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Kinematics of sea star legged locomotion

Citation

Ellers, Olaf; Khoriaty, Melody; Johnson, Amy S. (2021), Kinematics of sea star legged locomotion, Dryad, Dataset, https://doi.org/10.5061/dryad.2547d7wrq

Abstract

Sea stars have slower crawling and faster bouncing gaits. Both speed and oscillation amplitude increase during the transition from crawling to oscillating. In the bouncy gait, vertical velocities precede horizontal velocities by 98&[deg], as reflected by clockwise circular hodographs. Potential energy precedes horizontal energy by 16&[deg] and so are nearly in phase. These phase relationships resemble terrestrial running gaits, except that podia are always on the ground. Kinetic and potential energy scale as mass1.1, with the change in kinetic energy consistently two orders of magnitude less, indicating that efficient exchange is not feasible. Frequency of the bouncy gait scales with mass-0.14, which is similar to continuously running vertebrates and indicates that gravitational forces are important. This scaling differs from the Hill model, in which scaling of muscle forces determine frequency. We propose a simple torque stabilized inverted pendulum (TS-IP) model to conceptualize the dynamics of this gait. The TS-IP model incorporates mathematics equivalent to an angular spring, but implemented by a nearly constant upward force generated by the podia in each step. That upward force is just larger than the force required to sustain the underwater weight of the sea star. Even though the bouncy gait is the rapid gait for these sea stars, the pace of movement is still very slow. In fact, the observed Froude numbers (10-2 to 10-3) are much lower than those typical of vertebrate locomotion and are as low or lower than those reported for slow walking fruit flies, which are the lowest values for pedestrian Froude numbers of which we are aware. --

Methods

We took video of ambulating seastars, Asterias forbesi, in a plexiglass raceway. There were two cameras simultaneously recording from the side and from underneath at a nominal frame rate of 30 Hz. There are 128 pairs of  side and bottom films and 54 unique seastars each filmed one to several times. Open Source Physics software called Tracker 5.1.3 by Douglas Brown was used to track the apical point from the side and the mouth from the bottom. The data files give 5 columns: time, x position from the bottom view, y position from the bottom view, x position from the side view and z position from the side view. The name of each file indicates which seastar that file pertains to and also gives the DSC number that indexes the pair of side and bottom film data in each file.

There is also one file that gives the mass of each sea star filmed and the underwater weight of each sea star.

Usage Notes

The files are csv files with the first row being a header giving the column content and the units. Note that the zero value in each of the axis directions is arbitrary.

Funding

Bowdoin College Andrew W Mellon award to A. Johnson