Directional control in sea star locomotion with respect to light

Sea stars control hundreds of tube feet to navigate their environment with
a rudimentary nervous system. Tube feet are capable of responding to
stimuli without descending nervous commands and it is therefore
unclear to what extent tactic orientation emerges through the
collective action of the feet or is guided by central control. We
therefore performed behavioral experiments to test models of
neuromechanical control in a sea star (Protoreaster nodosus). We
found that animals moved rapidly along relatively
straight trajectories when exposed to light, but slowly crawled
along circuitous paths in random directions in the dark. To
remove mechanical interactions with the substrate, we measured the
kinematics of tube feet in inverted sea stars that exhibited crawling when
in contact with the water’s surface. The tube feet throughout the
body of these animals moved with power strokes in a similar
direction when the animals were exposed to light, which is
consistent with central control. This contrasts the variety of directions
for power strokes exhibited without illumination. These findings
support a model of navigational control where directionless body motion
emerges from collective mechanics in the dark, but is guided by
the nervous system when exposed to light. In this manner, sea stars
navigate through a combination of collective and central control.