How does tube foot coordination generate a novel bouncing gait in the Indo-Pacific seastar Protoreaster nodosus and the North Atlantic seastar Asterias forbesi?
Faculty Mentor: Amy Johnson
Funded by the Doherty Coastal Studies Fellowship
Recent work from the lab of Dr. Tatsuo Motokawa characterized a novel bouncing gait in the Indo-Pacific seastar Protoreaster nodosum (Ellers et al. 2014). Ellers et al. (2014) found the bouncing gait was associated with higher velocities than crawling locomotion, that kinetic and potential energy exchange were in phase (as it is for terrestrial running), and that groups of podia coordinated in pushing during a bounce. Since bouncing locomotion has only been observed (without interpretation) in one other species (Montgomery & Palmer 2012), our research asked whether the bouncing gait was more widespread in seastars by studying local seastar species Asterias forbesi and Asterias rubens. We also quantified how bouncing locomotion was associated with speed and podial pushes in these local species.
Field Collection and Naming
Seastars, varying in size, were collected from the Giant Stairs located on Bailey Island, Maine adjacent to Casco Bay. Each seastar was given a unique name that served as an identifier when filming and during data analyses. Once identified, each seastar was housed in individual baskets at Bowdoin College in seawater circulating flow tanks.
Seastars were filmed bouncing through a filtered seawater-filled Plexiglas® tank set at a temperature of 55°F. Two Nikon® DSLR cameras were set at the bottom and the side of the tank to film movement in the x, y, and z planes. The bottom camera used a wide-angle lens to have a fine resolution of podia moving across the bottom surface of tank. Added lighting was gained through the use of clip-on lamps surrounding the tank.
Seastars needed to exhibit the bouncing gait for filming. Originally, seastars displayed this behavior when a basket containing a number of seastars straight out the field was overturned into the Plexiglas® tank. If seastars did not bounce, the response could be elicited a number of different ways. Seastars were overturned underwater and were allowed to exhibit a righting response, using tube feet and arms to restore a normal upright position in which the mouth faces the surface they are walking upon. Following the righting response seastars generally began the bouncing gait. Also, a small amount of pressure applied for a short amount of time to the side opposite the direction of travel could also be used to evoke bouncing in a straight line down the center of the tank. Finally, seastars could be held upside down in the palm of the hand outside of the tank for a short amount of time and once returned upright would begin bouncing.
Films were exported to Tracker Video Analysis to track the movement of seastars; Tracker is a free video analysis and modeling tool built on the Open Source Physics (OSP) Java framework. Side profile tracks, mouth tracks, and podial tracks were acquired to characterize the bouncing gait. Data files were exported to Microsoft Excel© and Wolfram Mathematica® 9 for further analyses.
Motion analyses of videos revealed that the bouncing gait was present in the local seastars Asterias rubens and Asterias forbesi. The bouncing was similar to that of Protoreaster nodosus in that: (1) there were stance and swing times associated with podial pushes – tube feet sticking to and pushing assisting in the pushing during a bounce and swinging as the tube feet moved with the body (Figure 1), (2) bouncing was associated with increased horizontal speeds; (3) the relative magnitudes of increased speeds were similar to that of a human doubling to tripling their speed from jogging to sprinting (Figure 2), and (4) potential and kinetic energy exchange were in phase similar to terrestrial running where the highest displacement was associated with the highest horizontal velocity (Figure 3).
We also asked: what level of coordination do podia have during bouncing? Although it was difficult to obtain long cycles because podia were hard to track through multiple bounces, stance times of podia, or time that a tube spent attached to the substratum in a push, were calculated. On average podia pushed for no more than one bounce cycle (up and down movement) in three and generally began pushing a bit before a bounce began (Figure 4).