanism of the body segments. In robotics, coordination between multiple legs and body segments during turning for navigating in complex environments, e.g., narrow spaces, has not been fully realized in multi-segmented, multi-legged robots with more than six legs.

Method

To generate the efficient obstacle avoidance turning behavior in a multi-segmented, multi-legged (millipede-like) robot, this study explored three possible strategies of leg and body coordination during turning: including the local leg and body coordination at the segment level in a manner similar to millipedes, global leg amplitude change in response to different turning directions (like insects), and the phase reversal of legs inside of turning curve during obstacle avoidance (typical engineering approach).

Results

Using sensory inputs obtained from the antennae located at the robot head and recurrent neural control, different turning strategies were generated, with gradual body bending propagation from the anterior to posterior body segments.

Discussion

We discovered differences in the performance of each turning strategy, which could guide the future control development of multi-segmented, legged robots.

Synchronous multi-segmental activity between metachronal waves controls locomotion speed in Drosophila larvae

The ability to adjust the speed of locomotion is essential for survival. In limbed animals, the frequency of locomotion is modulated primarily by changing the duration of the stance phase. The underlying neural mechanisms of this selective modulation remain an open question. Here, we report a neural circuit controlling a similarly selective adjustment of locomotion frequency in Drosophila larvae. Drosophila larvae crawl using peristaltic waves of muscle contractions. We find that larvae adjust the frequency of locomotion mostly by varying the time between consecutive contraction waves, reminiscent of limbed locomotion. A specific set of muscles, the lateral transverse (LT) muscles, co-contract in all segments during this phase, the duration of which sets the duration of the interwave phase. We identify two types of GABAergic interneurons in the LT neural network, premotor neuron A26f and its presynaptic partner A31c, which exhibit segmentally synchronized activity and control locomotor frequency by setting the amplitude and duration of LT muscle contractions. Altogether, our results reveal an inhibitory central circuit that sets the frequency of locomotion by controlling the duration of the period in between peristaltic waves. Further analysis of the descending inputs onto this circuit will help understand the higher control of this selective modulation.

Automated, non-invasive Varroa mite detection by vibrational measurements of gait combined with machine learning

Little is known about mite gait, but it has been suggested that there could be greater variation in locomotory styles for arachnids than insects. The Varroa destructor mite is a devastating ectoparasite of the honeybee. We aim to automatically detect Varroa-specific signals in long-term vibrational recordings of honeybee hives and additionally provide the first quantification and characterisation of Varroa gait through the analysis of its unique vibrational trace. These vibrations are used as part of a novel approach to achieve remote, non-invasive Varroa monitoring in honeybee colonies, requiring discrimination between mite and honeybee signals. We measure the vibrations occurring in samples of freshly collected capped brood-comb, and through combined critical listening and video recordings we build a training database for discrimination and classification purposes. In searching for a suitable vibrational feature, we demonstrate the outstanding value of two-dimensional-Fourier-transforms in invertebrate vibration analysis. Discrimination was less reliable when testing datasets comprising of Varroa within capped brood-cells, where Varroa induced signals are weaker than those produced on the cell surface. We here advance knowledge of Varroa vibration and locomotion, whilst expanding upon the remote detection strategies available for its control.

Bioinspired claw-engaged and biolubricated swimming microrobots creating active retention in blood vessels

Swimming microrobots guided in the circulation system offer considerable promise in precision medicine but currently suffer from problems such as limited adhesion to blood vessels, intensive blood flow, and immune system clearance-all reducing the targeted interaction. A swimming microrobot design with clawed geometry, a red blood cell (RBC) membrane-camouflaged surface, and magnetically actuated retention is discussed, allowing better navigation and inspired by the tardigrade's mechanical claw engagement, coupled to an RBC membrane coating, to minimize blood flow impact. Using clinical intravascular optical coherence tomography in vivo, the microrobots' activity and dynamics in a rabbit jugular vein was monitored, illustrating very effective magnetic propulsion, even against a flow of ~2.1 cm/s, comparable with rabbit blood flow characteristics. The equivalent friction coefficient with magnetically actuated retention is elevated ~24-fold, compared to magnetic microspheres, achieving active retention at 3.2 cm/s, for >36 hours, showing considerable promise across biomedical applications.

Flexible locomotion in complex environments: the influence of species, speed and sensory feedback on panarthropod inter-leg coordination

Panarthropods (a clade containing arthropods, tardigrades and onychophorans) can adeptly move across a wide range of challenging terrains and their ability to do so given their relatively simple nervous systems makes them compelling study organisms. Studies of forward walking on flat terrain excitingly point to key features in inter-leg coordination patterns that seem to be 'universally' shared across panarthropods. However, when movement through more complex, naturalistic terrain is considered, variability in coordination patterns - from the intra-individual to inter-species level - becomes more apparent. This variability is likely to be due to the interplay between sensory feedback and local pattern-generating activity, and depends crucially on species, walking speed and behavioral goal. Here, I gather data from the literature of panarthropod walking coordination on both flat ground and across more complex terrain. This Review aims to emphasize the value of: (1) designing experiments with an eye towards studying organisms in natural environments; (2) thoughtfully integrating results from various experimental techniques, such as neurophysiological and biomechanical studies; and (3) ensuring that data is collected and made available from a wider range of species for future comparative analyses.

Multiresponsive Microactuator for Ultrafast Submillimeter Robots

Untethered submillimeter microrobots have significant application prospects in environment monitoring, reconnaissance, and biomedicine. However, they are practically limited to their slow movement. Here, an electrical/optical-actuated microactuator is reported and developed into several untethered ultrafast submillimeter robots. Composed of multilayer nanofilms with exquisitely designed patterns and high surface-to-volume ratios, the microrobot exhibits flexible, precise, and rapid response under voltages and lasers, resulting in controllable and ultrafast inchworm-type movement. The proposed design and microfabrication approach allows various improved and distinctive 3D microrobots simultaneously. The motion speed is highly related to the laser frequency and reaches 2.96 mm/s (3.66 body length/s) on the polished wafer surface. Excellent movement adaptability of the robot is also verified on other rough substrates. Moreover, directional locomotion can be realized simply by the bias of the irradiation of the laser spot, and the maximum angular speed reaches 167.3°/s. Benefiting from the bimorph film structure and symmetrical configuration, the microrobot is able to maintain functionalized after being crashed by a payload 67 000 times heavier than its weight, or at the unexpectedly reversed state. These results provide a strategy for 3D microactuators with precise and rapid response, and microrobots with fast movement for delicate tasks in narrow and restrictive scenarios.

Tardigrade stowaways: literature review of Propyxidium tardigradum and its first record in Scotland

Tardigrades are a phylum of microscopic invertebrates with a global distribution. Although our understanding of their systematic position and taxonomy has increased and continues to grow, their relationship with the other organisms that share their habitat remains poorly studied. One such organism is Propyxidium tardigradum, a peritrich ciliate which uses tardigrades for dispersion and as a substrate for reproduction. Here, we present the first Scottish record and tenth global occurrence of Propyxidium tardigradum, thereby expanding our knowledge of its poorly understood zoogeographic distribution. We also summarise the literature concerning P. tardigradum biology, proffer hypotheses regarding the Propyxidium-tardigrade relationship, and the apparent lack of heterotardigrade ciliate infestation. Additionally, we indicate a number of recommendations for the direction of future studies regarding the ciliate. Finally, we add a further three species, Milnesium variefidum, Hypsibius cf. scabropygus and Macrobiotus scoticus to the list of Propyxidium host species.

Do we all walk the walk? A comparison of walking behaviors across tetrapods

A walking gait has been identified in a range of vertebrate species with different body plans, habitats, and life histories. With increased application of this broad umbrella term, it has become necessary to assess the physical characteristics, analytical approaches, definitions, and diction used to describe walks. To do this, we reviewed studies of slow speed locomotion across a range of vertebrates to refine the parameters used to define walking, evaluate analytical techniques, and propose approaches to maximize consistency across subdisciplines. We summarize nine key parameters used to characterize walking behaviors in mammals, birds, reptiles, amphibians, and fishes. After identifying consistent patterns across groups, we propose a comprehensive definition for a walking gait. A walk is a form of locomotion where the majority of the forward propulsion of the animal comes from forces generated by the appendages interacting with the ground. During a walk, an appendage must be out of phase with the opposing limb in the same girdle and there is always at least one limb acting as ground-support (no suspension phase). Additionally, walking occurs at dimensionless speeds <1 v* and the duty factor of the limbs is always >0.5. Relative to other gaits used by the same species, the stance duration of a walk is long, the cycle frequency is low, and the cycle distance is small. Unfortunately, some of these biomechanical parameters, while effectively describing walks, may also characterize other, non-walking gaits. Inconsistent methodology likely contributes to difficulties in comparing data across many groups of animals; consistent application of data collection and analytical techniques in research methodology can improve these comparisons. Finally, we note that the kinetics of quadrupedal movements are still poorly understood and much work remains to be done to understand the movements of small, exothermic tetrapods.

Locomotion in the pseudoscorpion Chelifer cancroides - forward, backward and upside down walking in an eight-legged arthropod

While insect locomotion has been intensively studied, there are comparably few studies investigating octopedal walking behaviour, and very little is known about pseudoscorpions in particular. Therefore, we performed an extensive locomotion analysis during forward, backward and upside down walking in the cosmopolitan pseudoscorpion Chelifer cancroides. During forward locomotion, we observed C. cancroides to freeze locomotion frequently for short time periods. These microstops were barely visible to the naked eye with a duration of 100-200 ms. Our locomotion analysis revealed that C. cancroides performs a statically stable and highly coordinated alternating tetrapod gait during forward and backward walking, with almost complete inversion of the tetrapod schemes, but no rigidly fixed leg coordination during upside down walks with low walking speeds up to 4 body lengths per second. Highest speeds (up to 17 body lengths per second), mainly achieved by consistent leg coordination and strong phase shifts, were observed during backward locomotion (escape behaviour), while forward walking was characterised by lower speeds and phase shifts around 10% between two loosely coupled leg groups within one tetrapod. That is, during the movement of one tetrapod group, the last and the third leg are almost synchronous in their swing phases, as are the second and the first leg. A special role of the second leg pair was demonstrated, probably mainly for stability reasons and related to the large pedipalps.

Universal Features in Panarthropod Inter-Limb Coordination during Forward Walking

Terrestrial animals must often negotiate heterogeneous, varying environments. Accordingly, their locomotive strategies must adapt to a wide range of terrain, as well as to a range of speeds to accomplish different behavioral goals. Studies in Drosophila have found that inter-leg coordination patterns (ICPs) vary smoothly with walking speed, rather than switching between distinct gaits as in vertebrates (e.g., horses transitioning between trotting and galloping). Such a continuum of stepping patterns implies that separate neural controllers are not necessary for each observed ICP. Furthermore, the spectrum of Drosophila stepping patterns includes all canonical coordination patterns observed during forward walking in insects. This raises the exciting possibility that the controller in Drosophila is common to all insects, and perhaps more generally to panarthropod walkers. Here, we survey and collate data on leg kinematics and inter-leg coordination relationships during forward walking in a range of arthropod species, as well as include data from a recent behavioral investigation into the tardigrade Hypsibius exemplaris. Using this comparative dataset, we point to several functional and morphological features that are shared among panarthropods. The goal of the framework presented in this review is to emphasize the importance of comparative functional and morphological analyses in understanding the origins and diversification of walking in Panarthropoda. Introduction.